Leveling and extending a terrible existing concrete base for a shed.

[Damian Hemsworth]

Hi All,

I'm new to this group and thought I'd post a question (probably answered be fore so apologies, I can't find the answer).

I've decided to have a go at replacing my old shed which presently sits on a sloping concrete base that must have been laid by a drunk! Either that or one end of the base is slowly sinking (does this happen?)

the are of the existing base is approx 16ft by 10ft and slopes by about 3 i nches across the 16ft side. My old shed sits at a funny angle and the door doesn't close properly as there is a large triangular void at the top of th e door.

I want to create a 16 ft x 16 ft base in it's place to build a new workshop on, I'm reasonably proficient at DIY and have used concrete before (laying patios and fencing) but have never laid a concrete base.

The area behind the shed drops down about 2.5 ft, my plan was to run a spir it line level across from the highest corner and enclose the entire area wi th a retaining wall using dense 440mm blocks.

I would then chuck in a bit of hardcore and compress, followed by about 6in of ready mixed concrete and level off.

My question is, would I experience any problems with the new layer of concr ete being quite thin as I approach the highest corner? The thickness is lik ely to be only a few mm thick.

The alternative is to take out all the existing concrete and level this (wi thin a retaining wall) followed by a layer of concrete (6in) however this i s likely to be quite a bit of hard work using a sledge hammer. My project i s estimated to be around £2k so I'm hesitant to bump the cost up by hirin g tools if I can get by without.

Also, if I was to take the second option, is there any merit in concreting over a reinforced steel grid, Or is this unnecessary?

Any advice would be gratefully received.

Kind regards Damian

[Bob Minchin]

First find out if the current base is moving either as a complete item ie no cracks or just in a few places as indicated by cracks. No good building over a shifting foundation. Yes this can happen if for example under ground water flow (eg broken drain) is washing away under the slab.

Assume all is sound or you have to dig it up in order to be sound, then thin areas wont work. I'd say minimum 50mm extra at your highest point. Maybe just remove a small amount of existing concrete in the upper corner. Make up a cement and water slurry and put this down immediately prior to pouring the concrete to get the best bond where it is likely to be a thin layer. Make sure the shed is raised off the new concrete to allow air flow all around and under. Standing the shed on concrete lintels is a lowish cost way of getting bearers that won't rot or wick up and water. Put as much preservative on the underside of the shed as it will take. "full lead" creosote is good and still available if you look for it - Bird Brand in one good one. Soak the shed in creosote too as much as it will take and then some more. The smell soon goes and it will extend the life no end.

[Andrew Mawson]

Undermine the slab in one or two places to see what it's resting on.

If it has top soil under it, break up the slab, move rubble to one side, excavate to sub-sol, make up back to level with the rubble and possibly more, blind with sharp sand, lay a waterproof membrane and pour a new 100 mm slab over it.

If the top soil has already been removed and it's on hard-core, just break up the slab, blind with sharp sand, lay a waterproof membrane and pour a new 100 mm slab.

Re-inforce the slab in either case with A142 mesh. Sound like your site slopes. make sure that the slab is higher than the higher ground level to stop water entering the new shed.

Andrew

[Tim Watts]

That much concrete is hard work.

Why not build an extra load wall 6ft off the end of the concrete, then:

Make a floor of 4x2" decking timber, using packing plates under them to where the concrete is low (either metal shims or Wallbarn adjustable pinths - very good these, I used them).

Floor that with 18mm ply (and maybe some celotex).

Build floorless shed onto that.

OK - your floor will be 5-6" high but it will save a lot of work. 4x2" joists should span the 5.5ft gap between the end of the concrete and the wall without much fuss.

Just an idea....

[tabbypurr]

before so apologies, I can't find the answer).

n a sloping concrete base that must have been laid by a drunk! Either that or one end of the base is slowly sinking (does this happen?)

inches across the 16ft side. My old shed sits at a funny angle and the doo r doesn't close properly as there is a large triangular void at the top of the door.

op on, I'm reasonably proficient at DIY and have used concrete before (layi ng patios and fencing) but have never laid a concrete base.

irit line level across from the highest corner and enclose the entire area with a retaining wall using dense 440mm blocks.

in of ready mixed concrete and level off.

crete being quite thin as I approach the highest corner? The thickness is l ikely to be only a few mm thick.

within a retaining wall) followed by a layer of concrete (6in) however this is likely to be quite a bit of hard work using a sledge hammer. My project is estimated to be around £2k so I'm hesitant to bump the cost up by hir ing tools if I can get by without.

g over a reinforced steel grid, Or is this unnecessary?

If your old slab is subsiding it will most likely continue to subside, and likely break up. Either you can build on it using adjustable supports, or n eed to remove it and redo, digging further down this time.

No way is an inch thick concrete going to survive, even on top of the old s lab.

If you want it to last long term the only real option is to remove the slab & go down further this time. You could avoid that work with a bodge job, b ut that's what it will be, who knows how long it'll survive for.

Wooden bases of course avoid all the hard work, but they always rot away. O ne other possibility is concrete piers. Dig deepish holes, fill with concre te and build from those, avoiding using the old slab for support.

NT

[stuart noble]

I have proved to my own satisfaction with various projects that this is fundamentally untrue. Such advice pre-dates SBR (and probably even PVA).

[Brian-Gaff]

All I will say is that it might be wise to break up the base at the end where at the moment you would end up with just a skim of concrete. In my experience, such thin amounts get water in the join and with no real strength or weight there, it just crumbles away down to the old concrete. If you break it up on the thin end then I'd say this would work.

Brian

[tabbypurr]

the op's going down to a few mm! Can you get that to work?

NT

[stuart noble]

You can feather an SBR screed down to nothing. You may have to use plasterers sand at the thin end but concrete should be ok at >25mm

[Tim Watts]

Unless he puts a thick layer at the thin and and an even thicker layer at the other?