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Flat Roof & GRP - DIY or Not?


Tony K

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Hi All

My selfbuild is a single storey, L-shaped, 100m2 flat roof bungalow with GRP roof finish.

Quotes for the roofing are coming in unreasonably high when the roof construction is so straightforward I reckon I could build it myself. The question is though, how feasible is it to do GRP covering myself?

I've looked at videos online which make it seem like something one could learn, but is it really?

I will have about 8 or 9 small non-opening rooflights to fit. They will just sit above the gaps between the joists, but will need upstands in the GRP. 

Any experience of trying this as a DIY project?

Edited by Tony K
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Have a read though this, I cannot remember what I said.

I find GRP easy and versatile, but other find it problematic.

The biggest problem is keeping everything dry and at the correct temperature.  Why I always suggest laying up the roof sheets indoors, then only the joints and edges to do on site.

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I taught myself how to do GRP using on line tutorials (several different ones). As @SteamyTea says everything has to be bone dry and very temperature dependant but it is do able, might be best to get some help in (many hands make light work). I have done quite a few flat roofs in GRP and not had any problems. 

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I did a 30-ish sqm L-shaped roof with 4 rooflights on upstands, having taught myself from online tutorials and advice here.

 

The actual GRP wasn't a drama. However, the entire process took a lot longer than I expected - it takes a fair while to detail all the corners, upstands, etc both for the boarding out and then again for the GRP - and far longer than any likely spell of continuous dry weather in Edinburgh. So my boards got wet, and things went downhill from there...

 

It might be possible to go faster with more hands, but I think when you've upstands etc to do there's a limit on how fast you can go even with help.

 

Once I admitted weather-defeat and built a heath robinson tent over the entire roof and got it all dried again it worked OK and I'm very pleased with the end result.

 

So, if you can get the weather for it, or you follow @SteamyTea advice to laminate the boards inside first, or you build a tent from the get-go, then it's a great product and it's very easy to DIY. But there is scope for a lot of drama and stress if you time it wrong / take longer than you think you will.

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I had no experience either 

while ours was only 20 m2 I had upstand around the perimeter and the lantern 

As Joe has already pointed out 

Everything must be dry 

Semi skilled at best 

821115BF-6432-4991-99FC-8E4307AF1F72.jpeg

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8 hours ago, nod said:

Semi skilled at best 

Yes.

I could teach people in a day how to do it.  After that it is just practice.

 

Having said that, it is the subtle things like corner radii, varying thickness and multi-mix layups that catch people out.

And how to put it right after it has gone wrong, or is going wrong.

 

There is also a big difference between sheathing some plywood and making a chassis for a vehicle.

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Thanks all, very helpful.

My L shaped roof is in two sections at slightly different heights, so I can break it up into two separate jobs if I am struggling to get a long enough window of fine weather, though hiring a marquee might even be worth looking into if rain looks possible. Cheaper than scaffold I would guess.

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13 minutes ago, SteamyTea said:

I always suggest that people practice before hand.

If you can get yourself a small kit, and you have a bit of dry space, give it a go.

You will learn more in 3 practice runs than doing a roof for the first time.

 

Sounds like very good advice.

How big an area do you think I need for my practice run? 

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I have not done a roof this way but I  like the idea of pre sealing sheets of osb/ply with GRP (in the dry) then cutting/fitting them, you only need to cover screws/nails and joins then top coat the lot with grey.

Edited by joe90
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5 minutes ago, Tony K said:

How big an area do you think I need for my practice run? 

Get an 8 by 4 sheet of whatever is going to be your roof covering.

Then say it up into 2 by 2, make at least one upstand, and one external radius (the bigger the better).

Then have a play.

 

You can try different methods of applying it i.e. brush, roller, prewetted mat.

You can also screw a bit up and find out the best way to recover from it.

 

Also worth doing some bits in multiple seconds.  Mat usually has one cut edge and one uncut.  Use the uncut edge to overlap a cut edge.

 

 

Edited by SteamyTea
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Just now, SteamyTea said:

 

You can also screw a bit up and find out the best way to recover from it.

 

 

Ah yes, a constant possibility!

How am I most likely to screw it up do you think, and how best does one recover?

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6 minutes ago, Tony K said:

How am I most likely to screw it up do you think, and how best does one recover?

Most likely is putting in to much hardener and running out of time, but still trying to put it right.

 

There comes a time when you have to stop, and then, when it is fully cured, get the angle grinder out.

 

The other problem is not using enough resin and getting dry patches (this also happens when it sets hard before it is consolidated with a roller).

 

It is also not just temperature dependant, UV light sets it off to.  so working indoors and working outdoors can be very different.

But if you do a square metre at a time, with half metre overlaps (assuming at least 2 layer of 450GSM mat here), you will only be mixing up a couple of kilos at a time.

 

I would also make up something that makes cutting the mat easy.  I made a table with the mat on a stand.  That way I could roll the mat out and cut it with a Stanley knife (a basic one, don't try with a retractable blade one).  By having a dedicated mat cutting area, you will not get resin on the mat before it is used.

Edited by SteamyTea
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11 minutes ago, SteamyTea said:

If you lived down here I would come over and make a video of it, with @joe90doing the work.

 

Ha!

You'd be more than welcome to come to sunny Epsom and spend the day shouting orders at me while filming it!

Some people are into that sort of video. I'm told.

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4 minutes ago, Tony K said:

Epsom

Not so far from my Mother's.

 

Quite a few people around you still doing it.

https://www.yell.com/s/glass+fibre+services+and+supplies-surrey.html

You may be better off contacting a few and asking if you can buy some materials.  And don't forget acetone and hand cleaner.

Edited by SteamyTea
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1 minute ago, SteamyTea said:

Not so far from my Mother's.

 

Quite a few people around you still doing it.

https://www.yell.com/s/glass+fibre+services+and+supplies-surrey.html

You may be better off contacting a few and asking if you can buy some materials.  And don't forget acetone and hand cleaner.

 

Cheers.

Be interesting to see if any of them will quote for the labour. I found general roofers didn't want to put the GRP on as a standalone project if they hadn't built the roof. 

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  • 2 weeks later...

As a roofer I completed a GRP course, one day and about £90.

Was brilliant and you can then attain a warranty on the installed roof, its not hard, but there are a few tricks of the trade.

The course I did was through Sentinel, you can even view their manual online which is very useful

https://www.sentinelgrp.co.uk/assets/sentinela5installationguide2018.pdf

 

As said before, keep your boards dry and make sure you get your resin mix right, if you are struggling with the weather then roll out a thin coat of resin over the boards as soon as they are down as this will stop them from getting properly wet. Even if it rains they are easily dried.

 

One thing to mention...don't use ply the resin doesn't properly bond to it and will eventually delaminate. Only OSB should be used under GRP roofs.

 

If you have any questions I will try and help out.

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