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Martinr 01-07-2009 01:58 PM

Flat roof on block of flats - rebuild advice
I live in a 1970s block of flats with a flat roof, which is now beyond patching and needs stripping and rebuilding.

I'd really appreciate a bit of independent advice regarding the rebuild, as roofers have a vested interest in putting forward their own proposals.

We've engaged a building surveyor and he has presented his proposals; however, a trusted roofer I discussed them with is not convinced.

The decking is woodwool without a cement screed. In addition, the existing roof has a miserable fall of one and a half inches in 11 feet. It really is a flat roof.

The surveyor proposes:

1. Strip existing felt off.

2. Fit tilting fillets and OSB to give improved falls to valleys feeding to existing rainwater pipes.

3. Fit polyurethane insulation slabs on a vapour barrier.

4. Lay a high performance torch-on felt underlay and mineral-faced cap sheet.

5. New edge detail because new roof surface would be 150mm higher than present.

The roofer has commented that putting OSB over the existing decking (with the tilting fillets in between) is not a good idea - it could induce sweating; however, with the thermal insulation on top of the OSB/vapour barrier, I'm not sure this is a valid comment as the decking and OSB would now be in a warm roof in theory.

I also need to be convinced that the OSB can be properly attached to the existing structure.

The roofer also does not like the mineral-faced capsheet because he says it's difficult to get a good edge joint. He prefers (if I've understood it correctly) to use a 4mm high-performance capsheet painted with solar reflective paint after laying.

I'd be most grateful for any thoughts and comments on the above.

Many thanks


Martinr 01-10-2009 03:48 AM


Originally Posted by the roofing god (Post 26610)
SEEMS YOU HAVE A HAIR BETTER THAN 1/4" PER FOOT REQUIRED FOR MOST RUBBERIZED BITUMEN products,Certainteed Flintlastic S.A makes an excellent 3-part system with a granular cap sheet,They also have a torchdown model---we have no problems with lap edges on granulated,you should do a 45 degree lap cut,top & bottom selvages,then weld the lap-no problems

Yes, I'm in the UK and at the moment it's very frosty and cold. Meanwhile, I have the local agent of Icopal/Anderson roofing materials coming in the next couple of days. They offer a free service whereby they assess the roof and come up with a spec for its renewal, obviously centred around their own products. It'll be a good opportunity for me to get his comments on the proposals already obtained.

I also spoke to the technical people at Ruberoid and they too said that, certainly along the side joints, there should be no problem getting a good joint with mineral-faced capsheet, although they admitted that on cut edges care obviously is needed.



roofer 01-12-2009 03:51 AM

listen to theroofinggod,he`s got it right,funny that I only see his post in your blurb though,What`s up with that anyway ?

handyguys 01-16-2009 09:26 AM

Yea, I have done torch downs like you describe with the paint on and the granular. The granular will have a seam that you could see but it will be water tight. No worries there. If you have enough pitch that you wont puddle then I wouldn't raise things up, its more work and not really necessary. Just have all old work completely removed and any deck repairs made. If you have no insulation in attic/under roof now is the time to consider adding it.

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