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Hi

I’m building a garden room 4m x2m and trying to decide on what size timbers to use. I’ve already made the base out of treated 4x2 and 90mm pir. I’ve seen a few builds recently using 3x2 cls for the walls, is there much difference between 4x2 cls? Planning on 50mm pir for the walls and fit sliding doors.... Also planning 5x2 for roof at 400 centres as only 2m span, should this be treated wood?

Thanks

Many 'commercial' sheds use 3x2 for the walls (all ours do) and it's plenty strong enough for the purpose. Going to 4x2 increases costs and (imho only) the increased depth that you can fill with insulation doesn't make much of a difference.

Nowadays I try to use timber of the same size throughout (moved to rough-cut 4x2 treated) just to reduce costs as you can buy in bulk. and make savings.

We always use treated - but, then again, we live in a very damp part of the world so it's probably the only way to go - and the roof timbers not only depend on the span but on the shape. Peaked or sloped? Either way, 5x2 is good enough. I'd cover in OSB, membrane, drip shields and coroline (or your choice of metal sheeting in colours/profiles to suit).

Don't forget guttering.

Thanks mate, just ordered 3x2...sure it’ll be good enough for the size of the building