As the subject said, I want to put some decking over a ditch I have running across the bottom of my garden.
All the info and guides I can find talk about laying decking on flat ground and I don't know where to look for info on decking over a gap.
The deck will be about 5m wide and 4.4m deep. the 4.4m depth is the span of the ditch. Does anyone know where I can look to find out what size of timber I'm looking at for the joists, joist spacing and depth of concrete foundation I'll need?
I can't give definitive sizes, but you'll need something fairly hefty if you make it all out of timber.
Have you considered making an H frame out of 3 metal RSJs - with 2 going across the ditch and one going between them above the centre-line of the ditch? That way you would reduce the unsupported timber span by 50% - which would dramatically reduce the size of timber required.
To be honest, no I hadn't thought of that. I initially thought of wood because it was something I could work in myself, I'll need to buy in some help with metalworking. Any ideas on pricing? I'll have a hunt about for a local steel merchant.
This is effectively like an internal floor as detailed in Appendex A of the Part A building regs.
Assuming the 4.4 m includes a 150 mm of soild ground each side (i.e. concrete or brick - not earth), you have a 4.1m span and need for example 7" by 3" C16 timber (tannelised) at 400mm spacings. There are lots of alternatives in the appendix, that is just the one I used to span something recently.
If you go for a conventional floor type of construction with beams running over the full span and then board across the beams, The timber size will be large. For a floor joist in a house you would be probably looking at 200x75 (8x3") for that span - maybe even a flitch beam (i.e. a pair of beams with a steel plate down the middle) - but that would be to meet building regs where the maximum deflection allowed is relatively small, and the anticipated loading is quite large. You can probably tolerate a deck being a bit more bouncy than a floor, and you are less likely to load it up with wardrobes and the like.
That also assumes that the beam is only supported at the ends - if you could get some additional support like a couple of slanted posts between the under side of your deck and the sides of the ditch for example, you would be able to get away with thinner beams.
Alternatively since you are presumably not limited in how thick you can make the support for the deck, you could make a more complex "space frame" beam - rather like the jib on a crane - thinner timbers top and bottom, separated by a number of triangulated noggins that greatly enhance the stiffness.
It might be worth you downloading the demo version of Supabeam from the SDA website (this is a program that many architects/engineers seem to use for beam design):-
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will let you play with beam sizes and see how they perform.
Is there some reason that you can't put posts down into the ditch at, say, 1 metre intervals, and build on top of those? It would make the horizontal beams a lot smaller and avoid the need to put a heavy load on the banks of the ditch. Oak is an excellent timber for posts that are likely to be constantly wet and it is not a lot dearer than pressure treated softwood, which won't last as long.
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