What size plywood

[TYNE]

Hi im looking to replace the roof on my hut its 16ft by 8ft can anybody tell me what thickness of exterior ply they would recommend

[Rob]

Depends on the spacing of rafters or other support that the ply will rest on. Presumably you will also only need access for maintenance, rather than trying to stack stuff on there?

[TYNE]

There is 4 rafters Rob

[Rob]

Each rafter is spanning the shorter 8' distance? Are those the rafters in the centre area and there is separate support at the walls such that the gap between each is over 3' ? If so that would be quite a spacing and I would be looking at 19mm thick (3/4") minimum.

12.5mm would be quite bendy at that span. It might suffice, but you wouldn't want to be very heavy if you were working on the roof.

[TYNE]

Thanks for the advice Rob il go for the 3/4in atb TYNE

[bopdude]

Is it going to be weight bearing at all ? Is it going to have waterproofing ? Why not look at a Hardie backerboard

Might work for you

[heero_yuy]

Exterior grade flakeboard might be cheaper than ply. It's the stuff they put up round building sites.

I used interlocking chipboard on my garage extension, cheaper still but must be kept dry.

[Bulky]

osb is the stuff to use 1/2" is definately thick enough

http://www.josbdone.com/technical_info/what_is_osb.php

[Taf]

Go for OSB board... most DIY stores sell it. It's very good as an exterior board... and even better if you coat it with bitumen-type paint before adding roofing felt. It doesn't need to be thick for roofing except if you live in an exposed area where wind might rip off the thin grades. In any case screw it on, don't nail, making the screwheads flush with the surface by countersinking, then paint with bitumen.

Wickes used to sell bitumen-coated OSB, but it's messy stuff to wotk with.

[Rob]

1/2" would normally be thick enough for roofing on a garden shed type building. It's enough to support the felt, if you don't mind a little potential for sagging. However, would it be robust enough to allow you to get on the roof whilst you fit the ply and lay the felt? Not with a margin of safety given the span between supporting rafters of over 3' suggested by the OP.

[martyh]

18mm exterior grade plywood about £15 a sheet ,definately NOT any kind of fibre board i.e chipboard even if you are going felt it as the damp will turn it into wheetabix
i have replaced hundreds of roofs that use chip board or similar it is not suitable for the job
good luck

[alferret]

Exterior grade OSB\Sterling board x12mm will be plenty good enough, having used it many times myself & for some applications and clients at work, you'll find it cheaper than ext ply and it can take some weight too if you need to crawl on it.

You should be looking at around £7.50 for an 8'x4'x1/2" (2440x1220x12) sheet.