Bestest Flat roof

[ianfish]

Part of our build, well two parts will have a "flat" warm roof. 

 

My intent  is that they will have  GRP Fiber Glass top

 

An insulation over 100mm to 150mm

 

An insulation which also gives an acoustic element

 

A Vapor  Layer

 

OSB 18mm 

 

Joists I beam

 

15mm Plasterboard.

 

So fairly routine from what I can make out. My question is which actual materials have people gone for and manufacturer and why? 

 

What have peoples experience been of using their choice of material, there seem to be so many choices!

[Temp]

I would put 200mm of PIR above the rafters with OSB above and below the insulation. Then if noise is a concern fit sound insulation between the rafters, possibly resilient bars and two layers of plasterboard. No downlights.

Edited February 4, 2021 by Temp

[Onoff]

6 hours ago, Temp said:

I would put 200mm of PIR above the rafters with OSB above and below the insulation. Then if noise is a concern fit sound insulation between the rafters, possibly resilient bars and two layers of plasterboard. No downlights.

 

Where does the vcl go in that make up and how do you fix down through the "sandwich" to the joists, bfo special screws?

[TonyT]

https://www.ironmongerydirect.co.uk/product/spax-washer-head-wirox-screw-60x300mm-pack-50-310973
 

bfo screws

[Jtb]

Sorry to drag up a (slightly) old post.  I wanted to put 200mm pir on my flat roof - but I'm struggling to find cost-effective fixings that are long enough.  If it was 150mm pir then I can use 240mm screws from Toolstation at c.£37/100; from the link posted above, if I go up to 300mm screws, the fixing cost is £100/100 ie 3x.  My flat roof is about 75m2 so if I fix at 300 centres that's over £800 just in fixings.  Am I missing something?  

[Gus Potter]

Hello JTB.

 

Yes I had the same issue. My flat roof make up is:

 

EPDM rubber glued to 18mm thk OSB3 glued onto the top of two staggered layers of 100mm PIR (thus 200mm thick) on vapour membrane. Vapour membrane is on 18mm thick OSB3 resting on 195 x 45mm joists with 12.5mm plasterboard on inside.

 

I plumped for this as I was doing it single handed. The glue for the insulation / OSB3 interface is a poly urathane (PU adhesive), I used Insu stix, the can is in the photos. Very effective.

 

I did the membrane in two halves as it was too heavy and bulky to handle on my own.

 

 

The blocks are to weight down the insulation while the glue cures. And yes the pipes in the garden etc can do with a tidy up.

 

 

Roof lantern in progress.

 

I also opted for this solution as the weather is a bit unpredictable here. As there are no fixings then no potential thermal bridges, problems installing a fixing through 200mm plus insulation and hitting a joist etc.

 

Hope this helps flesh out your ideas. Only issue is you have a pretty thick roof so I'll maybe use some careful shaddow gaps etc to try and reduce the tunnel effect at the lantern. It looks promising at this point.. nearly ready to start messing with the plastering here.

 

 

 

 

[Big Jimbo]

1 hour ago, Gus Potter said:

Hello JTB.

 

Yes I had the same issue. My flat roof make up is:

 

EPDM rubber glued to 18mm thk OSB3 glued onto the top of two staggered layers of 100mm PIR (thus 200mm thick) on vapour membrane. Vapour membrane is on 18mm thick OSB3 resting on 195 x 45mm joists with 12.5mm plasterboard on inside.

 

I plumped for this as I was doing it single handed. The glue for the insulation / OSB3 interface is a poly urathane (PU adhesive), I used Insu stix, the can is in the photos. Very effective.

 

I did the membrane in two halves as it was too heavy and bulky to handle on my own.

 

 

The blocks are to weight down the insulation while the glue cures. And yes the pipes in the garden etc can do with a tidy up.

 

 

Roof lantern in progress.

 

I also opted for this solution as the weather is a bit unpredictable here. As there are no fixings then no potential thermal bridges, problems installing a fixing through 200mm plus insulation and hitting a joist etc.

 

Hope this helps flesh out your ideas. Only issue is you have a pretty thick roof so I'll maybe use some careful shaddow gaps etc to try and reduce the tunnel effect at the lantern. It looks promising at this point.. nearly ready to start messing with the plastering here.

 

 

 

 

Osb, insulation, insulation, osb, and epdm. My kinda flat roof. Looks great.

[Gus Potter]

Thank you @ianfish and conor for the compliment, it's really appreciated.

 

I have made a few practical mistakes.. maybe some technical ones too!

 

For example. I laid out the membrane for a while after it came in a back off the lorry  (it was much heavier than I thought being on my own) to let the creases settle out. It thought I had got them all but I failed so a bit of the roof still has a crease in it. It's still water tight and I may come back to it at some point. But at the moment I need to finish the rest of the job!

 

Every day is a school day!

 

Thanks again for the compliment.

Edited March 24, 2021 by Gus Potter

[Moonshine]

1 hour ago, Big Jimbo said:

Osb, insulation, insulation, osb, and epdm. My kinda flat roof. Looks great.

 

I am looking at not having the OSB above the insulation, and mechanically fixing the EPDM directly through the insulation.

 

Does the OSB above the insulation significantly add to robustness?

[PeterW]

6 minutes ago, Moonshine said:

 

I am looking at not having the OSB above the insulation, and mechanically fixing the EPDM directly through the insulation.

 

Does the OSB above the insulation significantly add to robustness?

 

EPDM is glued on not mechanically fixed...??

 

You need the OSB - even if it’s only 9mm - to provide a solid surface to bond to. 

[makie]

11 minutes ago, PeterW said:

 

EPDM is glued on not mechanically fixed...??

 

You need the OSB - even if it’s only 9mm - to provide a solid surface to bond to. 

 

You can mechanically fix certain EPDM's, tends to only happen on warehouses etc.

You can also do it directly on to the insulation, had an architect spec it on a job once. It's (expletive deleted)ing terrible, don't do it.

[Moonshine]

12 minutes ago, makie said:

It's (expletive deleted)ing terrible, don't do it.

 

****** goes off to change my drawings ****** ?

 

29 minutes ago, PeterW said:

 

EPDM is glued on not mechanically fixed...??

 

You need the OSB - even if it’s only 9mm - to provide a solid surface to bond to. 

 

i was getting confused as a product i was looking at used is a single ply PVC that is mechanically fixed without OSB

 

https://www.bauder.co.uk/roof-systems/waterproofing-systems/single-ply/pvc-systems

[Jtb]

On 24/03/2021 at 17:11, Gus Potter said:

Hello JTB.

 

Yes I had the same issue. My flat roof make up is:

 

EPDM rubber glued to 18mm thk OSB3 glued onto the top of two staggered layers of 100mm PIR (thus 200mm thick) on vapour membrane. Vapour membrane is on 18mm thick OSB3 resting on 195 x 45mm joists with 12.5mm plasterboard on inside.

 

I plumped for this as I was doing it single handed. The glue for the insulation / OSB3 interface is a poly urathane (PU adhesive), I used Insu stix, the can is in the photos. Very effective. 

 

I did the membrane in two halves as it was too heavy and bulky to handle on my own.

 

The blocks are to weight down the insulation while the glue cures. And yes the pipes in the garden etc can do with a tidy up.

 

Roof lantern in progress.

 

I also opted for this solution as the weather is a bit unpredictable here. As there are no fixings then no potential thermal bridges, problems installing a fixing through 200mm plus insulation and hitting a joist etc.

 

Hope this helps flesh out your ideas. Only issue is you have a pretty thick roof so I'll maybe use some careful shaddow gaps etc to try and reduce the tunnel effect at the lantern. It looks promising at this point.. nearly ready to start messing with the plastering here.

 

Thanks for this reply, sorry for being slow, I didn't get notified of the reply.  So you just bonded the entire roof together?  I like your thinking but I don't think the building inspector will let me get away with that, so massive long screws it is.

 

I've done an epdm roof once before, it was a really simple roof and it went well.  This time the roof has a few complications and I'm trying to decide between grp and traditional felt.  I'm concerned about the longevity of grp - I'm hearing a lot of stories of premature failure, plus cracking noises due expansion/contraction.  On the other hand, traditional felt is tricky for details and for the edges (I have a parapet detail).  I've seen a couple of roofs done in a newish system called Kemper but this is a bit unproven and is very expensive (£85/m2 against £55-65 for grp and £50 for felt).  Anyone got any thoughts on my quandary?

 

 

[SteamyTea]

6 minutes ago, Jtb said:

I'm concerned about the longevity of grp

Stuff we did 40 years ago is still working.

It does have to be done right, but then so should everything.

[joe90]

1 hour ago, Jtb said:

I'm concerned about the longevity of grp

tell that to boat builders that use it, loads of very old boats out there still floating.

[Gus Potter]

Hi JTB.

 

Yes I did basically glue it all together. I have little worry that the right glue will last as Steamy Tea and Joe mention.

 

The main thing for me is that it best suited me as I was doing it single handed and DIY. I avoided these long fixings, repeating thermal bridging and so on. Also, I have at the back of my mind how the insulation may creap. Most insulation, is speced with regard to it's compression stiffness but creep is not often explained.

 

For the creep behaviour think about a timber beam, it bends under the initial load but over time it creeps too, how much does PIR creep? If you take all the load off maybe it won't return to it's intitial shape. I wanted to avoid any issue of the insulation creeping over time.

 

Once you get to these insulation thickness'  (200mm PIR) I was keen to make sure that the insulation is not going to creep (call it shrinkage ) and pop the fixings up through my rubber roof in the long term.

 

I have a low parapet and found this quite simple to detail and do. The membrane continues up and over the parapet. Basically my roof is like a thick pond liner with a few edges that allow the water to run off. There is no noise as there are no areas of fixing stress concentration say.. thus no creaks.. so far.

 

The main thing for a long lasting roof is the quality of the workmanship and attention to detail.

 

 

 

 

[puntloos]

Ha, my complaint about all you guys is that nobody summarises the 'final conclusion'. 

 

Post 1: "I'm doing abc, xyz, solution yada yada"

Post 2: "You should do thisandthat"

Post 3: "And this"

Post 4: "Don't forget somethingorother"

Post 5: "THANKS ALL"

 

Where me, as an absolute beginner often can't quite piece together the actual bestest roof (for one, the topic started with GRP, which I was planning to opt for, but all you guys are doing EPDM )

 

@Temp: do you agree that:

 

GRP Fibreglass

OSB 18mm

200mm PIR insulation

A Vapor  Layer

OSB 18mm 

Joists I beam

15mm Plasterboard.

 

Would make a pretty good roof? ?

Edited April 3, 2021 by puntloos

[Mr Punter]

There is no bestest.  Some of the felt products are very good nowadays and lots of roofing contractors are familiar with them.

[PeterW]

25 minutes ago, puntloos said:

my complaint about all you guys is that nobody summarises the 'final conclusion'. 

Because there isn’t one.
 

Each situation suits a better solution - so you have to take all the parts and come up with the correct roofing system for what you’re doing. 

[puntloos]

9 minutes ago, PeterW said:

Because there isn’t one.

Each situation suits a better solution - so you have to take all the parts and come up with the correct roofing system for what you’re doing. 

 

Fair enough... I'm sure it all depends as per usual.. anyway I was mostly joking about my complaint, in a way it's probably a sensible statement to say "if you don't understand it don't try to build it" as well ?

 

[Temp]

6 hours ago, puntloos said:

Temp: do you agree that:

 

GRP Fibreglass

OSB 18mm

200mm PIR insulation

A Vapor  Layer

OSB 18mm 

Joists I beam

15mm Plasterboard.

 

Would make a pretty good roof? ?

 

Looks good to me.

 

[Oz07]

How are you all finishing these warm deck roofs at the edges. I did one recently that was getting on for 400mm thick all in. Cut the ends of the joists so they tapered to 100mm at the end on the eaves as per drawing but still leaves a thick fascia. I see some builders finishing the warm roof at wall insulation point, with a step down to a standard cold flat roof for overhang. Not sure how this effects condensation. 

[puntloos]

Also one dumb question perhaps but the GRP fibreglass itself, how thick is that? Or is it 'included' in the OSB it's attached to? A few mm wouldn't matter but if it becomes meaningful I'm sure people could complain..

[PeterW]

2-4mm max depending on the fabric or rovings that are used. 

[MP1]

Hi @Gus Potter

 

On 24/03/2021 at 17:11, Gus Potter said:

 

My flat roof make up is:

 

EPDM rubber glued to 18mm thk OSB3 glued onto the top of two staggered layers of 100mm PIR (thus 200mm thick) on vapour membrane. Vapour membrane is on 18mm thick OSB3 resting on 195 x 45mm joists with 12.5mm plasterboard on inside.

 

What thickness is your EPDM and what brand did you get? I am thinking about EPDM but I have never done it before.

 

My flat roof stack for a small, 30sqm outbuilding (office, gym, bathroom with steam shoer) is going to as below: