Prof

[Prof]

Hi!  I have a couple of projects lined up for this year and I will ask for advice on one of these soon.  First though is my request for advice about my 'Warm' flat roof that was installed last year in the summer.  Last week we had frost for the first time this year, and when I looked out of my bedroom window I could see every screw hole outlined in the frost, and upon closer inspection, I could see that each screw site beneath the GRP waterproof covering had sunk by about 2 to 3 mm.  As the sun slowly heated the roof and water started to form, these dips became full of water and then eventually the warming of the screws and their expansion (I Assume), returned the GRP to its normal evenness and no visible signs of the screws.  It is frosty again today and the same thing has happened. I am very worried about all of this, as to my thinking, if this happens regularly with cold weather, and possibly the reverse happening in the summer, every screw hole site will finally break down the GRP which is not designed to move like this and will eventually split and allow water through each hole.  I have told the roofer about this, but he said that he has never heard of this sort of thing happening ,  I think that he just used long metal screws, but will not know for sure until I see him.

I would be grateful for any advice as to how I should handle this situation.  Prof.   

[SteamyTea]

Not heard of it either.

How think is the GRP, thickness is often referred to as 'layers', ounces or gm.m^-2.

Also what are the screws holding together, could that be expanding and contracting more than the design parameters?

 

[Prof]

Hi Steamy Tea!  I do not know the thickness, but it is quoted as heavy duty GRP. with a high performance polyester top coat The screws are holding -from the bottom12mm plywood sub deck. Vapour control barrier, 120mm foil backed Celotex PIR insulation board18mm T&G smart ply screwed to existing joists .

[ProDave]

I have a suspicion that it is not the "screws have shrunk" rather the whole lot, except where the screws are holding it down, has risen.

 

This suggests that there is moisture under the GRP whether that is directly condensation on the underside of the GRP  or the plywood layer underneath is soaked, but whichever, that wet layer when it freezes expands, except the bit where the screws hold it down it can't expand so the dips form.

 

I don't know what that contributes to finding and solving the cause, but knowing it is probably wet under there might help.

[MJNewton]

I suspect it might be an optical illusion caused by a combination of varying depths of frost between the warmer screw head locations and diffraction of light from the meniscus of thawed water. Can you cover a screw head overnight and remove the covering in the morning? It’ll still have been subject to the same temperature swings  but I expect it’d look ‘normal’. All speculation of course.

 

Alternatively, if the dips really are forming might the screws be contracting in the freezing temperatures?

Edited January 28, 2022 by MJNewton

[Prof]

Thankyou all for your comments.  I really hope that moisture is not the problem.  I really cannot see how moisture could get into that top area, as it is sealed to the GRP.  how do I check this is happening and how can I approach the roofer ?  If he refuses to accept the photographic evidence, what is my next option, who can I consult if he refuses to do anything about this?

I will check next time we have frost and see if it is an optical illusion, but on close inspection by magnifying the pics, there really seems to be hollows around the screw sites and also every screw hole is visible because of the cold effect. The screws may well be contracting, this was my first thought, but if they are, what will need to be done to stop this happening, remove every screw and put new ones with insulation around them and then have the existing GRP covered over with another layer of GRP?

I will attempt to download the photos, but I am not very good at this sort of thing and it may not happen until I see one of my children!!

[SteamyTea]

1 hour ago, Prof said:

how do I check this is happening

You could tap the GRP lightly and listen out for a hollow sound.

Not, as you say, usual for GRP to delaminate from a substrate.

Having said that, I have known some MDF board to not bond, does not happen with ply very often.  It may be a case of a release agent used when the SmartPly was manufactured. I don't know the product well enough.

Even if the GRP has delaminated around the screws, it will probably be a long time before the material fails.

One way around it would be to use a hole cutter, with the screws as the centre. Cut a circle if GPR out  and repair locally with some flexible additive added to the polyester resin (assuming that is what was used). 

You may need to replace some screws.

[Iceverge]

Just a guess but i suspect that there may be some residual moisture content in the OSB smartply. That can't dry out because it's trapped between the Celotex and GRP.  The freezing causes it to expand.

 

Perhaps the screws being right through the material to the warm side of the house are conducting enough heat to prevent the OSB getting cold enough to expand locally. 

 

 

[SteamyTea]

10 minutes ago, Iceverge said:

Just a guess but i suspect that there may be some residual moisture content in the OSB smartply

If it got wet, and then the GRP was layed up before it was dry, that could cause a bonding problem. Means that the interface is uncured, and still sticky. Would have thought you would smell it in a sunny day.

Cut a circle out and have a look.

[Prof]

Thankyou all for your comments.  I really hope that moisture is not the problem.  I really cannot see how moisture could get into that top area, as it is sealed to the GRP.  how do I check this is happening and how can I approach the roofer ?  If he refuses to accept the photographic evidence, what is my next option, who can I consult if he refuses to do anything about this?

I will check next time we have frost and see if it is an optical illusion, but on close inspection by magnifying the pics, there really seems to be hollows around the screw sites and also every screw hole is visible because of the cold effect. The screws may well be contracting, this was my first thought, but if they are, what will need to be done to stop this happening, remove every screw and put new ones with insulation around them and then have the existing GRP covered over with another layer of GRP?

I will attempt to download the photos, but I am not very good at this sort of thing and it may not happen until I see one of my children!!

[Prof]

Thank you for your comments and advice.  I will discuss this with the roofer and see what he is prepared to do.  If he will do nothing, I think that I will at least have to get him to write me a letter stating that he is aware of a problem and is and will accept responsibility in the future to rectify any damage occurring from this perceived problem.  Those are my immediate thoughts, but no doubt I will have to refine them.  Being an old fellow now makes me less capable to handle these things.  I will let you know what transpires, and meanwhile if you have any other advice or thoughts please let me know. 

Regards to you all. Prof.