Post and beam foundations

[John Rumm]

I need to do a foundation for a garden building (log cabin style summer hose) about 3 x 3.6m

I was toying with the idea of a post and beam style foundation for it, since the location its going is clay soil with a tendency to get water logged, and there are plenty of tree roots in the area. So a traditional concrete raft would have to be a fairly hefty construction probably with some reinforcement in there as well.

Not having done one like this before, what are the best options for posts that are not going to rot? Treated timber posts, stub concrete posts, plastic tube full of concrete, or just build a blockwork pier on a pad foundation?

[alan_m]

The TV programmes on building log cabins in Alaska in very poor ground conditions and subject to earthquakes seem to favour cardboard formers filled with concrete with rebar down the middle. However, isn't the problem with clay and water one of heaving hence the need for raft.

[Capitol]

Daughter has just used this system for her new large shed 12M x 5M. The contractor used concrete filled holes topped with short wooden posts metposted to locate the base, before bolting the beams to the posts. There are a lot of sacred tree roots around, so getting permission was a pain.

[The Natural Philosopher]

Concrete with rebar down the middle IS a raft.

The alternatives are deep piles down to stable subsoil - or rebarred strip foundations - and then to accomodate surface soil movment you need wither to use a lot of expanded polystyrene under a cast slab, or block and beam and an air gap.

[Tim Watts]

Why not concrete pillars or piles, just above the ground and 2-3' deep, then span with timber?

My neighbour has just started this method for a small shed - 4 pads, about 1ft sq, 3' deep into solid clay.

He's planning on spanning with 6x2" timbers, then 3x2" between (it's about 8' long, 4' deep)

[John Rumm]

Those concrete fence post "spurs" designed for making a new base for a wooden post might do a reasonable job - they are about 4' long and ready prepared to accept something bolted to the tops. By the time you are down over 2' you are into fairly stable clay...

With getting on for 12 x 10' I would probably have to do a box of 9 posts... (or even 12 posts so you can double them up at the corners and have bolt holes in the right direction for the beams on both directions)

[Brian Gaff]

Yes and also of course if there is earth underneath the structure things do tend to grow and come up through the floor if its not concreted. also around here foxes seem to like to dig under sheds supported this way.

Brian

[nospam]

I cut cheap paving slabs into strips about 150mm wide, set the strips upright in quickset concrete then drilled them so I could bolt the timbers to them.

[Brian Simons]

Not when its piles.

That is what they have.

- or rebarred

What a slab does is go straight on the ground and the clay can do what it likes. It doesn?t move much when the slab stops it getting any water on it.

[harry]

I used 100mm x 100mm fencing spikes for the uprights for my garden shed. Cheap and quick. No problems in the last 15 years. A lot of weight in the shed. This sort of thing:-

[rick]

If it is clay ... just make a simple raft ... be easier than digging several holes.

150mm thick ... thickened to 200 mm on perimeter ... put a layer of reinforcing mesh in there 50mm up form the bottom ... and at least 50mm in form all edges. Some sub base, blind it with sand, lay over some visqueen sheet and a nice C15 mix on top trowel off and floor finish & foundations in one go and no penetrating damp.

[John Rumm]

If going for holes, then I would probably hire a small powered auger machine...

Yup that was my first intention (although the building will have a wood suspended floor, so don't need a floor finish as such), but thought I would look at alternatives, since a raft will require more excavation, and the location is probably a couple of hundred feet away from the nearest place you could get a concrete delivery, so that would mean shifting best part of say 40 barrow loads of volumetric mixed concrete (after the type 1 MoT etc for the sub base). Not to mention shuttering etc.

Anyone have a ballpark cost of a mix on site with barrowing service? (can't say I fancy loading that through my mixer and walking it a total of 3 miles!)

Whereas say a dozen 700mm deep piles with 600mm in the ground, would only be around 0.15m^3 which I could easily mix an barrow myself.