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?

[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?

[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!

[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?

[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?