Wooden shed foundation - how to prevent rot?

[Nigel Watts]

Between the floor of my garden shed and the concrete slab below it is a gap of about a foot. The floor is laid over joists, the bottom of which are two or three inches above the slab but don't touch it. The structure seems to rest on a few vertical wooden posts which extend down to the slab. The ends of these are starting to rot. The space around the shed is very restricted, with two adjacent sides quite tight against the brick walls which form a corner at the end of my garden.

I need to find a way of preventing the shed from sinking as the posts rot, and of preventing rot from damaging the rest of the shed. One obvious way would seem to be to get some packing between the bottom of the joists and the slab in a few places, but what should this be made of to stop the joists rotting?

Any ideas appreciated.

 

[Penners]

Given the constraints of the shed's location, I think I would probably build some brick pier supports under the floor joists, located at what seems like logical intervals to provide sturdy support for the entire structure. I would place a "mat" of DPC material between the top face of each pier and the wood joist, and try to ensure that this doesn't form a cup shape that would cause water to pool in it. Before doing any of this, I would (if I could reach) give the whole underside of the shed a good drink of wood preservative.

Finally, I would saw off the rotted ends of the existing supports and soak the stub ends in preservative, too.

This sounds like the sort of job that you'll want to get done before you have to lie of cold, wet ground to do it!

 

[Flyfisher]

Sounds like a good 'ship' spoiled for a ha'porth of tar - a good air gap to keep the shed off the ground but supported on timbers likely to rot with the inevitable damp ground.

I would use bricks to support the shed joists - engineering ones if you're concerned about frost damage. Two bricks on the concrete base, a layer of DPC and then additional bricks to get to the height of the shed joists. I probably wouldn't bother with any mortar for two layers of bricks but it would help to solidify things if you have to build higher.

If the shed joists are not an exact number of bricks above the concrete base then use a car jack to gently lift each corner just enough to slot in the top bricks and then gently lower it back down onto the new brick pillar. There should be enough 'play' in the shed to lift each corner a few inches. Do this for as many pillars as you deem necessary to provide a solid support for the shed. I'd probably then cut away the existing rotting timber supports to prevent any problems with the shed structure.

My shed was installed in a similar manner, although I cast four concrete pillars instead of using bricks and didn't bother with a concrete slab at all. It's still rock solid after six years.


Edit: Looks like Penners beat me to it - that'll teach me to be more concise!

 

[Penners]

Two minds with but a single thought, FF, and your contribution certainly added refinements to mine!

 

[Nigel Watts]

Thanks for the ideas.

What I am not sure about is how to reach the far corner of the shed. Currently access is very restricted, not least because many of the floor joists extend beyond the footprint of the shed to provide support for decking. As the softwood decking is now starting to rot, along with the extended ends of the joists, I am planning to remove all the decking and cut back the joists to the walls of the shed. I then intend to treat and possibly cover the joist ends. I have ordered an inspection camera to try and get a better view of what is happing underneath.

Once the shed has been stabilised I intend to reinstate decking in a more robust material, supported independently of the shed and in such as way that it can be removed from time to time to allow the shed's supports to be inspected and maintained in future. I have quite a bit of recycled 4x2 Iroko from an old conservatory which could form the substructure for the decking.

The complicated arrangement is a consequence of the way our garden has been laid out. When we bought the house most of the garden was a concrete slab. When trial pits were dug, the slab was found to be between two and three feet thick with builders' rubble underneath. My guess is that a building with a basement or cellar might have stood there and was later knocked down and capped off with concrete, with the rubble mostly filling the cellar. Rather than trying to dig up the concrete the garden was built on top of it, with raised beds and a raised lawn. The shed however sits on the original slab, which is now a foot or so below the new ground level.

Given the fad in London these days for creating basements under existing buildings I now rather regret that I didn't excavate. A basement den/workshop under the back garden could have been rather useful.

 

[mattice]

Can you jigsaw an access hole inside the shed between the joists to get access to the far corner and build up a brick plinth from there?

 

[88v8]

Can you poke a trolley jack under to lift the far corners, and by jacking up each corner in turn and inserting more and more temporary timber beneath - a wooden shed will take quite a lot of twisting, you would not need to agonise about lifting it unevenly - elevate the whole thing until you have room to get under and do the needful.
Then lower it back by the same means.

Ivor

 

[worms]

The structure seems to rest on a few vertical wooden posts which extend down to the slab. The ends of these are starting to rot..

Not quite sure how these are positioned and I presume they are not actually embedded in the slab?

While the brick piers suggested above might be the best solution, I would wonder about replacing the vertical wooden posts with a couple of horizontal beams, which could be slid in and hammered home with a sledge hammer. A couple of decent, treated replacement beams would last you another 25 years or so.

As an aside, I wonder if these fake wooden beams made of recycled black plastic would work in a situation like this? Do they stand up to long-term pressure?

 

[Flyfisher]

It's not easy to visualise the exact arrangement (pictures?), but if the shed support is also part of the decking structure then I can understand it will be trickier than I first envisaged.

It seems like the shed really needs to be moved out of the way somehow - either by lifting as Ivor has suggested or temporarily moving it altogether - in order to create some space for the necessary repairs to the decking/plinth structure. A lot depends on the size of the shed though.