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Flat Roof vs Pitched Roof Costs


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15 replies to this topic

#1 jamesm96

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Posted 20 April 2015 - 10:40 AM

Hi all,

Had an interesting chat with the builder we're planning on using to carry out the bulk of our extension build, and just wondered if I could get the 'general consensus' from everyone on here...

We're building a small 3m x 5m rear extension, and in an effort to reduce costs, we'd considered having a flat GRP roof and fixed roof-lights, rather than a pitched roof with velux windows.

However the builder mentioned that there's probably not much of a price difference, which surprised me somewhat! His point was that, for a fairly small roof, it'd be about the same amount of labour time (3 days he estimated), and although there are fewer materials in the flat roof, some of them are more expensive and therefore not much of a price difference.

Any thoughts, anyone?

Thanks very much!

Mike.

#2 joiner

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Posted 20 April 2015 - 10:46 AM

He's right.

Quite apart from the fact that flat roofs are more prone to 'problems'.

#3 tonyshouse

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Posted 20 April 2015 - 02:26 PM

Lifetime costings, flat roofs always loose! They always leak too.

Edited by tonyshouse, 20 April 2015 - 10:21 PM.


#4 jsharris

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Posted 20 April 2015 - 05:25 PM

I'd personally never, ever, have a flat roof, no if's but's or maybe's. They are always a problem, both in terms of limited lifespan, limited insulation, problems with potential interstitial condensation, plus minor maintenance issues like keeping them free from moss growth and leaves that both block the gutters.

I'd always go for a pitched roof, even if you can only manage a relatively shallow pitch, as it won't cost any more, will last a lot longer, will look nicer, will be relatively self-cleaning when it comes to leaves etc, will be easier to insulate well, will be easier to ventilate to prevent internal condensation, need I say more?

#5 tonyshouse

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Posted 20 April 2015 - 05:48 PM

anyone wanna stickup for flat roofs?

#6 SteamyTea

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Posted 20 April 2015 - 06:14 PM

I would consider one, if I designed and manufactured it from scratch.
But then I was in the composite plastic industry for 20+ years.

#7 tonyshouse

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Posted 20 April 2015 - 10:22 PM

Never seen wood deform or puddles then?

#8 Onoff

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Posted 21 April 2015 - 07:09 AM

I've a flat roof dormer, tbh really badly DIY done by the previous owner and I keep patching up with repair compounds. It's not got much of a fall, suffers from puddles and is a haven for moss (must try the copper wire trick). Think under the felt is ply but maybe only 12mm.

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I was debating, long term to convert to a pitched roof bringing it just under the ridge tiles and use EPDM as the roof covering. Wondering whether I'd need planning permission though.............

#9 Calvinmiddle

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Posted 21 April 2015 - 04:33 PM

But what are you all classifying as a "flat" roof? A roof that is perfectly horizontal? Why about roofs with a 1:80 fall? Or a 1:40 fall? And if they are "flat" roofs as well at what angle does a roof stop being flat and become pitched?

The flat(ish) part of my roof has a 1 degree slope so 1:57, would you call that flat?

Edited by Calvinmiddle, 21 April 2015 - 04:34 PM.


#10 joiner

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Posted 21 April 2015 - 04:43 PM

To get a 1:40 fall, you need a rise of 75mm over 3000 (3m) = 1.4o

And this would be also be my definition of a pitched roof...

http://www.dictionar...tched-roof.html

According to this calculator, to have a pitched roof of 10 degrees the rise would have to be just over 529mm...

https://www.carbided...s-trigright.asp

Edited by joiner, 21 April 2015 - 04:48 PM.


#11 jamesm96

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Posted 03 May 2015 - 06:48 PM

Brilliant! Thanks all very much! I've always preferred the look of pitched/tiles, but just assumed a flat GRP would be cheaper. Decision validated; we'll go with the pitch. Thanks a lot everyone.

Mike.

#12 sketch3d

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Posted 05 May 2015 - 05:39 AM

Good decision Jamesm96, flat roofs will always cost more in the long run and never look at good.

#13 coopers

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Posted 06 May 2015 - 08:53 AM

View PostOnoff, on 21 April 2015 - 07:09 AM, said:

I was debating, long term to convert to a pitched roof bringing it just under the ridge tiles and use EPDM as the roof covering. Wondering whether I'd need planning permission though.............

Onoff, have you had any joy talking to builders/roofers about this?. It's one of our ideas too, but has always been poo-poo'd by anyone we speak to.


Edit: we are not thinking EPDM though. We would plan to replace the current concrete tiles and dormer hanging tiles with slate or replica-slate.

Edited by coopers, 06 May 2015 - 08:56 AM.


#14 Onoff

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Posted 07 May 2015 - 06:30 AM

View Postcoopers, on 06 May 2015 - 08:53 AM, said:

Onoff, have you had any joy talking to builders/roofers about this?. It's one of our ideas too, but has always been poo-poo'd by anyone we speak to.


Edit: we are not thinking EPDM though. We would plan to replace the current concrete tiles and dormer hanging tiles with slate or replica-slate.

No, I'm too tight to talk to roofers or builders!

Seriously though my best mate is a carpenter and his son in law is a roofer so have ready advice at hand if needs be along with sounding any ideas out on here of course. But mostly I go with what feels right! The roof in the pic above is +6m wide, 8" deep ceiling joists on a 4" perimeter stud wall with 4"x4" corner and mid posts along with the internal stud walls. Atop the roof joists are shallow firings and then 3/4" ply of dubious quality. What they've done is build the little triangular tiled "ridge" out of 4"x2" on top of the ply deck. The whole lot is then felted over. Our CWS tank is for want of a better word "wedged" up this triangular space and its tiny - only 25 gallons, a pig to access and service too.

Going to a shallow pitched roof as shown would give me more space for a bigger tank and easy access to the ceiling to better insulate and install things like spot lights in the bedrooms. Because of the shallow pitch most tiles are out (I did actually talk to the roofing supplier on this). EPDM seemed like a good idea plus I have a possible source for the material as a freebie from a commercial source who I've done various favours for. Happened to be talking about it one day and they offered!

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#15 sandy80

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Posted 11 May 2015 - 06:44 AM

We've went with a pitch at our house and with a flat at my mothers - the cost were pretty much the same.

#16 Dudz

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Posted 11 May 2015 - 10:54 AM

I've worked in the building industry for more than 20 years as a carpenter and general builder. I would always advise my clients to choose a pitched roof over a flat roof even if the initial cost is greater. Reason: flat roofs will run into problems sooner than a pitched roof and as well as that a pitched roof is generally easier to repair (keen DIYer can usually refit slipped tiles etc) - with a flat roof you will inevitably need specialist tools and skilled labour to repair or replace properly.

Just my two penny's worth!