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Flat Roof Extension - Material Options


canalsiderenovation

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So as we await decisions on our revised plans I'm looking into flat roofs. There seems to be a few issues with the recent weather which has caused people with flat roofs some issues and it's worrying me.

 

We currently have a small flat roof area which is felted. When we purchased the bungalow just over 18 months ago the surveyor said the felt was knackered and gave it 3 months before we would likely have issues. Luckily we moved in last summer when we had the red hot summer so slathered on that black sticky stuff as a preventive (I can't remember what it was called but I do know it took about 2 weeks to get it off my skin)! So even now the roof is still intact and no leaks despite it being the original 1970s roof....  

 

Although this area will stay flat roof as it's all being extended along with the pitched roof too we are exploring alternatives as we will have a significantly larger area of flat roof (roughly 8m * 8m) and the existing flat roof will need replacing when we have the extension.

 

My concerns are getting something that is warm, I'm always bloody cold (ok so we will have all new heating ashp, windows etc) but it needs to be a warm.

 

We will also have at least 3 flat roof windows in the roof area. One for a bathroom and two along hallways.

 

Despite reading lots about GRP, rubberised systems, etc what worries me is those that are leaking after a short space of time versus our knackered felt with is still leakproof and whilst knackered, thin and on its last legs  it is doing it's job 40 odd years later (I say this with baited breath)!

 

So, my question is in terms of longevity, maintenance (it's a bungalow so fairly easy to maintain, sweep leaves off etc) and with roof windows, which flat roof system would you consider.

 

I know a bit about GRP but only in terms of boats as we are looking to buy a boat. Given what I've seen with windows fitted into GRP boats and the issues there this doesn't fill me with confidence!

 

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First rule to stop a flat roof leaking is ..... make sure it’s not flat ...!!! Get a decent fall on it, so water can’t find a place to settle. 
 

Also go over the top on structure so you get zero deflection and a decent ply or OSB layer -

2 layers of 12mm OSB can be better than a single layer of 18mm ply. 
 

A good built up and torched on felt (base layer plus top layer) will last 30 years if done right but getting the laps correct and the right falls is equally as important. 

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45 minutes ago, canalsiderenovation said:

[...]

So, my question is in terms of longevity, maintenance (it's a bungalow so fairly easy to maintain, sweep leaves off etc) and with roof windows, which flat roof system would you consider.

[...]

 

Any one. It doesn't matter.

What does is attention to detail. Nitty gritty finicky fussy attention. 

Choose one system and get it done by someone who knows what they are doing. 

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There are very few  issues with modern flat roofs GRP or Fibreglass 

Either system when done right will last for decades Ideally you Brad a small amount of fall 

But any standing water is unlikely to penetrate 

I see newly roofed commercial buildings with nowhere for all the water to run off due to the sheer size 

So if you use either and add a little bit of fall

It should be the last time it would need doing 

 

I used fibreglass on top of OSB 3

No need for expensive Marine ply 

anymore  

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17 hours ago, canalsiderenovation said:

Given what I've seen with windows fitted into GRP boats and the issues there this doesn't fill me with confidence!

That is a totally different problem.  Not one they have with the front window of the Intercity 125 trains.

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I have used EDPM on a large garage.  It is cheap and fairly simple to DIY.  I got it made as a single sheet but it was quite unwieldy.  Joins, junctions, abutments, upstands and penetrations can be quite tricky, but I think this goes for many systems.

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2 hours ago, Mr Punter said:

I have used EDPM on a large garage.  It is cheap and fairly simple to DIY.  I got it made as a single sheet but it was quite unwieldy.  Joins, junctions, abutments, upstands and penetrations can be quite tricky, but I think this goes for many systems.

 

Thanks.

 

We will have a builder and won't be diy'ing it but you raise a valid point and joins, junctions etc worry me especially as we will have three rooflights. It's also such a large area potentially 8*8m.

 

Edited by canalsiderenovation
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I have done loads of EPDM. Each to there own. Personally i love it. Bit of a pain to do in winter, as below a certain temp you have to stick it down with, glue on substrate, and glue on the rubber, You only get one chance to get it down without ripples, as it aint coming up again. Much easier in better weather, when the stuff you stick it down with is like PVA and you just glue the substrate. If you get a ripple, you can pull it back to get the ripple out. Going around roof windows is a bit of a pain but not really an issue.

Fibreglass. I did a large flat roof with a buddy a couple of years back, and to be honest i thought it was a right faff with movement joints etc around the parapet walls.

Only my opinion, but if your roof is ply, osb or whatever it will have some movement (expansion and contraction) in cold or hot weather. I know it may only be quite small, but my small brain says. The rubber will stretch with any movement = no prob. The Fibreglass willl be rigid = ???

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15 hours ago, Big Jimbo said:

The Fibreglass willl be rigid = ???

Not quite.

Expansion has often come up as a negative for GRP roofs.  Not known it to be a problem, but then, there are big roofs and very big roofs.

 

I think the main problem with GRP is that people think it is easy to do, and can be done by anyone. Because of that, we ended up with the Reliant Robbin (I used to employ some ex Reliant workers, they were useless).

It is like plastering, anyone can learn to do it in a day, then you just get better over the years.

Edited by SteamyTea
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On 13/11/2019 at 19:49, Onoff said:

I think @jamiehamy & @Tennentslager both used EDPM for their complete roofs without issue. Check out their posts.

We did 270m2 of EPDM on our roof (mostly 5degree pitch). We did the rubber in 6.5x6m sections heavy going but we managed okay. We used contact adhesive onto tissue faced insulation. As long as there arw two of you and you determine your approach properly, it's very straight forward. It was the right choice for us - GRP, despite being lauded on other threads, has one major downside - it's super weather dependent. There's a chap on here having a nightmare putting it down. Had it been EPDM, it would have been done a long time ago. Never under estimate that as a factor. I know for a fact had we chosen GRP, our build would have been literally delayed by about 6months (iirc we did the epdm in winter). 3 years later its doing fine. 

 

As said, build in a fall to avoid any pooling, carefully workout all your details and how you will do/materials. It's very straight forward once you get going. 

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  • 4 weeks later...

Bit late to this thread but we need to put our flat roof on in the next few weeks and still haven't decided on a system. Our architect specified single ply which I took to mean EPDM. But we've just had a quote from a roofing company recommending Protan. Never heard of it before but it looks to be a tried and tested system. Single ply but pvc based rather than rubber based (my interpretation from the website, I'm happy to be corrected). 

The quote was just over £8k for 95m2 of roof. Most of that is roof terrace or balcony that needs to withstand foot traffic. It's also in 4 sections so not the most straightforward job. I looked at the rough price of raw materials for a DIY EPDM roof and that is around £2.5k. Quite a big difference but I'm minded to go with the pros on this one. 

Any thoughts or experience of Protan? Protan website

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It’s a standard PVC welded membrane so needs proper detailing. I would expect a lot of the cost will be in stuff such as the pre-made corners etc. 
 

I don’t think that’s a bad quote for a roof anyway - assuming you’re putting down a ply base beforehand ..?? What is the insulation spec and detailing ..?? And how are you forming the falls ..?

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It's 120mm insulation. I'm doing the falls using firring on top of the joists (cross firrings to create diagonal falls. Then 22mm ply on top. The roofers take it from there although we are supplying the insulation. 

Overall there are walls and parapets all round, 11 internal corners, 1 external corner, 5 drainage outlets and one soil vent detail. It's a reputable company and I agree seems like a reasonable quote. 

 

3534 WD07G External Services and internal Penetrations, Roof Plan.pdf

Edited by DarrenA
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