Imposed Load on Deck

[The Medway Handyman]

Hi

Building regs mention the dead load for a floor as being no more than 0.50 kN/sq.m which I reckon is greater than the dead load for a deck, since 28mm boards are about the same weight as 18mm chip (I think), but there isn't any plasterboard (which is heavy).

The imposed load is 1.5kN/sq.m, so total 2.0kN/sq.m

Since there is very little furniture on a deck, anyone hazard a guess as to the total load?

And if its less, by what sort of % could you increase the span?

IOW if the loads is 20% less, would that mean the span could increase by

20%?

Guesstimates gratefully received!

I currently do this by experience, over engineering & the highly scientific 'FB' test, but I'd like to know how the maths work.

[dg]

The span tables in the approved documents/Trada tables underestimate the capacity of the joists by quite a bit. The main reason for this is to prevent cracking of the ceiling finishes. Another reason is one of useage and bounciness.

It would take quite a bit for the member to fail, so spans and spacings can be increasesd, but by how much needs to be calculated. Also you will lose some rigidity in the floor.

Another consideration is that the decking joists will get wet and behave differently to dry floor joists. They will be more prone to permanent deflection of over spanned

Whilst the loading of a deck is significantly less than a floor, it will still bounce and appear to be overspanned if spans and spacings are increased. It will not fail, but will not feel 'right' to the customer

All in all, it is best to keep to the tables or work by trial and error to see what section sizes are possible

dg

[Tony Bryer]

No because deflection (for a given load and member) is proportional to span cubed. And as the load increases in proportion to the span, a

20% extra span doubles the deflection (1.2^4). If the loading is 75% of the figure used in the span tables you can increase the span by about 8%.