DIY Doctor UK DIY Forums

Hi all, new member here.

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?

sealing the building from damp is a problem with your method.And the insulation is not needed and could be problematic,Try this way.
Lay 100mm harcore and compact.Shutter off the size you need 150mm bigger width and length than required,lay your mesh on pieces of 50mm broken slab.Then pour your concrete 150mm thickness.Then set your structure on top.
Paint the floor with buitumin paint, and once set lay 50mm sand&cement screed on top.You can also add a latex self level compound once set to give you a hard smooth finish.The exposed concrete outside can be sloped,and covered with gravel.

Hi welsh brickie

Thanks for your response. As I understand it the dpm isnt to prevent damp as such but to prevent salts from the ground leaching up into the concrete. All of the sectional garage manufacturers suggest putting the sheet DPM over the hardcore/blinding and pouring the concrete over the lot. However 300mm concrete/hardcore doesn't meet U-values for new builds (I know this is too small to come under BC but I also want to be warm(er) in there during the winter), so I'm not sure why you say the insulation isnt needed (I also know that 50mm XPS isnt as much as I really need, but its all I can allow depth wise, and reduces heat loss through the floor by around 75%).

If I understand you correctly, your method will reduce floor to ceiling height internally by 50mm, but the effective damp proofing is the bitumin paint which I guess you'd take up the concrete panel a short way as well?

the buitumin will prevent the salts aswell as the latex compound.The insulation is not need because it can crush underweight, also there is no other insulation in the concrete sections,the floor being higher on the inside will stop damp traveling through.
If you want to insulate the walls and roof try www.expandingfoamkits.co.uk

raise your meh on 50mm.pegs so the concrete completely surrounds the conc. make sure you hsve a minimum of 25mm of concrete to the slab surface this avoids concrete blister caused by moisture egress try using "leca" loose fill insulation can be poured in your formers
before you start the baseit might be worth laying a dpm beneath the whole slab before starting