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Roofing
Flat roofing for extension
Shortly, I would have to get a flat roof done for my single storey rear extension which is due to be 5.5m x 4.5m for the house I own. The architect has advised the below and as the height of the extension must only be 2.6m, I wanted to know:
1. What the total depth of the roof would be as I want to know what the total internal height of the extension interior would be as I am very concerned it would be very short.
2. Cost of getting the roof installed.
Architect notes:
It will be a flat warm roof deck. Covered with 12mm thick mineral chippings bedded in hot bitumen over 3 layers of roofing felt complying with BS747 set in hot bitumen over 140mm Celotex TC3000 roof board laid and joined to manufacturer recommendations on firings to provide fall of 1 in 60 on joist sizes and crs, 9.5mm plasterboard ceiling set with plaster no ventilation provided to roof space.
The felt roof would be on 150 x 50mm joists at 400mm.
203 x 133 UBs.
3 Answers from MyBuilder Roofers
Best Answer
Your architect does not mention the decking board, vapour barrier nor quilt insulation between joists?
Unsure why you you want to go with a felt,bitumen system when there are better modern materials with greater life expectancy - GRP / EPDM....which have no seams on a roof based on your sizes.
Based on your spec above your roof thickness (including plasterboard) will be 319.5mm plus say 5-6mm for the covering (327.5mm). You will of course need to allow for upstands (150mm) on the wall abutments.
That would give you a height from finished floor level to ceiling of 2272.5mm - however this would be along a drip edge. Most flat roofs have kerbs which also increase the height of the roof by another 20-30mm in the direction of the falls. This does not also allow for the UBs
Not sure why your height is so critical?
In terms of cost I cannot assist as we only deal in GRP (fibreglass) and EPDM (rubber) - both of which have a life expectancy of circa 50 years
If you go with the above system, maybe you should just run it by Celotex tech team to ensure you are spot on with the design (vapour barrier / quit between joists / decking requirements).
ALSO - ensure your builder uses the correct insulation - there are a few which read the spec then just get cheap standard solid insulation from their suppliers - the cost difference is immense.
Regards
Drew
Fibretechs - GRP (fibreglass) & EPDM (rubber) flat roofers
GLOUCESTERSHIRE & HERFORDSHIRE
Answered 25th Dec 2015
Hi loconte,as Fibretech stated there is no mention of the vapour barrier and his use of 12mm chippings bedded in bitumen,really? I haven't used this method since I was an apprentice in the late 80's . A decent high performance mineral felt will suffice rather than these outdated chippings.
If anyone has to access your roof or use it as a work platform the chippings dig into the roof especially in the summer, then come winter you have embedded chippings which loosen and leave possible holes and leaks.
I really can't believe an architect has specified this system.
Life on Mars
Regards
Steven
Pegasus Flat Roofing
Answered 25th Jan 2016
Your architect didn’t mention The size of your timbers and whether it’s going to be a warm deck or a cold deck roof
Answered 9th Oct 2019
