GRP - how to deal with non-standard angles

[Adsibob]

The shell of my ground floor extension has been built. It has a gable roof, more or less, with an 18 degree fall. I say "more or less" because it is rather bespoke and has some unusual features so as to conceal a gutter and skylight behind a parapet wall.

My builder has done the external deck with weatherproof ply, I think it's 18mm thick. We were then going to cover this with GRP before putting down trays for a seedum roof. Engineer has specified the rafters for this, structurally no problems. Because the roof was quite bespoke, my architect - who designed it - mentioned that it might be worth running the design by a specialist. I did and the specialist (who is a RICS surveyor and building engineer) made some tweaks which were quite sensible and also recommended we outsource the GRP and Seedum installation to a specialist roofing contractor that could give us a 20 year guarantee. Also sensible I thought. That contractor has now come and surveyed the roof. He has put the cat amongst the pigeons with the following:

 

He does not think that the "non-standard" angles we have where our concealed gutter and skylight connections are will work with GRP. As the cross section below shows, we have an obtuse angle that is probably about 110 degrees just to the right of the skylight and reflex angle that is probably about 250 degrees just above the gutter.

In both cases, I think the plan had originally been to use a 90 degree trim and bend it slightly to fit the angle, even though in both cases it would be out by about 20 degrees. The GRP contractor doesn't agree. He says:

 

I have not quoted for a GRP system due to the angles as the preformed trims are all set at 90˚ and will not work with this angles of the upstands / downstands. If GRP trims are forced into position there is a risk of them cracking when thermal movement occurs.

 

Instead he has quoted for the following:

 

The Triflex product is a latest generation PMMA resin system that forms a monolithic membrane that is elastomeric when cured and is the same waterproofing that we use on curved garage roofs. Triflex resins are more expensive than GRP but in my opinion Triflex is the correct product for this project.

 

My architect is quite surprised by this, because he thought GRP was very tough. Reading up on the forum, i see that others like @nod and @SteamyTea think it's tough too (e.g. here and here), although @markc did think it would crack if severely twisted. Does the forcing something designed for 90 degrees into a 110 degree situation, or perhaps even a 120 degree situation, count as severe twisting? If so, can't I lay the trims without twisting them and just have a slight air gap because it won't be flush due to the mismatch of angles?

 

If GRP is really out, then the problem with using the Triflex PMMA resin, is that it's more expensive in terms of materials, but also the Contractor reckons the way my builder has built up the ply wood won't work as a base for the resin and will need to be redone.  He says:

 

the current roof deck is plywood that has been exposed to moisture, the correct roof deck board for this type of installation are 18mm tongue and grooved OSB3

 

It did get a bit rained on when a tarp tent that was meant to be covering it leaked (turned out it had a few holes on it) but my builder dried it with an industrial sized hair dryer and it's marine grade ply anyway, so I'm not too worried about the moisture as i think it's just cosmetic damage - though if anyone thinks I'm being naive, please do shout!

 

We asked him what difference T&G OSB3 made vs Plywood and he said that the problem with plywood is twofold: (i) it could chip; and (ii) as it is not T&G it should have really been installed with a special tape under the seams. I'm guessing he means glass tape, but I'm not sure. So if we now want to go down the Triflex PMMA resin route, we would probably need to do the plywood deck again, either replacing it with T&G OSB3 or taking it up, putting down the tape and relaying it. It's about 22 square metres, though because of our first floor extension that sits on part of this roof and the rooflight, it's not just two simple rectangles of 11 square metres each, a bit more fiddly than that. Another option is possibly to add OSB3 onto the existing ply simply to save the labour of taking up the existing ply.

 

I'm sort of leaning towards finding a way to make the GRP work rather than start from the beginning so that we can get the 20 year guarantee, as we're low on cash and already very behind on the build and I'm sure if I tell my builder he's done the plywood deck wrong he will just blame my architect for not specifying OSB and we'll just get into a game of pass the buck. But I'm not leaning that strongly as it's not exactly a nice direction to lean in either way!

 

Any tips or advice greatly appreciated.

 

Edited June 3, 2021 by Adsibob

[Bakersdozen]

Hi Adsibob

 

You are correct in thinking fibre glass is incredible strong if used the right way - after all the build yachts with the stuff. However, it doesn't like sharp angles, you need to add some fillets so the fibreglass mat can flow in a supported curve.

 

As for the decking, yes plywood needs a small expansion gap that needs reinforcing with embedded fibreglass tape - think of the scrim tape plasterer's use on plasterboard. T & G also moves but the tongue protects the gap rather than the tape.

 

Marine ply isn't waterproof only the glue is, hence the need for varnish or an epoxy coating in boat building. The edges especially need treating to keep out moisture. Why did your architect choose that over structural ply ( EN314-3 - Class 3 for anyone who is interested)?

 

As you need to take the roof off anyway to tape it can you replace the ply with OSB 3 T&G, add fillets in the correct places, and use the ply somewhere else- or sell it on Gumtree?

[SteamyTea]

It is easy to work 'into an angle" but not over one.

So it would work fine with the top picture, but would fail with the bottom one.

Cured GRP will always be under stress if bend it, so using pre-moulded 90° angle is out on the lower diagram.

 

The ways around it are to radius the eternal angle, a 50mm radius should be OK.

 

Alternatively, get some trip made up that is the right angle.  A simple mould can be made on site by place a couple of strips of Contiboard over the angle and joining them together.

Then laying up on the inside of that new mould, release and stick in place.  Then the roof layup overlaps he edges.

 

 

 

 

 

As for being wet, it won't sure properly.

GRP layup has to be bone dry otherwise the resin will not cure properly.  Even a small amount of moisture in the timber can cause problems.

 

I have no experience of some of these newer resins, something I should look into.  Polyurethanes have a good track record and have been in service many years.  The main thing that plastics dislike is UV light, this is usually filtered out with a pigment, so should not be a problem.

Edited June 4, 2021 by SteamyTea

[markc]

agree with the above comments.

[Adsibob]

5 hours ago, Bakersdozen said:

As for the decking, yes plywood needs a small expansion gap that needs reinforcing with embedded fibreglass tape - think of the scrim tape plasterer's use on plasterboard. T & G also moves but the tongue protects the gap rather than the tape.

 

Marine ply isn't waterproof only the glue is, hence the need for varnish or an epoxy coating in boat building. The edges especially need treating to keep out moisture. Why did your architect choose that over structural ply ( EN314-3 - Class 3 for anyone who is interested)?

Thanks @Bakersdozen. So I've got some further info from site today. The plywood that has been used is WBP Plywood. I asked my builder why he didn't leave expansion gaps and he said that there are a couple of expansion gaps but mainly it's been laid "board to board" because WBP is resistant to expansion or shrinkage. Googling WBP seems to back this up (e.g. this website says: "because the WBP plywood layers are bonded with the grains running against each other the WBP plywood is highly resistant to expanding or shrinking, it is less likely to split when nailed at the edges, and the sheet strength is consistent in all directions."

I don't know whether the WBP used is EN314-2 (class 2) or En314-3(class 3). In an ideal world class 3 would have been used, but given the plywood deck will be protected by the GRP only has only had one or two days rain on it, which has since fully dried (they patched up the tarp tent protecting it and dried it with an huge industrial style hair dryer), I'm hoping that even class 2 will be okay, or am i being naive?

[Adsibob]

5 hours ago, Bakersdozen said:

As for the decking, yes plywood needs a small expansion gap that needs reinforcing with embedded fibreglass tape

If I were to leave the plywood as is, i.e. without expansion gaps, is it still worth applying fibreglass tape over all the seems? Any reason why this can't go on top of the seams rather than underneath it (which would require removing the plywood?