GRP/Rooflight Upstand help

[electrogear]

Due to having a flaky builder that's been very poor with advising me on my extension and only attended 3 days in the last 8 weeks, I've had to crack on and do the roof myself. I've finished the joinery and I intended to get the roof felted to match the garage roof which it connects to, however my roofer has also let me down and now tripled the price of the felting to the point where I might as well just GRP it myself.

So... a few questions:-

1) Having not initially planned for GRP I used 18mm WBP plywood. I understand this is not ideal for GRP, however I've heard that you can sand it to create a keyed surface, and make sure there's 3mm gaps at the joints. Any thoughts? Would I be better to over-board it with OSB3 T&G?

2) How do I do the GRP around the rooflight upstand? I've put a 2x2 45deg angle fillet around the bottom and the upstand is 2x2 with insulation in-between and 18mm ply either side. Do I need a trim to go over it?

3) How do I seal the GRP over the felt on the garage roof which it abuts to?

Any helpful thoughts would be much appreciated.

IMG_6373

• electrogear
• 26 Apr 2018

Plywood roof

 

[pilsbury]

1. Osb is recommended as it naturally has a good key. Your suggestion of creating a key seems logical. I’d be using super coarse paper to really rough it up. You are right about leaving gaps between boards. This is advised for osb too that isn’t t&g.

2. GRPing the upstanding is fairly easy. Put a little matting on the external corners first, then simply GRP around it. When the matting starts to absorb the resin it soon becomes very flexible to persuade into shape. As for trims, depends on you rooflight fitting. My panoroof lights just silicones straight into a timber kerb. I GRPd straight to the top of the upstand. But it all depends on how yours fits into place.

3. How to join the old field roof to the new? Not sure. How do your roofs meet? Same level? Step down? What direction is the fall? Photos would help for advice.

You’ll need trims for the roof edges and any wall abutments. I think GRP is pretty cheap. Got 40sq m kit off eBay for about £350. Trims were well under £100. Did in a day. Went surprisingly well. Just get all your gear ready as you don’t want to faff once you’ve done your resin mix.

 

[nickjb]

For my rooflight up stands I used preformed sections. You can get 90 degree angle (complete with 45 degree chamfer) and external corners to suit. Very easy. They we're tall enough to completely cover the vertical walls with no extra fibreglass. As a diyer it simplified the process and I could set all these bits up beforehand to speed things up when doing the glassing.

Mines all on plywood. Stuck perfectly. Tape the joins then go over the whole lot.

 

[electrogear]

For my rooflight up stands I used preformed sections. You can get 90 degree angle (complete with 45 degree chamfer) and external corners to suit. Very easy. They we're tall enough to completely cover the vertical walls with no extra fibreglass. As a diyer it simplified the process and I could set all these bits up beforehand to speed things up when doing the glassing.

Mines all on plywood. Stuck perfectly. Tape the joins then go over the whole lot.

Thanks. Can I ask where you got the trim from? I can't seem to find one for an upstand. Just to add, my roof is pitched against the upstand in two different directions, so the trim would need to be bent or cut to suit. Is that doable?

Did you rough up your plywood before application? And what do you use to tape the joints? Thanks

 

[electrogear]

1. Osb is recommended as it naturally has a good key. Your suggestion of creating a key seems logical. I’d be using super coarse paper to really rough it up. You are right about leaving gaps between boards. This is advised for osb too that isn’t t&g.

2. GRPing the upstanding is fairly easy. Put a little matting on the external corners first, then simply GRP around it. When the matting starts to absorb the resin it soon becomes very flexible to persuade into shape. As for trims, depends on you rooflight fitting. My panoroof lights just silicones straight into a timber kerb. I GRPd straight to the top of the upstand. But it all depends on how yours fits into place.

3. How to join the old field roof to the new? Not sure. How do your roofs meet? Same level? Step down? What direction is the fall? Photos would help for advice.

You’ll need trims for the roof edges and any wall abutments. I think GRP is pretty cheap. Got 40sq m kit off eBay for about £350. Trims were well under £100. Did in a day. Went surprisingly well. Just get all your gear ready as you don’t want to faff once you’ve done your resin mix.

Thank you very much for the detailed reply. My rooflight (madefortrade roof lantern) silicones directly on top of the upstand the same as yours. When you say you GRP'd to the top, did you go over the top edge too, or did you stop at the top edge of the vertical face?

The new roof joins the old felt roof at the same level, and the runoff unfortunately is along the seam of the join. I can probably put an angled fillet under the edge of the felt so that there's no standing water along the join. I'm just not sure what to use to seal the two together. Silicone? :/

 

[catlad]

I don't know were you are based but Glasplies at southport
are a good outfit.

 

[pilsbury]

Thank you very much for the detailed reply. My rooflight (madefortrade roof lantern) silicones directly on top of the upstand the same as yours. When you say you GRP'd to the top, did you go over the top edge too, or did you stop at the top edge of the vertical face?

The new roof joins the old felt roof at the same level, and the runoff unfortunately is along the seam of the join. I can probably put an angled fillet under the edge of the felt so that there's no standing water along the join. I'm just not sure what to use to seal the two together. Silicone? :/

To the uppermost part of the upstand but not lipped over the top. My roof light specified this. Then loads of low modulus silicone on top of the upstanding before dropping the roof light into the timber kerb. All squished out and smoothed to a good seal

Not to sure on your join. Someone who is a roofer on here will advise. Silicone joint there seems likely to fail to me.

 

[HERTS P&D]

Watch the following videos (approx. 5 in total)

Andy

 

[pilsbury]

Brilliant series of vids above from fixmyroof. Taught me what to do with mine.

 

[HERTS P&D]

Also taught me how to do mine.

Andy

 

[garyo]

#me too

 

[pilsbury]

#me too

You learned to GRP from the vids or got finger blasted by Weinstein?

 

[garyo]

You learned to GRP from the vids or got finger blasted by Weinstein?

Being felt has always been a risk in the flat roofing game. I had a hell of a shock when he turned up to quote though.

 

[geraldthehamster]

If you are DIYing this I would recommend you seriously consider EPDM rather than GRP. I am not a builder but have done two large flat roofs with EPDM, and found it a doddle. Do it well and it should last well over 50 years. Have a look at the "how to" videos here:

http://www.rubba-seal.co.uk/

GRP can get horribly messy. You are always chasing the cure time once you've added the catalyst and it will stick to everything. I considered GRP, visualised a forlorn pair of boots stuck to the middle of my roof, and went for EPDM instead. All in, including EPDM, adhesives and tapes, it cost me under £500 for a 25 sq m roof.

Upstands and roof lights are not a problem.

 

[pilsbury]

Not denouncing Rubber roofs, but I did a 30sqm GRP roof with a whole lot of upstands, angles, tricky bits in a day. Never done it before, all my info from those great YouTube vids. All in getting yourself fully ready before starting.