timber sizes / joist hangers

I've never got the hang of when timber is going to be the size you ask for and when it will be smaller, due to the method of sizing the unfinished product. And it seems to vary around the UK. This never mattered before. Now, I have on my plans, sizes like 75mm x 225mm joists to fit into matching joist hangers, and all the fittings should be tight. I will order the timber from a proper merchants, since some is graded C24 etc, and must not warp. So if I order 75mm wide joist hangers will the inside measurement be exactly that ? Assuming it is, I need timber that is exactly 75mm wide. However, a lot of timber is regularized and rounded on the corners, and is probably smaller than the size you ask for. What should I ask for to a) not look like an arse (no comments ...), and b) get the finished sizes I need ? In the Midlands. Cheers, Simon.
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In article

Basically sawn wood should be the size quoted. Planed wood starts out as sawn and varies slightly according to how much is needed to be planed off to get it smooth.
So constructional timber should be to the dimensions quoted.
--
*Seen it all, done it all, can't remember most of it*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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sm_jamieson wrote:

Nope, *standard* timber sizes don't vary that much around the country.

If you can find any 'natural' timber guaranteed *NOT* to warp, let the group know - BTW, that's why you use herring bone or solid strutting between the joists to try and stop them moving too much.

No necessarily - if the fit of the joist is too loose for your liking, simply stick a packing piece on side or the other before fixing the said joist!
BTW, why are you using 75mm (3") thick joists? The standard is usually 50mm (2") - unless you are trimming around an opening such as a stair well.

If you joists and exact size - then simply ask for 'dimension sawn' timber - and you will get what you want.
Cash
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wrote:

True I know. But for biggish beams like purlins etc, a bad warp could be enough to damage an aircrete wall as it dries, I would have thought. Most warps happen around knots in the wood in my experience, so C24 rated timber should warp less, since it has less knots in it.

Thanks, will do. Simon.
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Simon wrote:

What a load of cobblers - if the joists are that wet, then don't put them in until they dry out to a moisture content of around 18% - and if they are *that* wet, tell the timber merchant to stick 'em where the sun never shines and go and pick your own.

I would have thought.

Another load of cobblers - certainly in my years of experience (many( - timber will warp, twist, bend and shrink regardless of the amount of knots - but knots will weaken timber, that's why the number of them will affect the stress gradings that are used.

Cash
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wrote:

Have you seen B&Q timber ? I've got several lengths of 4x2 that are straight until they get to a knot near the edge, where-upon they diverge at a strange angle. They were straight-ish when I got them, have been sat outside but sheltered for 6 months. I built a partition wall out of them and didn't plasterboard for a few weeks and the partition was twisted a fair bit and I had to bend the plasterboard.
If your experience with timber is always reasonable stuff from timber merchants, well there you have it. But B&Q stuff is bad. I don't use it any more !
Simon.
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Most joist hangers nip up quite easily - and yes they will be exactly 75mm internally.
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