Metal roof on top of GRP

[tvrulesme]

I'm in the process of building an extension which has a metal roof (probably Greencoat PLX) specified. I'd like to kick the cost of the metal roof down the road a bit and was wondering if I can install Fibreglass coating now to make the structure weather tight and lay the metal roof on top of it at a later date?

 

In short, can you lay a metal roof on top of fibreglass (with structural underlay) or is this a no-no?

[BadgerBodger]

I see no reason why not. I have installed single ply (read Sarnafil or equivalent) underneath kalzip before (a long time ago mind) on a modular solution where we wanted weathertightness quickly to allow for quick occupation of a building. We completed the roof post occupation. The final covering was effectively cosmetic only. Just be mindful that the overall cost will be greater. 

[SteamyTea]

Most GRP roofs will be made with polyester resins.

The problem with polyester, as opposed to epoxy and polythene resins, is that the crosslinking of the polymers is not so great (why they are cheaper and weaker than epoxy).

This means that there is free MEKP (the hardener) and styrene monomer (used as a thinning agent and what smells), both of these are oxidants, so can cause steel to rust.

 

So while you can do it, and it will probably last a decade or so, it really does depend on the roof steel coating.

 

If it was me, I would look at using a polyurethane resin with glass reinforcement.

I would also make up a test sample and 'rapidly age' it.

[tvrulesme]

1 hour ago, BadgerBodger said:

I see no reason why not. I have installed single ply (read Sarnafil or equivalent) underneath kalzip before (a long time ago mind) on a modular solution where we wanted weathertightness quickly to allow for quick occupation of a building. We completed the roof post occupation. The final covering was effectively cosmetic only. Just be mindful that the overall cost will be greater. 

Thank you. Yes I'd be doing the fibreglass myself so costs wouldn't be too high

[tvrulesme]

55 minutes ago, SteamyTea said:

Most GRP roofs will be made with polyester resins.

The problem with polyester, as opposed to epoxy and polythene resins, is that the crosslinking of the polymers is not so great (why they are cheaper and weaker than epoxy).

This means that there is free MEKP (the hardener) and styrene monomer (used as a thinning agent and what smells), both of these are oxidants, so can cause steel to rust.

 

So while you can do it, and it will probably last a decade or so, it really does depend on the roof steel coating.

 

If it was me, I would look at using a polyurethane resin with glass reinforcement.

I would also make up a test sample and 'rapidly age' it.

Thank you, really appreciate the detailed answer. Would this still be the case with a structured underlay? I had assumed that the steel itself would not be touching the fibreglass directly

[SteamyTea]

3 minutes ago, tvrulesme said:

Would this still be the case with a structured underlay? I had assumed that the steel itself would not be touching the fibreglass directly

Not sure to be honest.

The styrene will evaporate off for a few years, and the fumes can still cause problems.  So really depends on the amount of ventilation between the two.

 

Just look at a polyurethane system, they have the secondary advantage that they can be used when the substrate is damp.

Would need to use a powder bound chopped stand matt, rather than an emulsion bound one, and that has the advantage that it does not itch anything like as much.

[BadgerBodger]

Considering @SteamyTeas comments you could consider a multitude of cheap “short term” solutions… EPDM, felt, pvc… the options are there… 

[SteamyTea]

1 minute ago, BadgerBodger said:

Considering @SteamyTeas comments you could consider a multitude of cheap “short term” solutions… EPDM, felt, pvc… the options are there… 

Or just leave it as a GRP roof.

[Iceverge]

https://polyroof.co.uk/system-applications/simulated-metal-finishes/

 

Or fake the metal roof effect with GRP. 

[tvrulesme]

2 hours ago, Iceverge said:

https://polyroof.co.uk/system-applications/simulated-metal-finishes/

 

Or fake the metal roof effect with GRP. 

It’s a listed building so the conservation officer would never let me get away with fake. The GRP solution would allow me to explain it as a work in progress 

[SteamyTea]

12 hours ago, tvrulesme said:

Greencoat PLX

Just had a look at the spec of that material.

Seems it is zinc coated, all be it not very thick.  Zinc nanoparticles can be used as a filler/bulking agent with unsaturated polyester resins.  Now this is reaching the limits of my knowledge of inorganic chemistry, but I suspect it is how some 'rustproof' paints are made i.e. the polyester sticks to the steel mechanically, then the zinc particles are pushed away because of the electrical charge difference, creating a coating of zinc.

 

If I am correct (I with we had a chemist on here, one joined and then never came back), then assuming there is minimum damage i.e. scratching during installation, and it is not in direct contact with the GRP, it should be OK.

 

The easy way to check is to get something that is zinc coated (corrugated steel) and whack some car body filler onto it.  Then heat and cool it in water i.e. hot tap, then cold tap, hot tap, cold tap repeat while observing.