Joist span

[Joe Shmoe]

In my new shed build, I've installed 2x8 C24 joists across a gap of 4.2m at 400mm centers.

The NHBC table say this is sufficient for 0.25-0.5 kN/m2 deadload.

I'm not really sure about deadloads and liveloads, and won't be having anything in the loft space (save for insulation) but I want to have 2 (possibly 3) layers of 15mm plasterboard on the ceiling.

15mm plasterboard weighs 9.8kg/m2.

How can I calculate if the joists are strong enough to carry 2 or 3 layers?

 

[Woody2Shoes]

One kN is roughly the equivalent force of a weight of 100kg

So, as long as the weight of the timber, insulation and plasterboard weigh less than about 25kg/m2 (times some safety factor that you need an SE to tell you!) you're probably OK as far as deadloads (but don't take my word for it!) but live loading is important to account for as well.

Cheers, W2S

I suspect that the main factor is not safety, so much as excessive deflection (a floor joist that deflects even a quite small amount gives a bad impression).

 

[MikeG.]

Individual elements of roof structure shouldn't be taken in isolation. If this is a raised tie, for instance, then there are all sorts of other factors in play. Or, if you are planning to use hangers from the ridge with a longitudinal binder you can greatly increase the capacity of the ties, but they need to be calculated, their frequency specified, and the fastenings designed. There's a reason that structural engineers exist......

 

[user22161]

Assuming you're talking about ceiling joists, the size seems to be overkill, this table shows 50 x195 strong enough to use as floor joists on a 4.13 span @ 400mm - http://www.home-extension.co.uk/tech2.html