False Pitch Roof - Insulation Approaches

[GoodDIYjob]

Why construct a warm flat roof and a cold pitched roof? Why not have them both as warm roofs?

Moving on from that, why have an area of air below the warm roof insualtion when it can be filled with insulation? This will allow the reduction of thickness of Celotex under the deck/tiles by using quilt to compensate for less Celotex, or will create a much better insualted roof by using quilt to supplement Celotex.

That's what I was alluding to in my first post, in the construction of a completely insulated roof structure.

Then there are no issues in venting, part venting, designing an interface between the two or even the potential for air leakage by having any sort of void - vented or not.

It just seems so simple.

It would solve my problems fairly easily if it is sound. It is not that different to what the designer has drawn. Just eliminating the air gap with insulation. Effectively the ceiling level insulation and any top up fill just becomes a continuous extension of the warm roof insulation between rafters (like under rafter board insulation would if it was mounted).

Wonder if anyone has modelled / proven / constructed or tested this. Would this condensation risk analysis do anything. Tyvek web site says they will do one and supro is specified. I'd have to get the air gap removed and convince the designer. It'd be nice to be confident, especially when it comes to building it (no post build issues), Building regs inspectors querying it, etc etc. False pitches aren't that uncommon are they...

 

[noseall]

The lantern well will be the most vulnerable part of the equation - condensation wise. Especially the upstand reveals. I'm baffled as to how this bit is insulated. It looks as though the architect is showing the roof light supported on insulation and that the whole lot is 'resting' on the trimmer steels. How is it all fixed?

I wonder about finishing that faux pitch with a seggy ridge too. Probably look neater and be more secure with a just lead flashing, bearing in mind that I reckon you would need another course of tiles at the top of that rafter and it will pitch the ridge unnaturally one way. Hence the need for an ugly mono ridge.

How is that flat roof drained?

 

[GoodDIYjob]

The lantern well will be the most vulnerable part of the equation - condensation wise. Especially the upstand reveals. I'm baffled as to how this bit is insulated. It looks as though the architect is showing the roof light supported on insulation and that the whole lot is 'resting' on the trimmer steels. How is it all fixed?

I wonder about finishing that faux pitch with a seggy ridge too. Probably look neater and be more secure with a just lead flashing, bearing in mind that I reckon you would need another course of tiles at the top of that rafter and it will pitch the ridge unnaturally one way. Hence the need for an ugly mono ridge.

How is that flat roof drained?

Thanks for you interest and reply. Really do appreciate all the input.

Sorry, my lack of roof detail experience may show, I do not know what seggy ridge is, depsite a google search!

The reason why plain tiles are used is to match existing roof pitch and elevation finish. I'll upload some more elevation views. What is the problem with the plain tile layout at the start of the pitch, is it there is not enough overlap of the ridge tile?

I agree re the drawing of the insulated upstand and condensation vulnerability. That was my totally worry when I first looked at all this before worrying about the area under the false pitch which I just relied on the designer for.

Some builders who have quoted have told me they just build a timber upstand and I have said "no way" to them. It is the weakest insulation in the whole roof structure and the highest point, and glass creating cooling when hot air hits it too..... It is a fair size too 1.5m x 2M. So the perimiter of the upstand is not insignificant.

At least he has shown it insulated but as you say, it appears to have no structure around it!! I can get him to add it in the drawing and notes. So I would be thinking a ply box section with uprights (section studs? every 400mm). And assuming a width of the upstand of no more than 90mm, 18mm ply box work, that leaves no more than 50mm for pir board, (comapred to 120mm in the warm roof) but I can't see any way around that.

That has to be far better than an timber kerb upstand right?

Do you want to add in your 2 pence for dealing with insulation detail of the false pitch and under the ceiling area of the false pitch corridor? Appreciate views. Ultimately, I expect I'll have to go back to designer (at least to change some detailing) but he wasn't offering me alternatives after I queried it initially...

 

[^woody^]

Lantern upstands (kerb) like all flat roof lights should be 150 minimum so that a proper seal can be made below the cill and reduction of splash and drip problems, and for future maintenance.

Then, bearing in mind that heat is flying out through the glass above, and there is only a bit of upstand section above the roof, you only really need 25mm of celotex lining the inside to take the cold surface away. More insulation would be better but lantern designers don't tend to allow for this in their cill widths. A ply box section would theoretically provide more insulation, but is not worth it in practice as the gains are minimal compared to the effort and cost.

The upstand is generally a couple of sections of 50mm timbers fixed together, and fixed on top of the double joists/trimmers. The depth of these timbers depend on the furrings and need to allow for 150mm above the deck at the higher end of the roof. So need to be quite deep.

BTW that drawing not only is a poor detail, but does not even show any furrings on the joists, nor a sloping lantern. You're getting a trully flat roof and lantern glass. That will be fun.

 

[tomfe]

I am not trying to be funny but what exactly did you pay for? Did you get building regulations drawing? If you did has your designer provided them?
Looking at the first image posted it looks really lacking in detail and would not be accepted under full plans.

 

[noseall]

I do not know what seggy ridge is, depsite a google search!

Seggy = segmental ridge.

The reason why plain tiles are used

No prob's with plains.

Do you want to add in your 2 pence for dealing with insulation detail of the false pitch and under the ceiling area of the false pitch corridor? Appreciate views. Ultimately, I expect I'll have to go back to designer (at least to change some detailing) but he wasn't offering me alternatives after I queried it initially...

The upstand design will need to be sympathetic to the space you have regards the roof window jambs and whether they can deal with the extra bulk, without encroaching onto the glazed area.

It could mean that the bulky upstand structure goes beyond the footprint of the roof light as opposed to the more favourable (that's why the architect drew it that way) roof light overhanging the upstand. ​

 

[GoodDIYjob]

Lantern upstands (kerb) like all flat roof lights should be 150 minimum so that a proper seal can be made below the cill and reduction of splash and drip problems, and for future maintenance.

Then, bearing in mind that heat is flying out through the glass above, and there is only a bit of upstand section above the roof, you only really need 25mm of celotex lining the inside to take the cold surface away. More insulation would be better but lantern designers don't tend to allow for this in their cill widths. A ply box section would theoretically provide more insulation, but is not worth it in practice as the gains are minimal compared to the effort and cost.

The upstand is generally a couple of sections of 50mm timbers fixed together, and fixed on top of the double joists/trimmers. The depth of these timbers depend on the furrings and need to allow for 150mm above the deck at the higher end of the roof. So need to be quite deep.

BTW that drawing not only is a poor detail, but does not even show any furrings on the joists, nor a sloping lantern. You're getting a trully flat roof and lantern glass. That will be fun.

I know about the 150mm upstand. In plans it is roughly 150mm. When I was talking about 90mm I was more talking about the width of upstand which you are terming cill width I think. I was just thinking if you made a 18mm ply box section with segment uprights you could get up to 50mm board in there.

So you would normally see 50mm timbers for upstand with a single layer of 25mm insulation then internal plaster compared with my more complicated ply box section? OK. I agree there is a lot of heat going through the glazing anyway.

I think when I view the plans I am also viewing with the specification text and how I know it should be done from my knowledge. Agree the section could do with a lot more detail. There are firings in the text specified at 1/60. And a box gutter to one end besides roof light with two outlets. No details of depths / falls etc but my calcs suggest firings would need to be about 105mm if allowing 25mm for the box gutter.

Yes all the roof lights require min 5 degree pitch so that is a given. Firrings aren't shown agree. Can ask him to and yes this is a building regs drawing from an ex building control guy and his plans do get through. I am planning on using a private building control inspector.

Thanks again. It does tell me I am adjusting things too much in my mind and the detail sections should have more detail / thought.

 

[GoodDIYjob]

I am not trying to be funny but what exactly did you pay for? Did you get building regulations drawing? If you did has your designer provided them?
Looking at the first image posted it looks really lacking in detail and would not be accepted under full plans.

Yes full plans. Although I' m probably using private but his plans do get through local authority. But as pointed out and also the things I am adjusting for in my mind, like firrings, roof pitch, no box gutter detail other than some text, there are a lot of things that should be tidied up.

 

[GoodDIYjob]

Seggy = segmental ridge.

No prob's with plains.


The upstand design will need to be sympathetic to the space you have regards the roof window jambs and whether they can deal with the extra bulk, without encroaching onto the glazed area.

It could mean that the bulky upstand structure goes beyond the footprint of the roof light as opposed to the more favourable (that's why the architect drew it that way) roof light overhanging the upstand. ​

Yes exactly. Most of the flat roof lights (5 degree pitch) seem to be about 90mm allowance including plasterboard. So 75mm without. So it is either a ply box section or as woody says timber with 25mm... My box section would seem higher performance , but maybe not worth it for another 25mm pir?

You weren't drawn on what to do about the insulation detail under the false pitch?

Many thanks

 

[noseall]

You weren't drawn on what to do about the insulation detail under the false pitch?

I would do it how he has drawn it. I would build my flatty. I would spike the rafters (faux roof) to the ends of the joists. Lay the flat roof insulation material so that it butts up to the slope of the rafters. Then construct a small ply upstand on top of the deck insulation and fix this to the rafter ends.
The remaining quilt can be fed in from below or between the rafters.

 

[^woody^]

I know about the 150mm upstand. In plans it is roughly 150mm. When I was talking about 90mm I was more talking about the width of upstand which you are terming cill width I think. I was just thinking if you made a 18mm ply box section with segment uprights you could get up to 50mm board in there.

So you would normally see 50mm timbers for upstand with a single layer of 25mm insulation then internal plaster compared with my more complicated ply box section? OK. I agree there is a lot of heat going through the glazing anyway.

I think when I view the plans I am also viewing with the specification text and how I know it should be done from my knowledge. Agree the section could do with a lot more detail. There are firings in the text specified at 1/60. And a box gutter to one end besides roof light with two outlets. No details of depths / falls etc but my calcs suggest firings would need to be about 105mm if allowing 25mm for the box gutter.

Yes all the roof lights require min 5 degree pitch so that is a given. Firrings aren't shown agree. Can ask him to and yes this is a building regs drawing from an ex building control guy and his plans do get through. I am planning on using a private building control inspector.

Thanks again. It does tell me I am adjusting things too much in my mind and the detail sections should have more detail / thought.

But the whole point of drawings is that they are accurate. Obviously that shows how it is to be built, but it also shows up how it should look like and line up, and any discrepancies are seen and designed out. Also the builder knows what to order.

What I see a lot of, is poor drawings (copy paste from the last job) and just drawings and not design. Then when the builder comes to build, things don't fit in as they should, as they are drawn, and then the client either gets a bodge or is hit with lots of claims for daywork to sort out the design.

For the roof light, typically cills are 150mm wide so by the time you have two timbers for the upstand, and the external overhang, there is no much left for insulation. So that needs designing properly, otherwise it looks a bit crap internally.

Non of this gets picked up by the building inspector, as it's all quality. The drawings will pass, and the inspections are typically about the structure and safety.

 

[GoodDIYjob]

I would do it how he has drawn it. I would build my flatty. I would spike the rafters (faux roof) to the ends of the joists. Lay the flat roof insulation material so that it butts up to the slope of the rafters. Then construct a small ply upstand on top of the deck insulation and fix this to the rafter ends.
The remaining quilt can be fed in from below or between the rafters.

Ok. interesting. Yes, that what I was thinking about the back of the strut for the pitch upstand, although to be fair he hasn't shown it or called out in text that it is formed in anything!! So another detail required....

What I was really referring to, when I asked you about the insulation detail, is the sort of mixture of having a warm roof, with insulation between rafters on the pitch and a cold roof insulation approach, by having insulation at ceiling level, immediately below. There is no ventilation from the pitch or would you want there to be. Have you seen it done like that with that insulation arrangement before?

Curiously, the ceiling section drawing looks like quilt but in the spec he has written 170mm celotex board. But wheather it is board or quilt it is still a normal cold roof set up?

Presumably the pitch will be unventilated, as no ventilation is specified, insulation is between rafters but looking though that pretty good guide by tyvec linked by a previous poster, he could do with a bit more detail on the pitch roof section, like eaves carriers, and is AVCL required under the false pitch warm roof (vapour layer required) etc etc. But then would that create a problem being above the roof level insulation...

You seem a bit more gentle and aligned to my designer's point of view! All the other comments on here a valid and the drawings could do with a lot more detail, which I will try to address with him and preparing a list as we speak!!!

 

[GoodDIYjob]

But the whole point of drawings is that they are accurate. Obviously that shows how it is to be built, but it also shows up how it should look like and line up, and any discrepancies are seen and designed out. Also the builder knows what to order.

What I see a lot of, is poor drawings (copy paste from the last job) and just drawings and not design. Then when the builder comes to build, things don't fit in as they should, as they are drawn, and then the client either gets a bodge or is hit with lots of claims for daywork to sort out the design.

For the roof light, typically cills are 150mm wide so by the time you have two timbers for the upstand, and the external overhang, there is no much left for insulation. So that needs designing properly, otherwise it looks a bit crap internally.

Non of this gets picked up by the building inspector, as it's all quality. The drawings will pass, and the inspections are typically about the structure and safety.

Yes, I'm totally with you. I really want to see the issues and iron them out, there are a few more than even raised here. Like there is another steel sitting on top of the steel shown above the rear elevation where the false pitch is. It is very very tight there. It will need chamfering, but will it fit even on paper without that detail and knowing 1/3rd of the beam can't be chamfered.

After this thread settles I really want to go back to him with all of the things raised and get as much of it as I can covered before work is started and give the builder another chance to look at detail.

So you think the roof stand kerb would be 2x50mm timbers side by side so 100mm timber before you start, I've been looking at a lot of flat roof lights, AA Glazing, roofmaker, Atlas I thought most were a max of about 100mm allowed for cill and leaving some overlap for glass on top. I'm going to go frantically rustling papers after this to check. So you don't like the idea of a ply box section insulated upstand? To get the thickness of insulation he has drawn it would have to be.

Really do appreciate the input.

 

[GoodDIYjob]

I could post the full plans up briefly in PDF then take down or PM anyone interested. I just wouldn't want this post to get to the designer before he had had a chance to sort out and before I have got an improved set in place. There are a lot of detailing issues but most appear around the roofing, section views we are focussing on and steel and connection issues which have come back from a structural engineer. Probably the most apparent to me is trying to fit the steel on top of the steel shown in the section posted up. Cheers

 

[^woody^]

So you think the roof stand kerb would be 2x50mm timbers side by side so 100mm timber before you start, I've been looking at a lot of flat roof lights

Earlier posts mentioned a lantern, not a flat roof light. Lanterns would tend to have cills, but flat lights would not and instead have some sort of proprietary shroud. So what this means is that the kerb could potentially be thinner (50mm timber ) for a flat light than for a lantern.

But whatever it is, it would be up to the manufacturer to specify the kerb requirements for their product. So that should be determined first, and then the kerb and the light well and structural requirems would be translated on to the drawing.

I'm not against box sections. In fact I made these as lintels for my timber frame instead of using double timbers, for the very same reason - to allow insulation within the box. But what I am saying is think about whether its actually necessary in context of the relatively small amount of exposed area.