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Pitched roof with flat section - detail?


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#1 raine

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Posted 29 August 2010 - 09:42 PM

Hi,

Just in the process of planning a new build which has a pitched roof with a central flat section to keep the ridge height down and increase the loft size.

I'm just researching construction, insulation and ventilation in preparation for later building regs submission. I think I've worked out all the eaves detail, including ventilation, but my research is coming up blank for the flat section.

Firstly, does this shape of roof have a name? Doesn't seem to be Mansard, or Dutch or anything?

Secondly, can anyone help me decipher the detail? In particular, what is the "ridge" (if you can call it that here) detail? I guess the flat section sits lower than the ridge, but how? The flat roof section will be supported by a steel ring beam (structural engineer to design), so maybe hang the flat roof joists off the inside of this, and sit the pitched rafters sort of on top of the outside of the steelwork with a birds mouth? How do the tiles finish? Short top course of tiles as with a standard roof, no ridge tile, turn flat roof covering up side of higher rafters and dress lead over the top of tiles and flat roof covering? How does the lead fix, and how do we provide ventilation to the flat roof and high-level ventilation to the pitched roof? What about drainage? Provide a slight slope on the flat section, maybe leave one tile out at the lower edge of the flat roof slope and create a lead-lined drainage channel out onto the main roof?

Sorry for the long and confusing message. If any of you could maybe point me towards some drawings or photos of the detail, that would be an excellent starting point for me, as I haven't been able to find a single detail on this anywhere. I'm surprised, as this is a fairly common construction around where I live.

Thanks in advance.

#2 raine

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Posted 29 August 2010 - 10:02 PM

Found a picture. This is the sort of roof shape, in case I haven't made it very clear (although probably not the same sort of construction).

Questions remain as per my post above.



#3 dr_rob

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Posted 30 August 2010 - 09:07 AM

Crown roof perhaps?
For starters:-
Ventilation:
Flat roof section - make it warm deck - remainder use a breathable membrane Tyvek or similar.
Create fall by using firrings to flat section.
Steel frmae will be designed to carry both the rafters and the flat roof joists. Use timber wall plates mechanically fixed to the steels for rafters to sit on.
Ensure your SE designs the steel frames as a portal frame rather than cranked beams.
Dr R Saunders MBEng, BEng(Hons), PhD
www.innervision-design.co.uk and www.sapcalcs.co.uk
Building Regulations Consultant, L.A. BCO, accredited SAP assessor, Designer of G-Calc 2010

#4 temp

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Posted 30 August 2010 - 09:23 AM

I think you should try and talk to a specialist lead worker - someone old school who does traditional/historic lead work. See what they recommend. Get them talking to your SE/Architect.

Would be nice to avoid having lead work visible from the ground but I can't think how to do that without also having the flat roof part lower. Then you could use ridge tiles and vertical hanging tiles. Problem is that would complicate drainage (internal gutters or drainag out through the gable?) and potentially form a leaf trap up there. See "(24) Vertical tiling junction with monopitch roof: V1" and V2..

http://www.clayroof.co.uk/educational_guides/vertical.pdf

#5 raine

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Posted 31 August 2010 - 03:17 PM

Crown roof - that's it! Thanks Dr Rob!

A quick search for crown roof on Google images shows the two attached photos, which come from a website for a cut roof contractor who happens to be very close to my local area. So he might be getting a call from me at some point :-)




It occurred to me that a crown roof (as I shall now call it!) is a very similar thing to the attached photo showing a window with flat & pitched roof below. The visible leadwork doesn't really detract from the look of the building here, so I can probably also have visible leadwork on my crown roof if required. Although in the case of the window in the photo, the flat roof doesn't sit below the top of the adjoining tiles, so it's not THAT similar.



So, for the roof covering itself, I will end up with something like the below "warm roof (timber deck)" drawing from the NHBC manual, with the "waterproofing" being a bitumen-based felt (unless one of the newer finishes such as Firestone EPDM is better for not much more cost?), the insulation I guess being Kingspan / Celotex to whatever thickness I need for the required U value, then what do I use for the vapour control layer here? All laying on plywood deck + firring.



No ventilation needed in warm deck flat roofs?

All this stuff doesn't show what to do with the tiles, so I will have a read through the Clay Roof Tile Council document that Temp sent, so see if that provides the answers. I'll also get back on Google Images for more pics of crown roofs in different stages of construction.

No gables on my house, hipped all the way round, so the drainage will have to come through some kind of channels as shown in the above pics.

My pitched roof sides need ventilating both top & bottom, so I need to find out the best way of ventilating at / near the top. I used ventilation tiles in my last project, for bathroom extractor ducts. Are these weatherproof and suitable for ventilating the roof void, or are they just for ducting extracted air?

Dr Rob - sorry to ask dumb questions, but portal frame vs cranked beams - what's the difference?

#6 dr_rob

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Posted 31 August 2010 - 03:43 PM

Warm decks - no need to ventilate
Breather membrane for the pitched section - no further ventilation required (see the products BBA cert).
Vapour barrier typically provided on the boards - check out the celotex/kingspans of this world, again their product data will give all the info you require.
Portal frame design will provide horizontal restraint forces which have to be accounted for elsewhere in the design (the feet want to spread in other words). Cranked beam design however, considers only the beam in plan view (i.e. as a simply supported beam) and no horizontal force is produced and hence accounted for.
Our SE is hot on this, insisting on the portal frame approach.
Dr R Saunders MBEng, BEng(Hons), PhD
www.innervision-design.co.uk and www.sapcalcs.co.uk
Building Regulations Consultant, L.A. BCO, accredited SAP assessor, Designer of G-Calc 2010

#7 temp

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Posted 31 August 2010 - 07:22 PM

I can't really improve on that, except to say that it is possible to build the whole roof as a "warm roof" not just the flat part (eg you could put the insulation above the rafters rather than between). It's a bit more awkward as special long fixings and tools are needed to fix to the rafters through the insulation. Also some details change at the eaves etc.

If you decide to put the insulation between the rafters on the pitched sections then go for vapour permeable as Dr Rob suggests but still leave a small gap between membrane and insulation. This isn't for ventilation (as it's vapour permeable) but to allow the membrane to drape slightly. That way any rain water blown under tiles can run down into gutters and not pool above a batten. If you want to avoid this gap then use counter battens to raise the tile battens off the membrane. All this is usually in the tile or mambrane manufacturers fitting guides.