"Padding" GRP to improve falls at raised edge / above rooflight

[andyscotland]

So, I am still plodding on with my GRP. Turns out the manufacturer "2 operatives in a day" thing might be viable for a small square, but miles off the time needed for an L-shape with 4 rooflight kerbs  and getting harder and harder to find a guaranteed-dry day. Got everything bandaged and the rooflight kerbs laminated a couple weeks ago but ran out of time to do the deck so gave it a thin coat of resin per manufacturer. Thursday was beautiful so took all the covers off, thoroughly sanded and acetoned ready to laminate : just as we were about to go mix the resin a torrential downpour blew in out of nowhere (we were soaked through in the time it took to get tools off the roof). Now built a tarpaulin tent above to let it dry properly..... ?

 

Anyway.

 

What this has shown up is that water is slightly ponding against the parapet/concealed gutter at the bottom of one sloping plane, and along the upper edge of the rooflight kerbs. Not masses, but more than I'd like.

 

So, I ideally want to put some sort of thin profile in both these places to create a bit more of a fall. For the parapet gutter just a long thin strip down to drip edge, and for the rooflights perhaps a sort of wedge shape leading from the centre of the kerb down/out to the corners.

 

These will be on top of the GRP trim / bandage, and below the main laminate (so ultimately encapsulated by GRP).

 

What can I use that will be easy to profile in quite thin bits / not attacked by the resin / not expand and contract unhelpfully? I know polystyrene will melt.

 

I have plenty of offcuts of PIR insulation - considering fixing these to the deck with the trim adhesive and shaping them with knife / rasp to get them to the profiles I need.

 

Thoughts?

 

 

[Jeremy Harris]

Polyurethane foam sheet is the stuff used with polyester/vinylester resins usually.  Available in fairly thin sheets from many GRP suppliers, like this: https://www.cfsnet.co.uk/acatalog/CFS_Catalogue__Polyurethane_Foam_Sheet_417.html

 

It can usually be shaped and sanded without too much hassle, although it's a messy job.  PIR should work as well, but may be a bit harder to shape.

[andyscotland]

16 minutes ago, JSHarris said:

Polyurethane foam sheet is the stuff used with polyester/vinylester resins usually.  Available in fairly thin sheets from many GRP suppliers, like this: https://www.cfsnet.co.uk/acatalog/CFS_Catalogue__Polyurethane_Foam_Sheet_417.html

 

It can usually be shaped and sanded without too much hassle, although it's a messy job.  PIR should work as well, but may be a bit harder to shape.

 

Thanks - I can probably tolerate a bit of awkwardness shaping the PIR offcuts vs buying something else, if it will otherwise work. It's only 5 pieces.

 

Will maybe have a go and see how I get on and fall back to PU if it proves too much hassle.

[SteamyTea]

Yes, PU sheet is the way to go.

You can get it with a glass fibre tissue covering, makes it much nicer to handle and uses less resin when you come to lay it up.

Any chances of some pictures.

Failing that, corrugated cardboard.

Edited October 26, 2019 by SteamyTea

[andyscotland]

@SteamyTea thanks. These show one of the kerbs and you can see the small sloped triangle / parapet in the background of the second one. Haven't got any post-bandaging yet as it was dark by the time I finished doing it, so was going to take the next round after sanding / before laminating on Thursday before the rain came...

 

I did think the parapet had a very slight fall, and it basically does but there's a little bump up where it meets the bigger rear section (that the rooflight is in) so it's holding water there.

 

The kerbs aren't holding much, I think primarily the issue is the bandaging at the corners is again just creating enough of a bump that a thin line of water (like a cable / thick bead of sealant) gets held along the line where the trim bends up away from the deck. With this I'm as much wanting to avoid a line of dirt/algae etc as being worried about the water itself, since the roof is overlooked from the house.

[SteamyTea]

So if I get this right, you want to make a "wedge" of PU foam to go around the window upstand, so that the water will always roll downhill, yes?

If that is the case, just get working with the saw, rasp and sandpaper on your celotex offcuts.

To hold it all in place, try sticking it down with gelcoat.

One think to remember is that matt does not like to wrap around an external corner, so a big radius is needed.  Much bigger than it looks like you already have.

Edited October 26, 2019 by SteamyTea

[andyscotland]

6 minutes ago, SteamyTea said:

So if I get this right, you want to make a "wedge" of PU foam to go around the window upstand, so that the water will always roll downhill, yes?

If that is the case, just get working with the saw, rasp and sandpaper on your celotex offcuts.

To hold it all in place, try sticking it down with gelcoat.

One think to remember is that matt does not like to wrap around an external corner, so a big radius is needed.  Much bigger than it looks like you already have.

 

Thanks very much. Exactly that - I'll get cracking (when it's dry enough to take the tarps back off!).

 

And yes re the corners - those photos are before I took the sander to radius off the corners, got them broadly the same radius as on the drip trim. The bandage / squares of mat still took a little coaxing over but formed to the shape ok once they were thoroughly wetted out.

 

And obviously now there's a layer over them the main laminate will have a bigger radius again.

Edited October 26, 2019 by andyscotland