Flat roof mould and bad ventilation in a new cold flat roof over a loft dormer.

I've never seen so much black mould and condensation and moisture in a new flat roof.
In this video, I look at how this has happened and the way to resolve the problem. With cold roof construction, the through ventilation is the most important thing even more important that the VCL vaper control layer.
Vaper control layers only controls the migration of moister into the roof structure; if there isn't the ventilation to clear what gets through then, you have a big problem. Now that this roof has been building the owner has to decide on how to redo the roof to put it back as a cold roof or to try and convert to a warm roof. Warm roof is definitely the way to go however this sill rais the roof, and that may need permission from the local authorities as this could be a planning issue.

[@MA-uh8ig]

Benefited a lot from this. My dormer roofs are open at the minute. I notice condensation here and there and realised that as in the video, the air was being blocked by noggins running accross floor of dormer. Took a long time but I drilled 4 inch holes and left them clear of the 100mm insulation. The condensation disappeared in a week. Thanks a bunch roofer guy.

[@kevindesilva1311]

Nice clear video. One thing I have found to add to you story is this can be caused because the roof timbers were soaking wet just prior to being closed in so the water drying out of the joists has no were to go when there is little through draught . I have also found this on warm roofs. In many other countries structural timbers have to be stored under cover , Where as in the UK they are nearly always out in the weather and are soaking wet.

[@SteveRoofer]

yes this is known as construction moister moister that's licked in at the time of construction

[@TheSnakehipster]

Totally agree, I have seen a damp roof enclosed and it didn’t need any further moisture introduced it started with extreme mould from day one which we know leads to rot. No air venting created the problem you see in this video.

[@willbee6785]

I just like this type of info. So clear. Well described & clear audio. A real life example of how it goes wrong. Well done.

[@JonnyPardoePhotos]

Your videos have saved my new build garden room I reckon. I had no idea that ventilation was required on my cold roof but luckily I came across your channel and videos just in time to change my plans. A day later and I would have been in bother. Thanks Steve. You the man.

[@effervescence5664]

Ah planning permission for warm roofs is such a pain. Not allowed to go above current ridge tile height but need those extra 4 inches, not like you can even tell from the ground anyway. Carbon neutral by 2050 they said, but we won't let you do it with modern tech or relax sensible aspects of planning to get there on older properties. Great content on this channel quite enjoying it.

[@bonchodikov6248]

Steve I am not a roofer but I am learning so good from you. I will show you the roof for my house before I close it in case you have some advice. Thank you so much for sharing you knowledge with us

[@nikolvesela5153]

Amazing video, Thank you Steve. I will get in touch when I will being my roof.

[@colingreen4625]

Great description in this video. Cross battening over ferrings would of helped, but front to back ventilation as you said definitely.

[@hazmat5749]

When I studied construction technology (many years ago) I was taught that that there is no such thing as a vapour barrier, only a vapour check. Moisture will always find a way through no matter how much care is taken - so correct ventilation is vital.

[@beatch42]

This is also why i don't put plastic on a ceiling. Vapor barriers have no place on ceilings. Only vapor retarders, like kraft that has variable permeance based on humidity, should be used on cielings

[@benchippy8039]

So good to see a British expert sharing knowledge

[@SteveRoofer]

Thanks

[@simplyr1728]

Great explanation & Warning - We used trapeze lights in our extension, rather than ceiling spots: Plus for most UK properties, I'd also recommend a dehumidifier to combat the moisture from cooking, showers & drying of washing over the rad's....

[@colinmiles1052]

Very informative - thank you. I shudder to think how many poorly constructed cold roofs are out there quietly rotting away?!

[@tac2180]

This guy totally knows his shit . I've learned a lot from your channel mate . I have some small patches of condensation building up on my soffit boards in the loft . Its - 4 outside at the moment but I think I need to put some soffit vents along the overhang .

[@The4Qman]

Very good video, learned a ton. Well explained, the best I've seen covering this kind of issue.

[@SteveRoofer]

Glad you liked it!

[@fuzmaginty5822]

fair play Steve your videos are well explained

[@Craigo5000]

Great video Steve. I've just had a cold flat roof dormer built exactly like this with front sloping roof but there is a chimney stack through the flat roof blocking even more airflow between rafters. There are no 50mm battenings to provide cross-airflow. Insulation consists of 100mm PIR in the rafters underdrawn with 30mm PIR -all PIR is foil backed. The BCO has said the firrings should now be notched every 300-400mm across all 16 rafters to provide cross airflow and a continuous soffit vent added on both dormer roof cheeks to compliment the same soffit venting on the dormer face. Considering I also have downlighters and the PIR has already been cut to allow wiring through it, should I now add an independent VCL on the underdrawn 30mm PIR before the finishing plasterboarding is added and does this remedial work sound like it will adequately prevent condensation build up? Many thanks

[@rodshop5897]

Nice clear video with good explanation and images. I have a concrete block building with a nearly flat roof that has an open soffit at one end and is closed at the opposite end where it terminates at the castle wall. Been trying to figure out how to insulate it, and now it seems like a warm roof is the direction I should be looking. The problem will be the expense of tearing off and re-doing everything above the decking. I'll look at your other videos and see what I can learn first. Thanks!

[@SteveRoofer]

Not a problem if you want to send me over some photos of your concrete deck roof perhaps I can help you

[@rodshop5897]

@@SteveRoofer I just found your video "thermal bridging of an insulated flat roof through the front facia" and your diagrams there look almost exactly like what I have. I am now figuring cost for putting a warm roof on. Cheers from the other side of the pond!

[@FreeskierSean]

I have a similar situation with my garage. I cannot do a warm roof at this point. I am now trying to figure out how I could allow the cold roof to vent properly. I have vented soffits at one end. Would making holes in the rafters at the other end to allow air to flow between the rafter gaps work? Or should I plan to install some kind of vents on the roof at the end opposite the soffits to allow through flow of air?

[@warrensummerfield1]

Really interesting 👍

[@coniow]

I was advised by a builder on exactly this issue years ago. I have seen some videos saying that an air gap of between an inch to 1.5 inches is advisable. Many I have seen take the insulation right to the roofing felt, with no air gap. at all! My intention on a 700 mm rafter is to flush fit 250 mm Recticel with the front to leave a 450 mm airway behind and over the top. Another 250 mm going across and on top of these, and left like that. The loft is just used for storage, not living accommodation, so the aim is to keep it less cold, (as opposed to warm), and dry. Thank you for this video, it reassures me that I am not too far out in my reasoning.

[@JohnnyMotel99]

The builder could have created a continuous air passage from the soffits to the flat roof, if he had allowed an air gap between vapour barrier and insulation.

[@garysimpson6611]

great video very informative 👍

[@SteveRoofer]

Glad you think so!

[@rogerparr9571]

Fantastic explanation and diagram. I came across it whilst looking for a solution to prevent moisture in a brick outhouse with a concrete roof, that i want to convert back to a toilet and utility room. (condenser dryer causing the problem). Any pointers would be great,

[@levismegenbir]

Based on the research I've done, it's important to consider the stack effect with your ventilation plan. Having the vent on one side of the joist cavity lower than on the other creates a passive, temperature-differential driven flow of air through the joist cavities. I'm having an issue with moisture accumulation in the joist cavity for my cold, flat roof even though I do have continuous venting on each side of every joist cavity. I'm trialing making adjustments to my build to promote the stack effect to see if that helps.

[@SteveRoofer]

Excellent point, so having sofit vents at one end the lowest possible point and then say an abutment vent at the other end at the highest point. Not only will you have thought ventilation you will have movement through warm air rising. Love to see what your doing please send some photos [e-mail]

[@marcvipond159]

Well explained.

[@richardwilson5842]

Another possibility is that the OSB roof deck was saturated with rain water when the roof membrane/ covering was installed. Thanks for your time Steve, great videos. Plenty of builders need to be aware of these issues.

[@dancinggorilla7]

If the deck were saturated then whatever was used on too for the roof I.e GRP or EPDM then that would fail and not bond. That’s not the case.

[@oreilly67]

Top video.

[@weedfreer]

hey, nice vid... particularly the use of CAD images to explain what you're seeing. Just wanted to ask, if I have a bedroom where I have patches of black mould appearing on the ceiling that corresponds to the soffit vents behind...what's best to do? I've recently been dealing with woodworm and, during this time, I had to strip out the existing 90s insulation to gain access through to the joists for preventative spraying. As the new absolute minimum thickness of insulation is 250mm, I have decided to implement a raised frame to place my loft boards back onto giving me the space required for the new 200mm (the deepest I could find for a reasonable price 😔) insulation to sit under. I have made the frame +50mm taller than the insulation which should give an air gap above it and, as such direct flow through to the soffit vents. However, I'm wondering...should I be looking at doing anything like spraying a small amount of foam on the back of the plasterboard near where the soffit vents are? Reason I wondered is because I was told it may be as a result of cold spots immediately surrounding the vents. Also, when placing the new insulation in the loft space, would I push it down into the soffit area or leave it with ventilation around it? Thanks for any feedback...I know I'm probably being a little cheeky asking you on here 😅

[@davidokeeffe2474]

Great video man thanks

[@SteveRoofer]

No problem 👍

[@adamkingdon8063]

Astonishing - the problems encountered by people that have paid good money to have their roofs renewed!

[@1234mascherano]

Great video.. just to confirm on the underside of the insulation, should I use a vapour barrier or vapour control layer. And which would you recommend

[@paulrumbold2436]

No air gap mush ? Flat roof problem all day . Needs to be a warm roof really . Cold roof design is bad great video by the way . Yes learnt a lot for once

[@buddytime7217]

As a loft converter of 25 years I can fully explain the issue on this building. A plastic vapour barrier will not let any moisture through whatsoever. On top of this the celotex is totally moisture proof. I know it has joints along the timbers but every building built now is insulated this way with generally no problems. I know you mention the house ambient moisture levels a house sits at 9% but this did not effect this roof because of the plastic vapour barrier and celotex. This was purely air flow its Vital. Raising the ridge lines is a no no in most loft cases.

[@SteveRoofer]

The correct Air Vapour Control Layer (AVCL) will let through vapour; however, it controls the amount hens why its called a control layer. For a domestic property, you would normally install one that worked well to 60% humidity as above. This is considered to be undesirable. If its not installed correctly, then it won't work. Celotex is totally moisture-proof however it has a high moisture resistance but again that's irrelevant if the AVCL is installed incorrectly.

[@Alite2012]

I like your video wanted to know with a garden Room can you do a warm roof from inside

[@HsingSun]

wow! These roofs must to replace asap. My roof is 37 years old which still look great. It would be bad materials and weather.

[@dad8922]

Fantastic explanation, learning alot with rheae videos!! I'm doing a cold roof now on my new extention what's the best way to install downlights?

[@SteveRoofer]

Anyway you can that doesn't cut into the insulation or the VCL so basically build a caverty under the the rafters ( this lowers the ceiling) or use slim LED down lighters

[@ge2719]

jeeze, thats so much water it looks more like the roof leaked. But then one year i noticed my outdoor plastic shed was dripping wet on the inside, with a pool of water in some plastic tubs that were up against the walls... I'd stupidly decided it was a good idea to tape up all the gaps in the structure, and evidently the gap between the roof and the wall panel was supposed to be to allow airflow in and to allow any condensation to flow out and into a sort of guttering system on the outside of the wall panels. Taping up the gap meant air couldn't circulate, and any condensation ran down the inside of the roof, down the wall, onto the shelving that was against the walls, into any boxes that were touching the walls. the whole shed had basically turned into a water condenser/collector.

[@user-yq4sq1wn2w]

Hi Steve Your videos are a life saver! Having discovered the exact same problem 18 months after the dormer being built. I'm likely going to rip everything down, clean, redo the insulation vapour barrier and remove the spot lights. I have soffit ventilation but no ridge ventilation. Will fitting mushroom vents at the ridge end solve the problem? Have you done a video of how to retrofit these into a GRP roof? Thanks

[@amonkeywithabanana4494]

Hey steven i put asphalt emulsion with Polly Esther fabric over my old tar roof I live in a very dry climate here in New Mexico us do you think that'll work I have a little bit of ponding water

[@agentcarbunkle]

Would a fan or a PIV in the room help reduce this problem, before it starts? I'm in the process of insulating my basement ceiling and i'm planning on adding two fans to remove moisture and PIV to bring in filter and heater fresh air. So for i have added to ceiling 100mm batt mineral wool insulation (actually previous owners of the house added this) flush up to t&g floor boards above, then I have added ecotherm 115mm which has two sides of foil filling out the floor joists completely. Then i have 12mm battens as a gap the opposite way across the beams and left gaps so air can flow. I have used 600mm x 1200mm waterproof tile backer boards as the ceiling finish. There was not any mould growth before in the ceiling between beams or on the underside of the floor boards but moist air was definitely escaping into the living room and made the lower walls sticky, like wall paper paste wasnt setting. The ceiling in the basement was covered in MFC Chipboard ! and had expanded / dried that many times it was all out of shape and bowing. Some mould was on this facing into the basement but not as much as you would expect. I'm trying to make the basement as dry as possible but at the moment when it rains around the bay window in the basement it forms puddles - then drains to a center drain over time or condensates. I'm hoping re-pointing my front wall in summer will solve this issue of a water puddles. The main issue where the walls of the cellar someone had the bright idea of timber partitioning the walls out from brick face to make them flat adding two plastic vapour barriers front and back of the timbers adding batt insulation in between it all then plaster boarding. The whole walls where sucking up the moisture and never drying out as the vapour barriers failed most had nails through them. The wood was also fixed to the floor and had practically turned to coal. Mold was growing in between the to the walls and vapour barriers mainly on the timbers. Fun job removing it all and bagging it up! Any way, long story short i was just wondering how much effect fans and PIV's have on the effect on mould growth. at the moment i have the window in the basement open all the time, and no moisture has collected on the ceiling like it was before when shut

[@abbbee8918]

Is it worth using a wood preserver like Sikagard to help control damp and mould on the underside of a cold flat roof? Obviously as well as a 50mm+ well ventilated gap. Would really appreciate some advice on this.

[@johnmurphy1466]

Hi Steve, I’ve just recently done away with the low ceiling in the kitchen which was installed purely to fit spot lights. The original ceiling is a vaulted lean to with a support beam midway which I favour as it’ll open up kitchen and looks far more interesting, the rafters are 4x2, no insulation as the original ceiling is old lath and plaster. It’s a slate lean to roof with a vapour barrier underneath with battens. Not ventilated. I want to strip out the lath and plaster also. What’s the best way to vent and insulate it. I’m guessing after watching numerous videos as well as your own that a cold roof is the way to go?

[@ShedDwellerMakerandRepairer]

By the model on the screen, You can't change to warm roof, unless of course you drop the joists. Because putting insilation on top of this joists would raise the ridge height, and planning very likely won't let you do that.

[@SteveRoofer]

Yes, this is the problem. People don't realise that the difference between warm roof height and cold roof height is very little 50 to 75 mm. If done correctly. Problem is most people don't do cold roof correctly, so the depth of a badly installed cold roof against the depth of a well installed. Warm roof is closer to 100 to 150 mm, so realistically people are measuring things incorrectly in their minds. But whatever way you do it you have to plan. How are you put your joists in from the very beginning if you put them in for a cold roof you can't convert easily to a warm roof

[@Evolve_windows]

Iv had my garage built about 2 years, recently insulated the roof from inside like this, all osb is going black like this and started to smell Am I best to insulate with dry therm and put some vents into fascia at backside of garage ? Is this moisture not from the cold brake between the installation and the cold osb? Same as you get with glass ?

[@Muhammad_asif78]

Hi mate Im thinking of getting our new build roof insulation with kingspan. The external roof is already done with the sandtoft double pantiles with breathable felt now what I notice though the guy who done the roof hasn't put in vent tills in the roof can u give me some idea please for the internal as well thanks. I really like your videos

[@robnelsonphotofilm]

I have wooden noggins between joists that need ventilation to make sure I have good airflow. Thinking of drilling a few holes to let the air flow, is there a specific size for this?

[@davidhogg1886]

@steve roofer : love the rotatable graphic you created. What bit of software did you use?

[@qamarh9713]

Hi Steve, watched a couple of your videos (not a builder, just a homeowner) and have learnt more about and the importance of roof ventilation then I'd ever known. Our dormer is being constructed and the builder was trying to figure out the venitlation as I've proposed a parapet style dormer so it has no sofit/fascia, which they've not had to do before, as the the standard dormers have a sofit/fascia, which is where the ventilation is. How would the ventilation work here and where would it be, while ensuring the parapet is kept to the minimum to avoid it going above the roof ridge (fyi the roof is slanted towards the ridge with drainage on the front of the house) - Appreciate any pointers.

[@SteveRoofer]

You ned to send me sone photos so I can have a look

[@hi2lenduffy837]

Is roofing felt ok to use as a vapour barrier? Also where is the best place to get an abutment vent

[@Droningonuk]

So what's the best way to fit downlights without upsetting the vapour barrier?

[@johncarr3855]

Hope the builder sees this.you.never stop learning. Cant believe this.happened.in.a.year.i dont.blame builder he went too far .

[@trevspark]

How about old buildings is it possible to insulate say a domer by putting up celotex and a vapour barrier just like this?

[@jeromeclatot1769]

Same question here would some packed insulation would have avoided that? By restricting the air flow completely if this is completely tight and dense packed?

[@johnclowes3894]

Just a thought. What if that foam insulation was not as thick. Surely then there would be a gap for air to flow. Does it really Have to be from bottom of joists to top of joists?

[@joshuacarvalho5405]

Hi there, I just had a question regarding Rockwool. Well I ordered some Rockwool aor my shed as I was recommended by a friend. Now someone else tells me that PIR is better as Rockwool can get moulldy. Now when I read the website and it said that Rockwool does not catchbmouls as it's inorganic. My shed is fully made of wood. I would really appreciate your insights. I'm contemplating if I should send this back and get PIR instead. Thank you for your help.

[@nicrobinson5543]

Newbie here. Looking to batten out my internal block wall and put Superquilt over, along with a counter batten and insulated plasterboard on top. Do I need to worry about mould for this project? Many thanks

[@stevecrowable]

Hi This is well explained thank you. I am thinking about buying a timber house and totally expect to have to do some work so what insulation construction would you advice for this for the internal and external walls. Many thanks Steve

[@SteveRoofer]

Yes can do bets to email me or call me details on video above

[@MichaelPickles]

I freak out a bit. I'm remodelling my bathroom. I'm making it as water tight as possible with foam backer board floor, walls and roof all taped up. I've added a mechanical heat recovery unit for the very purpose to stop mould and give good air changes.

[@SteveRoofer]

Sounds good

[@plummetplum]

I'm designing a dormer cold roof now, will a dry fix ventilation ridge tile system be enough to vent at the ridge. Also I will have a steel much like the above, will 37mm PIR underneath satisfy building control for cold bridging bearing in mind the steel will sit exposed to the air directly beneath the dry fix ridge tiles? Thanks.

[@roxy4963]

These sweaty Osb all shows EPDM covering. Is it the same for a torch on felt roof. Do they need an air cavity and cross venting? This is a cold roof from 2012

[@johnobrien2614]

Hi Steve If you are restricted in height above your flat roof could you have a build up of Vapor barrier under joist 9x2 100mm board insulation tight to the plywood deck above. 50mm on top of the deck Then plywood and roof covering.

[@SteveRoofer]

No.if you want to go down this route then the golden rule is to always have more insulation above the decking than below

[@uzohf11]

Do you know that there are mushroom roof vents that can be used to achieve the through ventilation?

[@seanmichaelo9777]

Could use Humidity sensors and vent axia extractors wired to a BMS system like in industrial roofing applications on sensitive units such as document storage or computer rooms or certain mod buildings I’ve worked on

[@SteveRoofer]

Or just vent the roof correctly

[@bonixos]

Great video!! Do you think that a vapour barrier under the deck could help in case of poor ventilation?

[@farmerjimmy7453]

No

[@kenkoh1724]

How much room between roof osb and insulation is needed for airflow?

[@xxxsmithxxxx]

In a cold roof, why leave a cavity at all. What would happen if you pressed the insulation right up against the timber deck?

[@ashleyedmonds6214]

Hi Steve , can you sandwich insulation between the plasterboard and the osb within the rafters so in theory you would have a warm roof internally..

[@SteveRoofer]

No refer to my video on hybrid roofs

[@pweller6500]

Hi Steve. I’ve watched a few of your videos now and find them very informative. I’m about to do a garden build with a pent roof and I’m going for a cold roof because of the height issues. I’ve just watched the video with the dreadful mould growth due to lack of ventilation for the second time now. After watching it the first time I decided not to install down lights. Now after watching it again I now feel that it should be ok to install down lights so long as I have enough air flow through the roof. I just thought I’d ask you for the definitive answer. Also, if I have plenty of air flow across the roof, do I still need to install a vapour barrier? It’s not a case of trying to save money. If you feel it’s still necessary then that’s how it’ll be. Many thanks in advance Regards Pete.

[@SteveRoofer]

Yes, venting is a good part of the equation but also air tightness from below is necessary and cutting in lights compromises that. It also compromises the thermal element as its really hard to achieve a good robust installation if you install spotlight

[@arbit3r]

If the insulation would have been pushed all the way up to the top. Touching the OSB,PLY. Would this have happened?

[@northernjimbo]

Isn’t it a fact that the front gable insulation needs removing, to allow the thru flow of air, rather than adding 150mm above ridge line...

[@mathewgreen9663]

Could it be possible that the moisture came from the unprotected pitched roof section. The ventilation would draw it up through into the flat roof void.

[@benleigh8361]

The other factor contributing to this issue could be “thermal bridging” by the wood joists. The insulation stops flush with the joist, so there is no thermal break between the interior and roof cavity. The joists will transfer energy between the spaces. The temperature difference will result in a phase change of the moisture in the roof cavity, i.e. “sweating”. Insulation needs to be applied under ceiling sheet rock, to properly disrupt this circuit. The tape and vapor barrier installed will stop air leakage from causing this same issue, but have effective R-value of 0, so won’t prevent thermal bridging. Ventilation you recommend is certainly necessary, but in a cool, humid climate passive drying is challenging, so preventing condensation is equally important. Check out #thebuildshow for interesting insulation info!

[@dancinggorilla7]

Thermal bridging wouldn’t cause this. This is 100% down to no/very little air flow as stated

[@bl3dse786]

I recently had a dormer done and might have put the insulation in a manner where I was blocking the airflow slightly. Secondly, I believe that the osb boards were applied and they were wet. How could I dry them after the rubber roof has already been placed on?

[@guyuba7524]

Try using a good quality dehumidifier indoors. Could make a considerable difference

[@OkeeeDokey]

Can you do a warm roof in hot climate say Florida?

[@anthonywilliamson915]

Could it help if the insulation board was pushed tightly to the roof level and leave a gap between the insulation and plaster board in the room just like a hybrid roof construction?

[@SteveRoofer]

no as you cant vent out the moisture

[@Dg-zj6jo]

i got same condensation rus down walt when its hot as its a consevaty but its a rubber roof i did not put a breather pipe in if i put 1 in the roof do you think it would help please cheers

[@SteveRoofer]

Probably you really haven't given me all the information I need is it a proper cold roof? If it is of course cold roofs need ventilation

[@ducaroamer1261]

Overkill insulation to meet ridiculous eco mandates equals sweaty property’s!

[@SteveRoofer]

No bad ventilation

[@subzero3054]

The only thing I learnt from the video was that there are still lots of good builders out there who unfortunately have little technical knowledge. Why don’t the planners request warm roofs and why did BC sign it off! Warm roofs are the only way to go!

[@videogalore]

It's just not always an option sadly - anything that's terraced means that your ridge height is already set and that any space is then reducing headroom internally.

[@subzero3054]

@@videogalore That’s utter nonsense. All it takes is a bit of forward planning.

[@videogalore]

@@subzero3054 I'm talking about loft conversions here, so the house exists, the first floor is already in place and the ridge lines through with the neighbouring properties. You have a lower limit on the floor level and an upper limit on the ridge, so I'm not sure what's nonsense about that?

[@Moultonist]

@@videogalore Yes I agree with you videogalore. Many loft conversions would be unviable if we had to use a warm roof construction because the new ridge line would be too high relative to neighbours and would be refused by planning. Match the ridge line and the internal headroom is too low. I totally understand why builders install a cold roof but it's definitely sub-optimal. And just ban the installation of downlights cut into ceilings. Everywhere. They're just stupid at any floor level when you consider fire, sound, insulation and ventilation. And it's quite difficult to change the bulbs.

[@kevlannigan1330]

👋 Steve so building a garden office/man cave 😂 so would this work ? From inside to outside Plasterboard Vapour barrier 100mm wool insulation 18mm osb Rubber roof And at either end of the soffits ventilation so air runs to one end to the other on each 600mm spacing between joists ?

[@klaeLIFE]

You would need need 50mm of clear void between the insulation and the OSB and vent it at each end

[@frederickbowdler8169]

If joist is running counter to vents from soffits are you best to counter batten with say 2 * 3 Inc batten so air can go to vents preferably on edge to promote really good 3 Inch gap also what vents shall I get how many?

[@SteveRoofer]

If you count batten then you will have lots of ventilation left right up and down ventilation everywhere so put in as many events as you can but if you're using mushroom vents we normally put them about 2 to 3 m apart when cross battening and you must have a face event or more vas mushroom vents the other end

[@SteveRoofer]

Yes but you put the VCL on the outside of the insulation

[@jamesthomson5774]

Would a solution to this be to add a vented dry ridge if keeping as a cold roof?

[@SteveRoofer]

Yes anything to increase the venteration. But more to the point on this building the internal RH relative humidity is much to high so he needs to do more to stop this from coming back

[@rafmo7413]

What a great video. Could the customer in this scenario put venitalition holes/turrets (sorry don't know what they're called) at the ridge end to allow a through movement of air, the flat roof will then have a series of ventilation pipes coming out of it but at least it'll stop the damp and rot. Also warm decks are great and always the way to go, but it depends if the customer has the hight available to do a warm deck on a loft conversion? Or is that not true? Also on a warm deck would the void underneath not need to be vented as it'll be warm due to the insulation being above? Love your videos, just discovered them.

[@SteveRoofer]

No you don't vent under a warm roof and yes more ventilation on the existing cold roof would have help stop the problem

[@rafmo7413]

@@SteveRoofer thanks, I'll def do a warm roof when I do my loft conversion

[@paulus650]

you show it having continuous tape -jointed foil backed kingspan with a polythene layer below that and yet you still state that humidity from the dwelling area will go up into that roofspace - how ?

[@JohnnyMotel99]

@@Ed-ti1ss if the tiled roof section is not vapour sealed on the inside, will not moisture make its way up to the flat roof? I know it’s blocked at the junction, but moisture has a way to find weak spots.

[@TacticalMove]

Sadly I think I have this very issue :-( (thanks for the explanation)

[@SteveRoofer]

Happy to help

[@masherco]

What product would you recommend as a good quality vapour barrier for a cold roof

[@subzero3054]

I wouldn’t recommend a cold roof, steer clear it will end in disaster.

[@plummetplum]

@@subzero3054 Not very helpful, some situations mean only a cold roof can be installed mainly height restrictions. But I get your point.

[@JohnnyMotel99]

If they replaced the rock wool batts with celotex under the tiled section, wouldn’t that allow an air gap between insulation and tiles. This could be ventilated via the soffits. Then there could be a clear path for fresh air to travel all the way across the flat roof section.

[@SteveRoofer]

You dont want to do that as you cood fee warm air into the flat roof from the front pitched roof. What you should do is vent both separately

[@cazzied5714]

I need to insulate a sloped roof in the utility room. Having got advisement from a builder he has said to insulate the rafters and to push the board as far into the pitch as possible. I’m not sure how it will get ventilated if I seal the ends or should I leave the ends open. I am worried it will attract vermin. I am capable of doing this but want to get it right. Also the walls need insulation. I plan on battoning and using insulated plasterboard. Should I also insulate between the battons or will the plasterboard be enough.

[@SteveRoofer]

You ned to levae an air gap above the insulation and vent the ends

[@cazzied5714]

@@SteveRoofer thank you for your advice.

[@MriSimplify]

What are your thoughts on the use of ‘mushroom vents’ adjacent to the ridge on the flat roof? Also your experience of using the Marley vented ridge tile and whether it’s adequate in practice?

[@Eventsandthings]

OSB? sure it will come to me

[@rabzful1]

Hi mate I have similar situation single storey flat roof it’s currently newly build and I have severe condensation problem it’s almost like leaks all over the ceiling since I’ve put my underfloor heating can you help thanks 😊

[@SteveRoofer]

send over info [e-mail]

[@CycloneCyd]

Surely those downlights are compromising the vapour barrier? All downlights let air through - it's needed to cool the bulbs and this will bring air from the room into the ventilation gap. The answer, of course, is to NOT have downlights cut into the ceiling - find another lighting solution (there are plenty).So, in short, downlights letting warm moist air into the cold space and no through ventilation to take it away.

[@SteveRoofer]

Yes the lights are also the problem

[@neilrollason]

I was led to believe PIR insulation has a vapour barrier built in to it?

[@bg7632]

What happens if you have the insulation butted right up to the back of the OSB board?

[@richardthomas175]

I was under the impression that would solve the problem. Pity he didn't respond.

[@KKTR3]

So having done a warm roof flat roof do you have any problems with also insulating underneath?

[@SteveRoofer]

no if you dont use that much insulation

[@Duck1985]

What is the best way to install down-lighters in this rigid cold roof insulation?

[@CycloneCyd]

The best way is ... DON'T. Find another lighting solution.

[@simonjavens6775]

@@CycloneCyd agreed

[@alexhenderson529]

Hi Steve which application did you use to make these drawings? Thanks

[@SteveRoofer]

Are used SketchUp I'm using a free old version from about 2017 I think you can still find downloads for it however there are some new versions out now some free some paid for

[@jamesthomson5774]

Would be helpful to say how you would add the correct ventilation

[@SteveRoofer]

I cant cover everything on one video but I do cover how to vent a cold roof on other videos

[@roymichaeldeanable]

When I go up inside my loft of a 1911 house I can see light at the bottom of the tiled roof...so think my loft has plenty of air whistling through

[@SteveRoofer]

Thats ok its good ventilation. its a problem if you look up and you see light

[@royjakobsen5343]

The Osb is not good to use eighter.

[@Kimberly-ul1ph]

I am not educated in roofing at all. Please tell me what is "RSJ"? Thank you

[@SteveRoofer]

And RSJ stands for rolled steel joist basically and I deem normally painted red because that's red oxide to stop it from rusting I should say to help stop it from rusting

[@ShedDwellerMakerandRepairer]

Having an external vent in the eaves of the flat roof allows cold moist air to enter the roof void. When the sun shines on the top finish it warms the air/moisture inside. Then at night when the surface cools, so does the deck, the warm air trapped in the void then condenses. That is the cause of that mould. If the cross ventilation is suitable, from eaves to ridge, then you dont even actually need a vapour barrier.

[@SteveRoofer]

Can agree with the situation of allowing cold moist air in a lot of people. Don't realise that in bad weather this happens. However, it only happens in what's known as the wet period and wet period is only a matter of weeks or months. The rest of the year is the drying period and as long as the train period can try the roof it should keep on top of the wet period. However, if the imbalance internally and externally is wrong, then it all goes pear-shaped. Cold flat roof construction not a good idea. Did you know that I can look at your videos by clicking on your name? Sometimes I do that because your name is slightly interesting, I did it I love that video you have watched about Dave Brubeck - Golden Brown I just listened to it twice absolutely fantastic

[@ShedDwellerMakerandRepairer]

@@SteveRoofer thanks for the update. How does one do that with ghe vids?

[@SteveRoofer]

@@ShedDwellerMakerandRepairer dont undertand

[@ShedDwellerMakerandRepairer]

@@SteveRoofer you said you can lok at my videos that I have been watching?

[@SteveRoofer]

@@ShedDwellerMakerandRepairer yes