Flat warm roof insulation

[richard at home]

Looking for some input on insulation for a warm flat roof.

All the insulation companies are selling composite roof deck boards consisting of PIR foam, vapour control layer and a thin (5mm)sheet of ply.

If I want to weatherproof with EPDM, I have read that I would need to add an additional 12mm ply (or OSB) layer on top of the deck.

The composite deck boards cost about £90 a sheet, plus the extra layer of OSB (£15).

It seems that buying the individual components, 18mm osb, vapour control layer, 120mm PIR foam sheet will work out a lot cheaper? (like £40 a sheet cheaper)

Am I missing something?

[roofer]

As long as the insulation is foil faced, you can use your combination. Google warm roof construction. If you have the vapour under the insulation, you can use a standard pur board, as long as you seal the vapour to the top membrane making an airtight parcel of the insulation.

So, 12mm ply/vapour/insulation/18mm osb/membrane.

[richard at home]

roofer said:

As long as the insulation is foil faced, you can use your combination. Google warm roof construction. If you have the vapour under the insulation, you can use a standard pur board, as long as you seal the vapour to the top membrane making an airtight parcel of the insulation.

So, 12mm ply/vapour/insulation/18mm osb/membrane.

Hi Roofer, Thanks for that.

some more questions!

What happens at the edges of the parapet walls? Keep the OSB back by 25mm. Run the VCB right up the side of the insulation and right up the wall to under the flashing where it meets the GRP trims?

Top layer of OSB stops 25mm short of the parapets too.

Is going for a self adhesive VCB worth the extra money?

Is all glued assembly better than using telescopic screws? (I would assume so due to reduced thermal bridging but will the glues/foams bond better than screws going all the way through the whole sandwich)

I'm actually planning on going for a GRP roof.

I've googled this to death so just looking for the fine details where the parapets are:




I think this is correct?

[roofer]

Your drawing is correct . However, you show no slip layer between the grp and deck. This will lead to thermal shock, or hearing cracking noises when the roof heat cycles. That's not to say I am recommending grp, because I'm not. S/A vapour is fine, as long as the laps are sealed.

[richard at home]

roofer said:

Your drawing is correct . However, you show no slip layer between the grp and deck. This will lead to thermal shock, or hearing cracking noises when the roof heat cycles. That's not to say I am recommending grp, because I'm not. S/A vapour is fine, as long as the laps are sealed.

That's very interesting Roofer. You are the first person to mention a slip layer!

Now I have googled it and found loads of posts about banging (in a bad way) GRP roofs!

So can you explain this some more?

I have yet to see it used in any of the many youtube vids showing how to lay GRP!

[ATTAK Z]

Roofer is correct as usual ... I would use a single ply polymeric membrane such as Sarnafil rather than GRP

[roofer]

richard at home said:

That's very interesting Roofer. You are the first person to mention a slip layer!

Now I have googled it and found loads of posts about banging (in a bad way) GRP roofs!

So can you explain this some more?

I have yet to see it used in any of the many youtube vids showing how to lay GRP!

GRP moves ( thermally) at a different rate to timber, therefore, the GRP will try to tear itself off the timber. This is what causes the noise.

I spent the latter half of the 80's removing GRP roofs from some large roofs, the moisture trapped under them was surprising.

A proper roofer doesn't fit GRP.

Attak's suggestion is a better option.

[anonymous-user]

You shouldn't really use any wood based board under the waterproof layer in a warm flat roof.

This is due to the risk of interstitial condensation forming on the underside of the roofing membrane (even if you have a good vapour control layer under the insulation).

Ideally, use the fibreglass faced insulation boards and stick the roofing directly to the boards rather than using OSB or plywood between.

So construction would be:

Waterproof roofing layer bonded to
glass fibre faced rigid insulation board
vapour control layer
deck
joists
ceiling

[richard at home]

ATTAK Z said:

Roofer is correct as usual ... I would use a single ply polymeric membrane such as Sarnafil rather than GRP

So EPDM or hot welded PVC is a better bet than GRP?

How is Sarnafil different to EPDM? I've read some of Sarnafil's website and can see it's a composite re-enforced PVC.

Other than that, what are the pro's and con's with EPDM?

[roofer]

For you application, EPDM will do. The problem is usually the quality of the people that fit it. Real roofers don't fit EPDM

There are many forms of membrane, TPO/PVC etc etc. Sarna needs fitting by an accredited company to get the warranty. Makes it expensive.

[C Lee Farquar]

Could I tag onto this thread rather than start another?

We have four copper lean-to roofs to construct working to 2008 regs. 6 degree pitch with my maths.

Our current spec is Copper, ventilated felt underlay, 18mm WBP nailed through 105mm Celotex Tuff-R deck, joists, plasterboard. 0.2 W/m2k.

As the design is a few years old and we would ideally like a thinner roof structure, I've asked the architect to have another look at it.

As there is clearly significant expertise on this forum, I'd welcome your views.

Also, is a lead flashing OK on a copper roof?

Thanks

[roofer]

If it's a new build over a habitable space, you will need a 'U' value of .16

If you are refurbishing, you will get away with .18

120mm will give you . 18. I suspect you will need to go cold roof of you want to keep the height down.

Lead flashing is fine with copper, although won't look very good.

[C Lee Farquar]

Hi Roofer. It is a new build but BR were passed in 2008 so we have a higher u value. I've checked with BC that we don't have moving goalposts.

Would you consider a cold roof yourself?

[roofer]

Cold roof is fine if you can get the ventilation in. Plans expire after 3 years, confused how BC are ok with old regs unless you put foundations in in 2008? Result if they are ok with it !

[richard at home]

roofer said:

For you application, EPDM will do. The problem is usually the quality of the people that fit it. Real roofers don't fit EPDM

There are many forms of membrane, TPO/PVC etc etc. Sarna needs fitting by an accredited company to get the warranty. Makes it expensive.

Thanks for the info again Roofer!

So to help me decide about GRP vs membrane vs Bitumen vs some other... (I'd actually like lead but the 6k quote put me off!)

The roof I want to cover is an Orangery roof, so 7.4m x 3.8M with a big hole in the middle! The actual roof area is only 15sq meters as the lantern is 5.4m x 2.4m

I'm planning on arranging the falls to go all the way around the out side gradually dropping to the middle of the long side. This means that at the corners where the fall goes round a corner, there will be a small 13mm step down.

Is this the right way to go about it? I wanted to go GRP as I thought it would go over the 'uneven' surface better than an EPDM?

[C Lee Farquar]

roofer said:

Cold roof is fine if you can get the ventilation in. Plans expire after 3 years, confused how BC are ok with old regs unless you put foundations in in 2008? Result if they are ok with it !

I can't get through ventilation, I'm OK to the gutter edge and to one flank. The other flank and rear wall are 700mm thick walls and built, so I guess that rules out a cold roof?

Planning was granted 2004, Building Regs in 2008 and started in 2010. BC are happy to go as passed or better. It's been handy as I was able to negotiate putting in a stone window that I wouldn't get through now.

[roofer]

C Lee Farquar said:

I can't get through ventilation, I'm OK to the gutter edge and to one flank. The other flank and rear wall are 700mm thick walls and built, so I guess that rules out a cold roof?

Planning was granted 2004, Building Regs in 2008 and started in 2010. BC are happy to go as passed or better. It's been handy as I was able to negotiate putting in a stone window that I wouldn't get through now.

Soffit vents at every joist space will suffice as long as you have scissor bracing between joists. Free standing kerb at rear with lead flashing will let air in .

[roofer]

richard at home said:

Thanks for the info again Roofer!

So to help me decide about GRP vs membrane vs Bitumen vs some other... (I'd actually like lead but the 6k quote put me off!)

The roof I want to cover is an Orangery roof, so 7.4m x 3.8M with a big hole in the middle! The actual roof area is only 15sq meters as the lantern is 5.4m x 2.4m

I'm planning on arranging the falls to go all the way around the out side gradually dropping to the middle of the long side. This means that at the corners where the fall goes round a corner, there will be a small 13mm step down.

Is this the right way to go about it? I wanted to go GRP as I thought it would go over the 'uneven' surface better than an


EPDM?

I would use Asphalt in that scenario, about 2 k

[Woody3]

Richard - I'm practically at the same stage as you - waiting for my posi-joists to arrive now, we (with a lot of help off Roofer) narrowed it down to Asphalt and Sarnafil.

Asphalt was going to be a go-er, but I wanted something similar looking to lead...

Sarnafil, came in at around £2k incl VAT - 5m x 5m roof, less 3m x 2m lantern, so 19m2 overall, plus upstands (parapet wall and lantern) plus a couple of outlets. I think EPDM was about £600 cheaper, but Sarna looks much better. We never got a price for Asphalt, so have decided to go with Sarna as it was in budget and achieves the look that we were after. I'm unsure if it would work in your scenerio though...

Edited by Woody3 on Wednesday 5th August 17:10

[C Lee Farquar]

roofer said:

Soffit vents at every joist space will suffice as long as you have scissor bracing between joists. Free standing kerb at rear with lead flashing will let air in .

Thanks for your help, much appreciated.