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Flat roof material choice

Discussion in 'off topic' started by Whaleblue, Aug 27, 2019.

  1. Whaleblue

    Whaleblue Southbound

    A quick question for those that might offer some wisdom.

    We’re embarking on having a reasonably large single storey extension (about 40 sq meters in an L shape around the back and side of the house). It will be a “Mansard Roof” as I understand it's called - short pitch then flat across the top.

    Our builder prefers GRP for the flat roof, but I understand they can suffer (sometimes loud) thermal expansion noises.

    I’d prefer rubber (EPDM) as we had it on our old house’s rear extension (15 square meters) and it was faultless.

    What’s the best way to go?

    I’ll obviously discuss with the builder, but good to be armed with some additional knowledge.

  2. sq225917

    sq225917 situation engineer

    Not fibreglass, were just rebuilding the conservatory and kitchen extension from the ground up because of a grp roof that has leaked for years.
  3. Yank

    Yank Bulbous Also Tapered

    paper and tar
  4. Mike Reed

    Mike Reed pfm Member

    Not sure what's wrong with a standard torch-on felt roof of good quality. These are far more durable than 'stuck-down' felt/felt & shingle. You mention pitch and flat, but I can't envisage that, unless you mean it's a flat roof which must be at an incline of some degrees to allow drainage.

    One oversight of my builder for my conversion/extension was to use Celotex insulation, but to allow unfilled gaps. On snowy days I can see where those gaps are as the snow melts. I did point it out but it was plastered while I was out; a constant regret.
  5. hp1

    hp1 pfm Member

    I like glass fibre roofs but would only use it on a warm roof due to the expansion. ours is 40 feet long and only bangs a little as its decoupled from the joists.
    I think the main reason they can leak is down to what it is laid on , alot of builders use sterling board and the resin can run through the matting(as it is thin anyway because its formulated for large horizontal areas)in to the coarse boarding leaving pin holes in the matting.

    I used wpa soft wood ply to lay up on and used a 2 degree pitch to avoid lying water , its been done 5 years and looks like it was done yesterday. Its a 50 year minimum job if done correctly.
    Rockmeister likes this.
  6. sam_cat

    sam_cat C'est Crounchifique!

    Can it not be build with some slope? Even 10-15 degrees makes a HUGE difference to the liklihood of leaks.
  7. Whaleblue

    Whaleblue Southbound

    Yes, it will have the same slight slope that all “flat” roofs have.
  8. hifinutt

    hifinutt hifinutt

    I remember my mum paid 6 times more for a GRP roof !! crazy . I have to get a roof done soon , i always go for felt, bitumen roofing. its reasonably cheap and lasts for a good 20 years . we just had our flat roof done on a dormer . I have a garage roof i have keep going now for well over 25 years by painting it regularly with aluminium paint

    I have a property where two flat roofs connect , one has EPDM , its like flimsy rubber , i am afraid to even walk near it in case it pierces . i wouldn`t touch the stuff , its horrible , but others may like it
  9. Whaleblue

    Whaleblue Southbound

    I should add a little context. Part of the extension is an annex for my mother to move into. We plan to fill the gap between the flat roof and the interior ceiling (around 60-80cm) with thermal and sound insulation to keep it warm, and keep the noise of heavy rain to a minimum. I assume that makes it a cold roof?
  10. Bart

    Bart pfm Member

    We got EPDM, I have no expert knowledge in this area but it looks neat and came with a longer guarantee than anything else. I believe from some literature I came across that it is used for large non-domestic structures.
  11. Whaleblue

    Whaleblue Southbound

    Thanks all. No clear answer, so I’ll dig up costs and go from there.
  12. dweezil

    dweezil pfm Member

    For a typical short span mansard you could be looking at 5 degrees; doesn't sound like much but it's an ideal application for composites if your regs allow.
  13. JimR

    JimR pfm Member

    Roughly 10 years ago, at my previous house, I opted for a new, apparently better, material for the flat roof of my integral garage. I seem to recall it may have been the EDPM mentioned above.

    The product was great. The problem was my insurer. They had never heard of the product so decided that they were no longer prepared to provide ‘buildings’ insurance. Indeed, I had a hell of a game finding anyone who would.

    So beware if choosing any product new to the market.

    PhilofCas likes this.
  14. Whaleblue

    Whaleblue Southbound

    @JimR, we had it on a rear extension, and over the garage around that same time, and our installer said they’d been fitting it for around 25 years.

    I’ll have to double check, but it was made by Firestone if I remember correctly. It was very durable, they gave a 25 year guarantee, and we had no issues at all with it - hence the thought to use again for this job.
  15. MikeMA

    MikeMA pfm Member

    Zinc looks good if done properly and will last for decades.
  16. venton

    venton pfm Member

  17. PaulMB

    PaulMB pfm Member

    It is called a Mansard after an 18th century French architect called something de Mansard, who built a lot of them in Paris. Just thought it might interest you if you did not already know. They were usually strongly sloped, like about 30 degrees. But regarding your roof, what about copper sheeting? With insulation underneath. The material is a little expensive but it should last forever.
  18. stevec67

    stevec67 pfm Member

    Copper? Jesus, round here the nails wouldn't have time to cool down before someone stole it. Same goes for zinc. Lead flashing is generally being replaced with composite for this reason.
  19. Colin Barron

    Colin Barron pfm Member

  20. fur

    fur pfm Member

    look at the drawings, they show where the insulation goes in relation to the roofing material. If they touch then torch-on probably a bad idea. If theres a solid layer of eg wood under the felt then a standard roof will be fine. What is the builder to stand on while working ? expanded foam ? I doubt it.

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