Warm Roof Overhang & Fascia Detail For Rendered Wall

[HHHAMSTA]

Hi All, 

 

Like others, I have searched high and wide for information before posting on here though I'm looking for validation on a couple of roof details please, hopefully these can help others also if correct as I was struggling to find the information myself.

 

We're having a roughly 9 x 4m extension on the rear of the house, warm flat roof with a few roof lights and 450mm overhang.  I was looking for a detail which allowed for a reduced height of the fascia, to avoid the store front look, which also kept everything level when viewed from the garden.   I understand the joists running through can be a possible cold bridge though this isn't any different to the eaves on a shallow pitch roof so I don't see any issues here.

 

 I've attached details for the overhang and also the side fascia's.  One side wall will be rendered and the other brick, though ultimately the same.

 

Above checking there are no glaring errors, can you confirm please:

 

1) VCL is in the correct position.  Wondered if the cavity insulation needs to be considered though not seen it as a requirement on any details, just the roof deck.

2) The soffit is fine unvented.

3) I'm showing additional cavity batts at the eaves to help reduce cold bridging.  Good or bad?

3) For the side fascia detail, we will be rendering so thought it would be better to have a soffit for something to render up to.  Can the soffit width be reduced to say 20mm and/or just render up to the inside face of the fascia, or is a soffit preferred?

 

Thanks in advance for any responses.

 

[JohnMo]

You have a nice cold bridge where the steel goes through the block work.

 

So you will have to wrap the whole steel with insulation.

[Gus Potter]

36 minutes ago, HHHAMSTA said:

Above checking there are no glaring errors, can you confirm please:

Hiya.

 

Good post. What you are doing is hard and taxes the mind. When doing a detail like this it's common to feel.. am I making an idiot of myself here and am I making a big mistake? Well done you for asking the question and posting your detail. Hat off to you as you have identified a big elephant in the room .. how do you marry up a warm roof at the eaves with the walls below, and how do you make / detail the transition between a beam and the rest of the wall.

 

These details are very, very tricky. Every job is different and very hard to.. to get everything perfect. This is what we face as designers.. we balance what we really want with what is achievable to build on site and cost efficient. Next we try and construct an arguement that will satisfy the building regs... we are not cheating.. we are innovating, understanding and improving design. The regs don't tell you how to do this and you won't find much stuff on the internet.

 

Here are my thoughts, I'll make some assumptions as I can't see all the detail.. what is below and what structural loads are involved.. cut me a bit of slack too?

 

For all. Let us start with buildability. The rule of thumb is simple stupid works. This drives down cost and then as you drive down the cost you can then reintroduce the things that really matter, we offset heat losses/ bridging by improving else where. If you value engineer this you also get to consider the extra stuff you also want.. be able to spend more on the things you see visually.. like a better kitchen.

 

Ok the top detail where you have the box gutter. If you think about the sequencing of the works.. make it simple. You do the structural roof and then add bits on once you have selected your roofer and taken advice. Try and build in some flexibility in case you have to change roofing contractors or go for a different membrane / system. Keep your options open as prices are changing.

 

Can you plumb cut the rafters at the outside line of the flange of the UB. Most joiners are ok to do a checked (birds mouthed) rafter but you have quite a complex shape.. I'm not shitting you.. a lot of modern joiners just can't do this simple stuff... they work with battery powered tools that have no poke.. they will faint if you ask them to cut a check they way you have detailed cut along the grain.. cut by hand with a saw? Frankly a lot of them need a dose of National Service.. or something like that.. for those on BH that have other views.. Some of our European neighbours have national service.. I say this out of frustration with the building trade at times. 

 

Next scab on some 95 x 45 C16 timbers at 1200mm centres say to every third rafter. Now you have less repeating bridges, rather than every rafter they are only one in three. To the ends of the scabbed timbers attach a runner timber behind the fascia, make it straight and that gives you a continous piece of timber to fix the gutter brackets to.. a get out of jail free card when you come to fix the gutter and fascia...

 

For me I would say.. put the main roof on first and the ply, run the VCL under the upstand edge kerb and decouple the job, extend the VCL as far as you can. Give the roofers a flat bit of ply with continuous VCl below and let them form the kerbs the way they want to. That also lets you change horses / insulation thickness if you want to change the kerb detail once you get the prices back.  It's not just a buildability thing as follows.

 

On the top detail I would introduce a strip soffit vent (although not technically required on a warm roof.. but we are innovating here and designing using first principles) just behind the fascia as I know that in real life there needs to be a compromise between the SE and Architectural Design.

 

You have this dead zone where it is really hard in practice to marry up the warm roof with the wall below and still make things work structurally. I'm not surprised you can't find any details on the net.. BC themselves often have no clue either. But this is not their resonsibility, the basic regs are some 500 pages long, then you have on top of that loads more regs. BC are there to do their best..don't knock them without good reason.. delve into it and at the end of the day it is you responsibily to comply..with all the standards that the regs refer to. If you want to protect your asset just do it properly and don't try to be a smart arse, innovation.. yes go for it, use first principles, save money in places and spend on the things you enjoy.

 

If you accept the weak spot between the wall and roof then condensation / the dew point will happen but how often you will hit the dew point where you have water in the structure.. the objective is to say how often and how do you remove that as quick as you can once the weather / temperature changes. The air tightness is hard to detail and complex... think about first what is the simplest way to do this that the builder will actually do / deliver at a sensible cost.. next think about.. if I ask for complex stuff how do I make sure it will get done and do I need to be there every day to check the builder has done what I want? How will that impact on your relationship with the builder?

 

The alternative is that you pay someone like me to preform a Clerk of works function.. that is very expensive...and will have a knock on effect on the prices you get at tender stage. Keep it simple so everyone is in their comfort zone. My view is to say.. you can draw stuff to death with complex detail but it is never going to happen on site on a self build job where every penny is a prisoner. Go for simple stupid, you can enforce that easily on site without someone like me turning up.

 

Let's just accept that some water vapour is going to get into the eaves and condense from time to time. What can we do to get that moisture out before it cuases harm to the timbers.  My starting point is to often look at a soffit strip vent, extend rockwool out half way say over the soffit so you shift the dew point outwards, examine the location of the building, the prevailing wind, take view on that, make an Engineering decision, check it is buildable and say.. well I can rest easyish now.

 

Ok, have a go at re detailng this with a solid bit of timber behind the facia and running the rockwool out half way over the soffit. Now you have shifted the dew point outwards. Put in strip vent to ventilate that. Have a go at thinking the construction sequence through.

 

One last couple of things. Don't use OSB to form the box gutter, spend a bit more on doing it in a WBP ply.. you are asking for trouble here if you use OSB. Make sure you have a good support over the gutter area as this is where folk step when accessing the roof and point load the roof... like the window cleaner... think ahead.. in real life where is the roof going to get loaded / subject to trafic on a regular basis?

 

Hope this gives you some food for thought, even if to say.. no Gus it's not for me. Design is often about identifying what you don't want and this lets you concentrate on the things you do want to achieve.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Dave Jones]

you can drop the war5m deck into the joists but then you must alutrix/vcl underneath and down the inside blockwork to seal it. No putting wires or downlights. You would need as service void underneath and then plasterboard.

 

 

[HHHAMSTA]

Thanks for the comprehensive reply Gus and others.

 

"I'm not shitting you.. a lot of modern joiners just can't do this simple stuff... they work with battery powered tools that have no poke.. they will faint if you ask them to cut a check they way you have detailed cut along the grain..."

 

I did a 15 degree mono pitch roof at the front of my house including a set back porch with overhanging soffit, it took a bit of calculation to get everything right and ensure the flow of water into the gutters at the eaves, due to the low pitch, but it worked.  New blade in the saw sliced through the 7x2's down grain as had to cut the ceiling joists to match the roof line. I'd be very surprised if a chippie couldn't do this, they do much more complex cuts.  I can square it off though thought this is better structurally.

 

If time is spent on detailing a correct design, certainly in my industry, the rest of the project runs much smoother, and it shouldn't matter who picks up the work as they can follow the design which has been validated and provided.  Obviously things come up during construction which might need adjusting or people have their own way of doing things though the designs should be followed unless there is genuine reason not to. I would like to think tradesman cut corners only when they don't quite understand what they are doing or doing something for the first time, else why else would anyone do something incorrect.  Self builders often have more time to study and understand how to do things correctly.  Proper detailing and planning will always reduce risk at the construction phase.  

 

"Next scab on some 95 x 45 C16 timbers at 1200mm centres say to every third rafter. Now you have less repeating bridges, rather than every rafter they are only one in three"

 

I ran the rafters through for additional strengthening to support any access to the roof, as you mention.  The cut section is still 125x47 so would provide suitable support for a person to walk on the roof.  Span table states a 125 x 47 is good for 2.4m at 400 centres and I understand projections should be no more than 1/3 of the rafter length so at only 450mm this is well below this.  Would reducing to one in three, therefore at 1200 centres, impact the structural element too much?

 

Is your thinking for the 4x2 bolted to the 9x2 to reduce cold bridging or to avoid a "complex" cut?  The 4x2 would need to extend well beyond the UB internally to be properly bolted to the 9x2, at least two times the projection(?), so would this actually reduce cold bridging? Or is it improved as the 4x2 would be high in the ceiling void and not supporting the plasterboard?   When thinking about the cold bridging element I compared it to a standard pitched roof at the eaves where there is very little insulation or space between the roof (tiles), the rafters and the internal plasterboard.

 

"To the ends of the scabbed timbers attach a runner timber behind the fascia, make it straight and that gives you a continuous piece of timber to fix the gutter brackets to.. a get out of jail free card when you come to fix the gutter and fascia"

 

The gutters won't be be fixed to the fascia due to the gulley and hidden down pipe.

 

Can you elaborate why the WBP Ply is better than the OSB3 please as I have seen a lot prefer OSB3?

 

Iteration of details below adjusted to some of the suggestions so far.  One with continuous joist and one with additional 4x2 rafter for projection. 

 

I welcome any additional input please!

 

 

 

 

 

 

[saveasteading]

11 hours ago, Gus Potter said:

work with battery powered tools that have no poke.. they will faint if you ask them to cut a check they way you want

My dad could saw through 6 x 6 oak with either hand, from above or below. It looked effortless  which may be the point.

[Temp]

If your soffit is something like 12mm WBP and the joists cut accurately you probably don't need the batten.  Or you could fit noggins between the joists to provide support. Saves the batten depth.

[HHHAMSTA]

Thanks Temp, the batten just made it up to a standard 400 fascia but yes, looking now cutting the joists lower would also work.

[SelfBuildSmurf]

How did you get on @HHHAMSTA?

I have just finished framing the roof and getting ready to install the deck. 

 

I would be interested in knowing more around the detail you used for the hidden gutter and soffit/fascia. I too am wondering if it will need venting.