sarnafil grey membrane Or firestone EDPM or GRP

[vin_noob]

Hi All,

Planning a single storey extension in Cambridge, a dining and redoing a ground floor bedroom. Both will have flat roof with a small pitch.

Quite confused as to which one to go for. Just wanted to get any feedback on membrane vs edpm vs grp.

Thanks

Vin

[Eddieslofart]

See previous re EPDM.

Sarnafil needs installing by e registered installer/co, which will push the price up, and at the end of the day, it’s a sheet of PVC.

BUF again, needs installing by a pro, and if you want insurance backed warranties, registered installer/manufacturer.

Asphalt, excellent, but quite possibly the most expensive.

There are many variables, but get it right first time, and you won’t be back here in a few years asking what’s the cheapest way to fix a leak.

[V8RX7]

I looked at all of the above and then went with insulated steel roofing.

I used Kingspan Lo pitch after I couldn't find anyone who could satisfactorily fit Kingzip (it looks like standing seam zinc if you squint).

We fitted it ourselves to an 8m x 8m roof in a day - it's a great product IMO particularly if you over look it, as I didn't want to look at any of the other products as IME they can bubble and look awful.

[vin_noob]

@V8RX7 Metal roof sounds quite interesting, no idea why I did not think of the same. May I ask how much it costed? And by any chance do you live in the vicinity of Cambridge, so that I could ask for your guidance?

Thanks

[V8RX7]

IIRC the sheets for approx 70m2 insulated with 40mm celotex was £3600 (I had to keep the profile low and used rockwool underneath)

Two of us fitted them in a day, with another day messing around with gutters (we adapted plastic) and lead work to the existing (the standard metal details are a bit crude)

I'm Midlands based.

There are many systems around - not easy finding the info / sellers as the Reps aren't interested in something the size of a Tesco porch.

I started off wanting a standing seam roof but wouldn't pay the price, which led to sarnafil and then to Kingzip etc

I was pleasantly surprised that metal roofing is considered "standard" these days by Insurance companies.

[C.A.R.]

Lots of options

Single Ply (Sarnafil is a brand)
Pros-
Cheap (flat area)
Aesthetic (looks clean and neat)

Cons
Expensive details (any penetrations, drip-edge)
Easily punctured with foot traffic

2-layer Felt
Pros
Robust even for foot traffic
Acoustic absorption
Cheaper detailing

Cons
Less attractive

Liquids (PMMA or PU)
Pros
Hard wearing
Continuous, seamless finish

Cons
Any variations in substrate mirrored on finished surface
Good systems are expensive

EPDM has it's place, but if you've any spotlights or penetration or anywhere there would need to be a joint between two sheets I'd advise against it.

As with any industry; there are a lot of cowboys out there and any waterproofing product is only as good as its installation - particularly with liquid systems.

I have a good roofer in Cambs I can recommend.

[vin_noob]

@C.AR. Thanks for the input. Would be great if you could share the details of the roofer in Cambs

[Equus]

V8RX7 said:

I had to keep the profile low and used rockwool underneath

Be very careful with this as a solution: it's asking for trouble with dewpoint.

Just about the most basic principle of thermal insulation is that you have the high vapour resistivity on the inside (warm) face.

Insulated steel cladding has a completely vapour impermeable layer (plastic coated steel) on the outside, but gets away with it (most of the time - it is still prone to condensation forming on the inside face) because it also has an equally vapour impermeable (plastic coated steel) layer on its inside face.

...and then someone goes and adds another layer of highly vapour and moisture permeable insulation on the inside face, ideally positioned to move the dewpoint to where it will do most damage.

anonymous said:

[redacted]

Summary:

I believe that all the common types of flat roof finish can work satisfactorily if detailed and installed correctly, but that you need to exercise extreme caution because the flat roofing industry, particularly at the domestic end of the market, is second only to the tarmac trade for being infested by π keys and cowboys.

Slowfart (being a purveyor of built-up felt flat roofing) believes that built-up felt is God's greatest gift to mankind, despite it also being the most defects prone of all building - never mind roofing - techniques known to man (admittedly mainly because of the aforementioned π keys and cowboys).

[Eddieslofart]

Proof ?

[V8RX7]

Equus said:

V8RX7 said:

I had to keep the profile low and used rockwool underneath

Be very careful with this as a solution: it's asking for trouble with dewpoint.

Just about the most basic principle of thermal insulation is that you have the high vapour resistivity on the inside (warm) face.

Insulated steel cladding has a completely vapour impermeable layer (plastic coated steel) on the outside, but gets away with it (most of the time - it is still prone to condensation forming on the inside face) because it also has an equally vapour impermeable (plastic coated steel) layer on its inside face.

...and then someone goes and adds another layer of highly vapour and moisture permeable insulation on the inside face, ideally positioned to move the dewpoint to where it will do most damage.

I added a vapour barrier behind the plasterboard - 2 yrs, no problems. I checked recently when fitting my (completely illegal) woodburner flue and there is no dampness whatsoever.

[Equus]

V8RX7 said:

I added a vapour barrier behind the plasterboard - 2 yrs, no problems.

Yes, that will work provided the vapour barrier is perfect, and offers at least as much vapour resistance as the metal insulated panels.

It's still not a solution I'd advocate, though - it's definitely one that's asking for trouble.

The other rule of thumb is that your underdrawn insulation should offer no more than 1/3rd of the thermal resistance of the panels, but again that's not something I'd want to rely upon.

[V8RX7]

Equus said:

Yes, that will work provided the vapour barrier is perfect.

It is, I did it.