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Groundwork & Foundations

Questions about workshop base...

I'm building a new workshop/bike store/man cave in the back garden, having outgrown the garage.

The workshop will contain my machine tools (milling machine @ 400kg and lathe @ 250kg + benches)

Overall dimensions of the slab is 6m x 4m, the workshop itself will be constructed of sectional concrete panels and will be 5.85m x 3.85m externally with a low pitch box-corrugated metal roof. Walls and roof will be insulated and boarded with OSB.

I have dug the hole approx 275mm below the datum level and 75mm larger all round, the finished height of the base will be 300mm. The undersoil is London clay and there is a mature chestnut tree approx 3m away which has been severely cut back and I have removed a significant number of roots (some over 4" dia).

The plan is to put 125mm compacted hardcore in the central 5.4m x 3.4m to leave a 300mm footing, 25mm blinding sand, 1000 dpm, 50mm insulation and then pour concrete over the lot, embedding some A142 mesh centrally in the top 100mm. The edge of the slab will be bevel troweled for 75mm all round to aid water run off.

Once the formers are removed and the concrete is set I'll fill the 75mm gap round the edges with pea shingle to aid water dispersion.

Questions I have, in no particular order:

1. Is it sufficient just to lay the mesh on 2" broken paving on top of the insulation, or should I form it down into the footings by bending at right angles and going down vertically one mesh (200mm). Is there any further benefit in returning it horizontally at the bottom?

2. How far should I extend the insulation - the same area as the hard core or out into the footings? And if the latter, how far?

3. I plan to extend the DPM up the side of the slab (between the concrete and the shingle) but how do I terminate it? Just cut it off 25mm below the top of the slab?

4. Is this over the top? The mill weighs 400kg over a 0.5m x 0.5m base so is quite a point load. (about the same loading as one wheel of a Rolls Royce , so I am following 'garage' rather then 'shed' thinking )

The ground rises towards the back LH corner of the shed, being approx 500mm higher in that corner then the front RH corner (datum point). I am considering erecting some shuttering and pour a concrete retaining wall across the full 4m of the rear and 1m to the LH side. The wall will be 250mm wide at the bottom, sloping up to 200mm wide at the top with some reinforcing mesh. Is this wall necessary?

2 Answers from MyBuilder Groundworkers

Best Answer

1. By our regulations, mesh need to be min 50mm from dpm or in yr case insulation, between footings
2 and 3 dpm has to sit between footings over crushed compacted hard core. Dpm will go up to dpc level and will terminate outside internal wall. Dpc will sit on it.
4. It is not over the top because the ground is really sloped.
Craig M


Answered 21st Feb 2013

do not run the insulation over the footing, carry the mesh on small pieces of 50mm paving, no advantage in this case returning the mesh down into the footing as no risk of lateral movement, as for the retaining wall it would be easier to lay 100mm concrete blocks flat in normal bond this would give you a very strong wall 220mm wide.


Answered 13th Oct 2012

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