Shed base on exisiting sloping concrete

[Dean]

I'm planning on replacing our shed. The current one (over 60 years old!) has the main vertical timbers set in a 4" concrete floor slab. The floor, though quite solid, is uneven and on an incline of about 80mm over the long length of the shed (9'x6' pent).

I'm trying to figure a way of creating a durable and level base from this for a new shed (a standard pent 10'x6' with an integral floor). I'd like to avoid ripping up the old concrete and/or pouring too much new concrete.

Without pouring a new slab, most of the ideas I have come up with involve building various layouts of dwarf wall either with or without an additional

2"x6" timber frame on top (in attempt to cut down on the number of walls needed).

The problem is that with the fall on the existing concrete, I would need to cut alot of blocks. I'm considering the various 440mm concrete blocks and wonder which are best in terms of suitability and ease of cutting. Also will it be hard to get a finished level surface building like this? Bearing in mind my setting out skills aren't bad but I've not done any brick or block laying. It wouldn't be more than 1 course.

I'd be very grateful for any advice.

[Jeremy Nicoll - news posts]

I acquired a shed recently which was sited on gravel. The suppliers laid timber posts on the gravel first, then the reinforced floor of the shed went across that so that the shed floor rests on the posts at regular intervals. Although that might not provide as much ground contact as there would be if the shed floor had been laid directly on, say, slabs, it also means that there's much better air circulation under the shed.

Even heavy rain drains away through the gravel pretty quickly, so - I hope - the base timeber will rarely be all that wet.

In your situation I'd consider a retaining wall around your base, and filling the void with gravel.

[Harry Bloomfield]

Dean explained on 07/09/2008 :

I'm not clear on whether or not you are indicating that the new shed will over hang the existing concrete base or not.

Generally get the wood lifted clear of the base with anything which will prevent moisture tracking up through it - bricks, roofing tiles and etc.. Put plenty of these in place, both around the perimeter and at regular intervals under the floor joists. Air circulation and keeping the timber dry is the key to preventing rot.

[The Medway Handyman]

Concrete fence posts are good & very easy to lay.

[Dean]

The extra foot of the new shed will be up hill from the existing base and overhang existing patio slabs (these stand a bit proud of the concrete. I planned on supporting the underside of the floor of the new shed to a level

50mm above this patio, this then determining the height of the required dwarf walls across the sloping slab giving a max wall height of about 200mm.

This is a benefit that occurred to me when I thought of dwarf walls. I thought of using large concrete blocks or regular bricks. Then hiring a block splitter or angle grinder for a day. I'm wondering what type of bricks will be easiest to cut or split. I think I'll struggle to reliably split big 440mm blocks and I prefer to use a splitter rather than disc cutter.

The under the floor of the sheds I looked at (and can afford) seems to be 2" running long ways under thin t&g floor. Six dwarf walls would give support about every 18". I wonder is that enough? The shed isn't going to contain anything very heavy but will get fairly packed with stuff over time.

[Geo]

I have just cut (in half) a few dense concrete 440mm blocks. Marked all way round with a pencil then used 4.5" angle grinder with diamond blade to cut all round (depth about 1"). A bolster chisel and lump hammer separated the two halves. Takes 3-4 minutes per block max. One block from Wickes cracked /before/ I used the hammer - but nowhere near the cut. A second block is partially cracked. I then bought blocks from B&Q and these have been no problem to cut. Geo

[Harry Bloomfield]

Dean presented the following explanation :

Every 18" is quite adequate. The 2" sounds a bit on the thin side for a large hut though. The front over hang of 12" doesn't sound as if it will be a problem, even if it were to be left unsupported.

[Dean]

I have a particular shed in mind locally so I'll ask if I can have a stronger floor.

Thanks to all for the advice.

[Dean]

Thanks. That's reassuring. I'll give it a go.

[chris French]

In message , Dean writes

When I put our old shed, on sloping ground I used half concrete blocks bedded on the ground and then put treated timbers (fence posts as it happens) across these and then the shed was rested on those.

It was a 10 x 6 shed, IIRC I put timbers across every 2 feet supported at the ends and in the middle with concrete blocks.

You could probably do something similar if you don't want to lay lots of blocks.

[meow2222]

Shuttering and pouring more crete on would surely be the simple option, eliminating all masonry cutting.

Pad stones, either concrete blocks, or perhaps poured in situ raised pads, would also work, as you suggested. 2x2 floor joists are thin, and only workable if given frequent support. Blocks, bricks & slabs of various thicknesses are available, everything from 1" to 4". Such a mix could much reduce your cutting needs. Pouring pads in situ would eliminate all cutting, and the concrete needed is small enough to be bucket mixed.

NT