Fibreglass roof deck expansion gap and which timber?

[heliox]

Helping a mate replace his flat roof, 100m2.

It looks like roof has joists too far spaced causing cupping in boards and failure of present GRP roof. (very badly laid).

Three questions

1. GRP Manufacturer recommends OSB3 sheeting or Ply, OSB3 is cheaper but are there any downsides to using it as a roof deck for GRP? (I think the GRP will take very well to OSB3 because of its rougher surface compared to smooth WBP ply?)

2. Can anyone explain this: all manufacturer guidelines say leave an expansion joint between building and roof deck, one says 50mm gap which I commented I thought was way too big. we got talking and my reckoning was if the roof is going to expand by as much as 50mm then surely this would move the nails going through the deck into joists? or even delaminate the GRP?

3. As this is a DIY job (not bothered how long it takes) we were going to use 5 min expanding pu glue between 2.4x1.2m board joints, then scrape off the excess, figuring this will give a strong waterproof joint before overlapping with resin,matting, topcoat. So in effect the whole deck is sealed every joint. any issue with this or is it better to leave the boards butted tightly but dry (no glue) to allow some movement?

Many thanks all

 

[dilly]

1/ Joists normally 450 centres

2/ osb3 with tonge and groove to all 4 sides, then all expansion goes to edges,you can then bandage all joints proir to fibreglassing.Or use finn forest ply 2/3 face with a expansion gap of a 2mm between boards and you must bandage b4 fibreglassing(boards expensive but excellent).

3/25mm at edges should be enough for expansion,you will also need to put in a expansion joint on the 50 metre mark.

Also remmember to stagger lay the boards.

 

[catlad]

You do not need to seal the joints in the osb3 as the resin does that. There is a profile for the expansion joint. I have only used the recommended osb3 for fibre glass its a manageable size and you do not have to work to the next joist unlike 8'x4' sheets so saves time and waste.

 

[cotswoldbuilders]

Osb 600mm x 2.4 are great for carrying up 2/3 storeys up a ladder for flat roofs.
t&g lot quicker to put down, we use them all the time.
On the epdm course, instructors prefer osb, rather than ply.

 

[heliox]

1/ Joists normally 450 centres

2/ osb3 with tonge and groove to all 4 sides, then all expansion goes to edges,you can then bandage all joints proir to fibreglassing.Or use finn forest ply 2/3 face with a expansion gap of a 2mm between boards and you must bandage b4 fibreglassing(boards expensive but excellent).

3/25mm at edges should be enough for expansion,you will also need to put in a expansion joint on the 50 metre mark.

Also remmember to stagger lay the boards.

Thanks for the advice, we are going to use "metrodeck"
http://www.cfsnet.co.uk/acatalog/Metrodeck_Roofing_Resin.html

Have anyone experience with that?
Will this still need the expansion at the 50m mark as this is supposed to be flexible? in fact the roof is in two sections largest being around 80m2

I'm abit confused, if we put an expansion joint in at 50m mark then from catlads post we need to use a special profile which is the E280 Expansion Joint on this page:http://www.cfsnet.co.uk/acatalog/CFS_Catalogue__Roof_Trims_550.html

Can you confirm I got this right: this gets fitted between two areas of boards, let say 40m2 each side, and I assume it falls with roof toward guttering, what happens when you get to the fascia boards, do you have to cut out a U shape in the fascia board and in the fascia trim to accept the expansion joint? and then bandage over and top coat?

Also is it just stuck down with silicon or construction adhesive before bandaging the edges? or to you omit bandage and just stick it down and overlap edges with the laminate and topcoat?

Thanks again

 

[dilly]

Cfsnet the place your buying from will give you all the help and instructions you need as i think they have a seperate roofing section.As far as roofing resins go from general suppliers there all there abouts the same.Is this the first time doing a fibreglass roof ?

 

[catlad]

I don't know when you are thinking of doing this roof but I have just put my grp stuff away till spring. I have not done a roof that has needed an expansion joint but if you want the water to get into the gutter then notch it out Grp is very flexable like that. The profiles are nailed with 20mm clout nails every 250 mm.

 

[heliox]

Yes first time doing a grp roof. (not put us off though, worse that can happen is we make a pigs ear of it).

Catlad, Thanks for advice, funny we were just talking about it this morning and both agreed that its getting too cold and damp now and will probably have to patch up until spring.

Thanks all, we'll post some photo's up when we're done.

 

[catlad]

It was when you said " not bothered how long it takes " I thought how are you going keep it dry for a start, also you are supposed to put the top coat on the same day ideally and within 24 hours. You can take a coarse for a day and get a lot of questions answered thats a big roof to start with as the last people found out.

 

[heliox]

It was when you said " not bothered how long it takes " I thought how are you going keep it dry for a start, also you are supposed to put the top coat on the same day ideally and within 24 hours. You can take a coarse for a day and get a lot of questions answered thats a big roof to start with as the last people found out.

Yes I see what you mean, by not bothered with time, I mean dont mind spending extra time on better detail, I dont mean days weeks

We'll use some DPM for temporary protection.
(I suppose we could use breather membrane too and staple down and pull back up when needed, easier to roll out and I got some rolls here)

Anyhow thanks all, much appreciated.