Badly Installed new cold flat roof sweating on the decking.

[@AJ-ds5gf]

I used the same mushroom vent on my cold roof, after considering using abutment vents (e.g. an upstand with over-fascia vents) across the two ends of the roof. I personally didn't like the idea of having abutment vents so I went for mushroom vents but I had to do one thing that this roof didn't do... added extra 3x2 timber strips perpendicular to the joists to allow the air to move over the joists i.e. 'cross ventilation'. That's the only way to make those mushroom vents work in a cold roof.

[@SteveRoofer]

Yes, cross batoning is essential to get a good flow. Most people don't do this because it pushes the hight of the roof up. If you had that spare hight in the first place you and everybody else shoud have opted for a warm roof. Warm flat roofs are much easier to construct and thus have fewer problems. A badly constructed warm roof will probably work. A badly constructed cold roof definitely won't work.

[@andrew1caldwell]

Great video Steve. I'm researching retrofit insulating a small garden workshop with a cold roof (8ft x 7ft) . The roof is 75mm rafters, osb deck and bitumusous felt. The walls on onside of the rafters is single skin concrete block rendered and other side is shiplap & timber frame. My plan is to insulate the roof using circa 25mm boards in the rafters and then plyboard to maximise height wput hich is circa 2m. I'm not into design over substance so happy to fit lights onto ceiling ply or walls rather than recess into cold roof space. The main problem is to ventilate the small cold space between the rafters. I was thing I could vent the barge boards on timber side under guttering and drill hole and put vents between each rather on the breeze blocks side. I think this is workable rather than replace the roof with a warm deck for economy reasons. I will need to work in this space during the winter probably with a small electric radiator. Crazy or sensible idea?

[@SteveRoofer]

Will probably. work and will defiantly be warmer

[@CardboardBots]

So how does one put in lights in an insulated ceiling under a flat roof? Have insulated batting over the electrical box and avoid pot lights?

[@SteveRoofer]

As long as you dont brake though the insulation and the VCL vapour control layer you can fix spotlights

[@preyashdesai]

@@SteveRoofer In case of warm roof, does it still create condensation below the first layer of osb boards (the on that goes straight on joist)?

[@SteveRoofer]

No as that is warm

[@Ratsotone]

@@SteveRoofer So how are you do you manage to install spotlights to a ceiling without breaking through the VCL and a vapour barrier??

[@syproful]

Why is it that there is not even a thin layer of xps or pir under the epdm ? It’s only the logical thing ? I’m really wondering.

[@SteveRoofer]

People are just not thinking.

[@dalturbo]

Steve how would you sort this issue of spot light fitting and insulation on a cold roof? I'm in the process of this right now and totally understand the vcl / insulation cannot be breached. I'm thinking of fitting ultra slim led lights which are 20mm deep as an alternative to traditional spot lights, what would you suggest

[@SteveRoofer]

I will be doing a video on this shortly have been getting info together for the past week and came across these videos which are really good kzbin.info/www/bejne/gXTNgGN9o9iehLc&ab_channel=THERMAHOOD

[@peteramberley9952]

Was the mushroom vent not a soil pipe or extractor vent seem as there was a token one ?

[@SteveRoofer]

Yes, it could have been. Luckily I have been asked to go and do a roof inspection and a report so I will have more info on this soon.

[@philmillard4605]

Hi Steve, do you think a warm roof which is obviously unventilated would be acceptable for a small utility building, this building is of old construction with minimal insulation and runs white goods appliances which can create condensation, the room is vented with a humidity fan. Many thanks and Regards Phil

[@SteveRoofer]

Warm roof is the only way to go you wont get condensation on the back of the decking as it will never get that cold too many other things will get cold first IE doors windows walls

[@roulettered56]

So.. how do you get ventilation AND insulation?

[@syproful]

You only need ventilation because there is always moisture in the initial process of constructing the roof. And so that you don’t get dead air that starts to the condense/ vaporize cycle. The only correct way of doing things is putting osb on the joists and insulation ontop of that. Then the epdm. All the rest is DIY stuff.

[@22bittugill]

Hi Steve , I am just wondering to know that is it possible to make existing fibre flat roof which got wool insulation between the rafter to warm deck and can we use 75 or 100mm insulation to Crete the existing flat roof with wool insulation in between rafters Wittgenstein removing old fiber roof and wool insulation.

[@preyashdesai]

Isnt the spot light ventilation (cold air with moisture) is an issue on warm roof as well? From my understanding there is no ventilation in wram roof like one in cold roof. (Please correct me if I am wrong). btw. I find all your videos very informative. Thanks

[@SteveRoofer]

No as the gap between the joists is not vented and is internal same air and condition as in the room below

[@preyashdesai]

@@SteveRoofer Aah that makes sense.

[@leondimaiolo]

Hi Steve I had my flat roof constructed about 2 years ago in pretty much the same way as the one in this video and I have exactly the same issues. I have a cold roof finished with a GRP system and the light fittings have been cut through the insulation I am starting to get water dripping through the light fittings and initially thought that there was water getting through the GRP. could this issue be fixed by just constructing a warm roof directly onto of the existing roof?

[@SteveRoofer]

Strickly yes however if you're going to spend that kind of money do the job properly because there is no guarantee that it will work. By fixing say 120 mm of insulation over the top of the existing roof you will definitely raise the temperature of the old roof decking and by doing this you should stop the water from condensing there. You will have to make sure that all external ventilation has been thoroughly sealed up. Now the holes where the lights are should be left open in fact the more holes for lights the better. The problem with what you are trying to do is that the cold flat roof installation between the joists is stopping the warm air from getting to the void above and keeping the old roof decking warm. Generally speaking, we would always remove the cold flat roof insulation before converting a roof to a warm flat roof. Please send through any photos you have

[@jimbow2310]

Steve great video. Do you think a lot of builders make the mistake of thinking the foil on foil back plaster board is a vapour barrier?

[@peteramberley9952]

The cut out for the light breached the vapour barrier then,especially the heat source from the light

[@SteveRoofer]

Thanks I've answered this above for you right at the end

[@SteveRoofer]

Yes if you call any of the insulation suppliers and ask them " Can I use the foil as a vapour control layer" ( I have called them all and asked) All of them come back with something like this. " Yes, you can, however, it's part of the thermal makeup of the insulation. As long as you tape all the joints you can use it as a vapour control layer" Although this is true it's also very missing leading. How do you tape joints at the bottom when the sheets are laying on the joists? Have you ever seen a sheet of insulation without a cut, bash rip in it! Most importantly the vapour control layer (VCL) also acts as an (AVCL) air and vapour control layer and should be bonded at all edges to stop air from migrating up into the insulation above. This means bonding to a wall vertically if abutting a wall. All other walls AVCLs should be joined and sealed to the flat roofs AVCL to form a building envelope. No wonder things are going wrong. Peter Amberley,s commented below is correct cutting a big hole in the AVCL and then fitting a hot light that makes the movement of air grater and basically pumps it into the vented space that's cold! nobody is thinking (Except you guys)