New Coroline Bitumen Roof - What size ply or OSB as support

[gh1975]

Hi everyone,

I have a prefab concrete section garage which from a walls and door point of view is structurally sound and water tight. The roof however which is asbestos cement corrugated rubbish is now leaking like a teabag, so a replacement roof is needed.

The garage is 3x5m with a flat sloping roof. I am thinking of using the Coroline Bitumen corrugated stuff that Wickes sell, but to me it seems pretty flexible and would sag without the correct amount of purlins.

So my idea is to board the roof first with either ply or OSB fastened to the purlins, then overlay the corrugated bitumen and nail using the correct nails.

So I have a few questions?
1) Material: OSB or Ply?
2) Thickness: Would 18mm be overkill or would a thinner board be ok, as there is very little weight in the bitumen sheets.
3) Would the timber cut condensation down? or would i be better fastening something to the underside of the wood? - I guess only time would tell with this one

Thanks for any advice anyway!

 

[ladylola]

A lot depends on the spacing of your rafters, My workshop has a coroline roof covering and I boarded it with 10mm or 3/8" ,shuttering type ply and I have rafters spaced at 16" centres and haven't had a problem with sagging. I felted the roof with that silvery bubble type of stuff that I got from screwfix (can you tell I can't remember the trade name ? ) before fixing the coroline. The underside of the ply is covered with 1/2" tentex or insulation board and there have been no condensation issues.

 

[gh1975]

A lot depends on the spacing of your rafters, My workshop has a coroline roof covering and I boarded it with 10mm or 3/8" ,shuttering type ply and I have rafters spaced at 16" centres and haven't had a problem with sagging. I felted the roof with that silvery bubble type of stuff that I got from screwfix (can you tell I can't remember the trade name ? ) before fixing the coroline. The underside of the ply is covered with 1/2" tentex or insulation board and there have been no condensation issues.

Thanks for your advice. I think from memory the steel purlins are at 24" centres so i think 10/11mm osb or ply should be thick enough like you have. After all there is very little weight in the coroline and they manufacturers dont tell you that you need such a base material anyway.
Cheers for taking the time in answering, much appreciated

 

[ladylola]

If you want to be absolutely certain then just thicken up the ply. I have seen a garage roof done with this stuff and it was clear by the amount of sag that it had been fixed at the ridge , wall and one purlin and nothing else