Marking and measuring valley jack rafters using diminish to work out lengths

I use a simple method for working out the diminish of valley jack rafters and this method also works for jack rafters and also if you double the diminish for cripple jack rafters
Valley jack rafters travel from the ridge board to the valley rafter, they will get smaller each time by an equal amount this is called the diminish. In this video I talk about how to work out the diminish and demonstrate how I do it
I hope you find this useful.
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[@terencecottington4273]

Can't get enough of this channel. Always learning something new even though I am not a builder or roofer. Just love the attention to detail. Put simply, it's absolutely amazing watching good craftsman work. We need more like the Robin Clevett's of this world...

[@ukconstruction]

Super nice comment Terrence, thanks for taking your time to watch mate, we need more Terence in this world too!!

[@terencecottington4273]

@@ukconstruction I think one Terence is enough. That's what the wife says anyway 🤣🤣🤣🤣

[@brapamaldi7666]

Same, I'm not a builder, dont build any roofs and mostly use metal instead of wood, but I've been a fan of this channel since seeing Robin on Skill Builder many moons ago. So awesome watching him work. If I ever do want to build with wood I know where to come to learn how to do it.

[@RobNorman08]

I learned to just times the common rafter run by the jack centres eg in your case... 1.4142 x 450 = 636.39 rounded to 636. loving these videos i have to say!

[@3house325]

Well said Sir! I used this method on site and had to explain over and over again to my workmates. When my men understood this, they were laughing out loud "Is this how it works? I'm gonna use this all the time now!" Well the intercept theorem is one of the useful things you can use from math class on the jobsite!

[@disklamer]

Simply gorgeous. Your presentation is so wonderfully mellow, I wish everybody could roll like that, we'd have a better world for sure. Another way to figure this all out with minimal calculations is by setting out the spacing, measure the shortest and longest, and divide the result equally. IE set out your 400 or 600 along the top, measure the shortest one plum to the valley, divide along the diagonal by the same number to get the positions, and divide the difference by the same number again to get the diminish. It's always interesting to me how many different methods you can use to get the same outcome, and how different approaches work for different people.

[@RI-uv3lm]

Superb. Also highly recommend your earlier video, on roofing, rafters, hip rafters, jack rafters, to those interested in learning the geometry of it 👏👏👍

[@davidcort2857]

Thanks for another great video Robin.

[@py_tok5589]

1m/cos45 = 1.4142 m, I figured out your explanation very straightforward, this is quite useful for builders, good stuff Robin

[@andy2a2m]

i not good at 3 dimension, would you mind explain it a little bit more please? i have been thinking for couple hours and draw it on my paper and still do not understand the formual. in 3:12 the jack valley rafters are spacing at 450 centres, that is the horizontal measurment, then 3:55 he talk about the common rafter, how it that related to the 450 spacing?

[@py_tok5589]

@@andy2a2m trigonometry, right angles, check the formula when side are equal and pitch 45, check the Pythagorean theorem when botch side are 45 degrees therefore it will equal side. no that complex

[@shaunglendinning]

Great video and tutorial Robin. Appreciated as always. Cheers.

[@barnescarpentryservicesuk2811]

Good video robin you make it easier to understand the roof look tidy mate 👍 like a work of art

[@dabruscar]

Perfect. Another quality demonstration buddy.

[@timberman1713]

Nice job, congratulations!

[@philipjones9297]

Work of art ❤️

[@gilesj2215]

your vids are like asmr to me ! so relaxing and learning too ! Love it 👍🏻👍🏻👍🏻👍🏻

[@ukconstruction]

Thank you Giles

[@pierceferris]

Good old trig, as an engineer I’m finding this quite fun to follow the practical techniques used here…

[@thetallcarpenter]

It really is simple Robin, and a really great explanation bud. I cant believe that we use the same method of keeping the valley jacks up on the valley rafter👊 I just tack a half inch rip all the way down the valley and then remove it once all the jacks ate fitted👍It looks amazing

[@ukconstruction]

I have said this before Del, we are cut from the same cloth!!!

[@christaylor5291]

Hi del. I have set up a string from top valley jack to bottom valley jack and offer up the remainder to this. Same end result.

[@thetallcarpenter]

@@christaylor5291 That's another great way Chris, and gives super straight/flat valleys. I don't do many roofs with valley rafters, but always find then a treat to do. Robins look really cool, are super neat and have those brilliant scarfs in. Imagine what someone will think, when looking at these joints, many years from now. Its this kind of traditional carpentry and being able to execute it so proficiently, that only a 'born to be' carpenter will get. Its, literally, a sin to cover it up and a shame its not in some great big roofs loft space so it can be seen every time you get the Christmas decorations out🤩

[@georgelfc1]

Just like to add a quicker way to get the diminish in robins example !! Rafter center ÷ cos of pitch = diminish ( 450 ÷ 0.7071 =636 ) ,easier as u have all the info eg pitch of roof and rafter centers

[@paulcloona9331]

@@ukconstruction you have excellent videos. I left my own trade years ago. And went roofing with a carpenter, I loved it and learned quick. I don't roof much now, but I have learnt more great bit and pieces and tips off of you. Top class man. Look forward to your videos 👌

[@bricklayersworldwithandy6277]

Top class that, well explained 👍

[@kevinbarton8036]

Love ur work and ur boys. I’m a Steelfixer. And don’t won’t my work to be covered in concrete. Keep it up 👍

[@billrimmer5596]

I am starting to get it a little better. I have a thick head and your terminology in the UK is different. U guys went metric. We didn’t. I like the term diminish. I am visualizing the roof sheathing sitting on all those rafters. Thank u for the instruction!!

[@tedcopple101]

Roof sheathing is not something we do regularly in the UK. Normal it's over felted with a breathable membrane then timber laths fastened on to carry a roof tile of various types. In this case a plain (flat) clay tiles.

[@billrimmer5596]

@@tedcopple101 Thank u very much for the info!!

[@kevinbarton8036]

I just won’t to pick u up and give u A hug. I’m not o poof. Just love ur work

[@carolewarner101]

🤣

[@nazar9794]

Next level robin

[@robertmorris5214]

Your attention to detail is fantastic, and love the way you explain thing's many thanks

[@johncoppock3823]

Great video, really enjoyed that one. For anyone that wants to do the maths you just need the Tangent of the angle to get the ratio, so Tan(45) in your example. Aside: other way you could do that is Pythagoras triangles so if the two square sides are scaled as one the the diagonal will be square root of one squared plus one squared i.e. square root of two. Think the App though is a great idea to keep it all simple. Now I'm really impressed how you get the cuts on the ends right and preserve the correct lengths on the jacks. Noticed the top edge of the jack's sticking proud, but you explained in previous video how the centre of the top face of the valley rafter is the point you are aiming at not the edge of the rafter. Thanks again for great video in fantastic series.

[@bigmonkey999888]

Merry Christmas and a happy new year to you all from Steve, Beeston, Leeds UK

[@ukconstruction]

Merry Christmas to you all too Steve, thanks for your message mate!!

[@badninja1971]

Aaah, someone else who uses the 1414 method. Also great for squaring big/small areas. Easily divisible or multiple method. 👍

[@stormyabraham]

What a master craftman you are Robin.

[@rossmorrow3013]

I SO wish I would have had you as my Maths Teacher, instead of the intolerant fool who if it couldn’t be understood after two explanations, well then cane it into you, instead of explaining where understanding triangles and geometry are applicable in the real world

[@williejcullen]

Well done son knew how to do it but didn't know how to explain it 🙄

[@danielspendley8202]

Would love to work with Rob such a clever man!

[@mauricekirwan8076]

Band of brothers

[@christaylor5291]

One thing I would like to add Robin. What I have done over the years with valley rafters depending on roof covering, although necessary keeping the valley rafter lower so valley jacks meet centre of valley rafter for plain tiled roofs with same pitches, with a GRP vinyl valleys for concrete tile and slate roof coverings the valley rafter can be brought back up inline with the roof plains so you have an instant line to set valley jacks either side as the GRP vinyl will seat itself on the valley rafter nicely. It also brings the valley rafter up inside the building so you don’t have to cut as much out of the pir insulation that will cover the valley inside the building! This seems to not be a problem for you as the same depth timbers are used but if the rafter material was 150 and the valleys were 200 ( old skool rule of hip, valley and ridge material is 2” inches wider than common rafter material) this sometimes can be an issue. Great detail as always in the video posts. Much appreciated.

[@ukconstruction]

Hi Chris, I talk about the position of the valley rafter top edge and keeping it down in the previous video in the channel about making and fitting a Valley Rafter, as you say its really important to know the roof covering and make the correct allowances

[@MrTrapper28]

I wondered why you hadn’t taken on Carol Vorderman as your apprentice, now I know.

[@mesolithic86]

hey Robin. Im a carpenter, love my job always make sure things are done correctly and always want to make myself better. Ive done many roofs but i always USE to measure my jacks individually. For me this has to be the most benefitting video from you yet and i am 100% taking this on board. Its actually quite simple when you talk it over and explain it. This is definitely how i will be doing my jacks from now on. Would love to work along side you. can't remember the giants name but he is lucky to be learning on the job with you. keep up the good work! :)

[@davidwebb2318]

There is no need to divide the 1.4142 by 10 if you use the centre to centre distance in metres (450mm = 0.45m). All you are doing is dividing one number by 10 and multiplying the other by 10. Just use the 1.4142 ratio and use the centre to centre spacing in metres.

[@carolewarner101]

That was a bit tricky...not sure I got my head round that entirely. I'll have to watch that a few times over.

[@bobbobby5724]

One word fantastic

[@amazing451]

Is there a reason why figure 10 is used to divide the common rafter as you said at 4.40 on the video?

[@TheDangerous66]

Me to 😍

[@joe2mercs]

If the roof was just a straight run the load from the roof tiles (slates) would be distributed across a series of parallel roof rafters centred at 450mm. However with the presence of a valley the load of the roof is effectively being supported by just one long valley rafter with all the Jack rafters transferring their load into it. At what point do you need to compensate for this extra loading by doubling up the valley rafter timbers?

[@user-cq5kl9wi2x]

Great video. How comes the jacks line up at the bottom of the valley?

[@MrSylvane]

These videos are really helpful. Would someone be able to help me size a valley rafter/beam?? I can't find anything useful on the web!

[@qwertyuiop124172]

Hello Robin, Brilliant video. This is best method for someone knows about maths. If someone has difficulty with mathematics, is there any other method please?.... I know maths and I understand very well. God bless you Robin. Best wishes from Leicester ENGLAND. Ahmed

[@ra4yu]

few methods come to mind If you google roofing calculator theres sites that let you print out 20 pages of documents that will detail every angle, length and cut. Great for simple roofs but fall apart a bit the more complex it gets. You could install your first 2 rafters by measuring them, note the difference in length and that gives you robins 636 which you can use to cut the rest. Draw a full size mock up of your centers and roof pitch(or rise) on a scrap of plywood or the floor and measure the diagonal. Traditional roofing squares with a fence are great, cut your longest rafter first then use it to step 450. Bit hard to explain but you could probably find a video of it. Roofing square are great you can do complexity roof, but theres a lot of technique rather than math and if you dont do it all the time its easy to forget or mix things up. You could even guess, most roof coverings arent too sensitive to centers so as long as your close and the corresponding lengths either side of the valley are the same then it doesnt matter much, it might just involving re-cutting a few lengths and a bit more manual handling to test fit.

[@masonsmith858]

Where did you get your roofing square gauge, did you make or buy it? Nice.

[@rossbuchanan7632]

And this is where the arithmetic falls down for me. I end up swearing, crying, shouting, throwing things....Thank god for the blue book, says I. Your method looks simple, if I could figure out how these apps things work. My only criticism of your film is it could have been much longer-I'd be happy to watch you set out the whole valley/ridge/valley, jack by jack . You are pretty good at this

[@py_tok5589]

c24 valleys?,look like the valleys sizes I specify when designing

[@andyb3331]

My Brain hurts!!!! 🤣👌

[@jonathanreynolds2227]

I'm still not clear whether the diminish from common rafter to the first jack is measured from the centre of the common to the centre of the slant cut on the first jack? Or to the long side of the jack, or the short side? Or from one side of the common to the same side of the jack - and in that case is the diminish different for the first jack compared with the later ones?

[@gregwalters4656]

Hi Robin, Does this method work on a bastard or irregular hip when the angle isn't 45 degrees?

[@JimBob-lz1gy]

Can you just do Rafter length minus the spacing used, so Rafter length minus 450 or 600 each time, then it doesn’t matter what pitch, as long as the corner is 45 ie the building is square.

[@PETERCYNANWILLIAMS]

Nice job, although I have never seen a valley blade with a scarf joint is this structurally sound?

[@ukconstruction]

This joint is fully supported, loom out for an upcoming video about this

[@antonyporter5045]

hope you show all the valley jacks in place

[@ukconstruction]

will do

[@peterleishman431]

🏴󠁧󠁢󠁳󠁣󠁴󠁿👍

[@lloyd4011]

Love the maths of it! Just curious, at what stage is the build now? What's the time difference between the video release and the build?

[@ukconstruction]

Hi Lloyd, we are about 9 weeks ahead!!

[@lloyd4011]

@@ukconstruction Brilliant! Hopefully the bit of wind the other week didn't cause you and the lads too much grief.

[@olafbigandglad]

Diminish - more usually known as the formula of common difference.

[@dermotkelly6946]

Hi Robin , Do you ever use a roofing ready reckoned, I have used it for over 35yrs am I missing something? Keep well.

[@ukconstruction]

Hi Dermot, I have never used the roofing ready reckoner, I have one somewhere! How do you enjoy using it??

[@dermotkelly6946]

Hi Robin, Thank you for your reply, I find the blue roofing reckoned a super piece of kit it maybe old fashioned now but if you have one and after you have worked out the simple maths they show you it is super. There are some very useful diagrams in the book to fall back onto when all seems lost 😁 , the Paddy’s used it all the time when I was a kid and swore by it. I think your lads would find very useful if not old fashioned and it is small enough to tuck in the glove box in the van. Keep well.

[@gordonclark7632]

Back to school and pay more attention to the maths teacher. Although 60 years back in time maybe wont work.

[@paulyoull5472]

Robin, is it the same mathematics for jacks on a hip ?

[@ukconstruction]

Absolutely Paul

[@shane4013]

I think robbin was cheating there he had a calculator on the floor 😂😂 mastermind is looking for a new mathematician 😎😂👍

[@mrsbradpittiful]

I guess no need to put a string line on the hips if they fit so snug with that diminished equation?

[@therealdojj]

Hope you've not wasted that bit of egger protect, there's a shortage you know 🤣

[@richardjreynolds6166]

Is it standerd to have the jack's pertuding above the hip, where they marige? could you pleas pontificate about this assembly for people like me who are ainle about this craft and would like to be informed, of such things. I also welcome education in reply, to my coment.

[@ukconstruction]

This is so the roof batten or sheathing meets exactly flat in the centre of the Valley rafter, I do explain this in the video called fitting valley rafters, hope that helps and thanks for watching!!

[@richardjreynolds6166]

@@ukconstruction Ahh yes I remember now, thanks!

[@jamesblair4709]

You need a white board on site for this! Also good for setting out the days tasks, and having a visual for all on site who’s doing what.

[@tonywentworth3225]

Gone for a lie down🤔

[@MrThebigredone]

Math never lies

[@edwardholmes91]

Indeed... a fellow RRBuildings subscriber? 🙂

[@MrThebigredone]

@@edwardholmes91 Yes indeed, another terrific channel alongside this one, Stud-pack, A&E construction, Charlie Collison, Essential Craftsman and Skill Builder.

[@michaelmorgan7862]

Hi Robin, how do you get on with the customer ? Looks very expensive order of building ,they must be living there still ? That scaffold must of cost 20 k + it just baffles me to why folk do things in backwards order

[@chriskehoe3180]

Also common difference

[@johnmorrissey1675]

👍☘️🇮🇪👋

[@mattmanning9267]

Did he ever get the app working

[@Mushyboy007]

Tease….just a quickie…

[@py_tok5589]

another way, cos45= S/L, where S= 450 mm then => L= S/Cos45 =636.39 ===== 636 mmm

[@andy2a2m]

isnt that 45degree is dihedral angle against the floor?

[@daversj]

Not trying to be critical but you somehow make it more complicated than it is. Holding the framing square flush on top of last common will let you mark the long point of the longest valley jack in seconds. Then just measure length from ridge. If you showed that, the concept is easier for the untrained to understand. Faster and no math to screw up. The framing square was designed for this.

[@carpentryzucconiltd7875]

Basic carpentry. 1.4142 its the first number an apprentice have to stick on his head, forever.

[@CapeCornwallCarpentry]

Carpenters really need to learn trigonometry. If the spacing is 450mm just type in to scientific calculator: 450/cos(45)=636.

[@steveandrew2139]

Wonder what your client thinks of you using a decent offcut as a blackboard! Also doing videos during work hours 😂

[@cuebj]

Video adds value to property - shows potential buyers quality of the structure that will be covered up. Reason I took photos of our 1930s semi as I gutted and rebuilt it. Given usual standard of these houses, pictures will help children sell it when we die!

[@Trevor_Austin]

You cheated. You used a measure and built everything level and square.

[@py_tok5589]

if you dotn mind Robin,what is the app you developing? perhaps we can exchange private email I am bit curious, I also develop apps but for structural calcs. Earlier I developed a bespoke U-value calculator

[@ukconstruction]

Hi mate, my email address is [e-mail]

[@py_tok5589]

@@ukconstruction Thank you, I will send you by Monday , a video draft demo version created on python and hope you can give me your feedback,Regards, Ernie