Workshop slab and DPM

It looks like I may be able to build my new workshop come spring, *just* within the 15 sq.m limit (so it doesn't have to be 1m from next door's garage-on-the-boundary, Grrr...), and wonder if anyone has any suggestions for putting a DPM / DPC in - the slab will (I think) be 6" concrete with 2 layers of steel fabric, sat on 2" of expanded poly (warm feet in the workshop - ooh, lovely!) - is there any advantage to taking the DPM below the insulation, assuming I don't poke holes in it? At present I'm hoping the walls will be concrete block with more insulation/vapour barrier/ply-or-platerboard on the inside, should the DPM come up from the slab to surround the first course of blocks, or would it be better/easier to put the DPM internal to the walls and slab, and screed over? I'd rather avoid any brittleness from a thin screed as there'll be 2 tons of lathe (and, funds permitting, a ton and a half of milling machine) sat in the 'shop once it's up, hence the need for a Substantial Slab!

Thanks in advance for any suggestions, Dave H.

Reply to
Dave H.
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I struggled with this question for ages until someone pointed out that the DPM under the slab is there to protect the concrete from losing water, from gaining water (if it rains or the water table rises) and from gaining unwanted salts from ground-water whilst it is in the process of going off.

Once the slab has cured, the DPM continues to protect the slab from sulphates (if you didn't use sulphate-resistant cement, of course) and chlorides.

So, don't think of the DPM under the slab as being a barrier to stop rising damp and don't worry about the edges and interfacing with the DPM in the walls. That's a different DPM for a different purpose.

I would consider:

------------Slab-------------- Mesh - Mesh - Mesh - Mesh I I I I

Reply to
Dave Osborne

Thanks Dave, I wasn't aware that that was the reason for the DPM! I'll probably go with 2 layers of mesh as a belt-and-braces measure to be sure the slab's rigid enough to take any movement / the loads applied and bring the slab forward at the door so I can make a slight ramp for moving machinery in and out. Being the last build on the estate, our garden's mostly crushed and compacted hardcore already...

I looked at the pavingexpert site, useful but nothing that exactly fits - lots of garage and shed slabs without insulation, or insulated with cavity walls! I think I may have to go with a DPM on top of the concrete slab and up to a DPC and then a thickish (2"?) screed or concrete on top of that - a bit of a sod, as it'll involve 2 pours/levellings and the top layer might be a bit more brittle than the slab - a consideration when rolling 2 tons about on scaffold poles and applying prybars to lift and manoeuvre it!

Dave H.

Reply to
Dave H.

Why would you want a DPM on top of the concrete? If for some reason you really do, bitumen paint would be better re firm support of a thin layer above.

NT

Reply to
Tabby

here it was slab, DPM curled up at the edges, insulation also up at the dges.. screed with mesh, done.

Reply to
The Natural Philosopher

15 sq m - that's 5 x 3m which is the size of the workshop I have had for 20 years. At that size you are, I would have thought, be highly unlikely to have any really heavy equipment - I doubt somehow that you are going to install a Wadkins Bursgreen RS wood turning lathe weighing nearly a ton, so what is with this 6" of concrete and re- inforcement for. Totally, totally unnecessary - you are seriously over-engineering. 4" good plain concrete on proper base is all that you need, with the dpm between the blinded base and the concrete. I've done that now 5 times on sheds of about that size and I still use 3 of them with no signs of failure.

And why the added complication of insulation when you can go to Costco and get interlocking rubber floor mats which =A330's worth would give you a good 1 cm's worth of real foot comfort - ie your concrete could be warm (how is it going to be warm in a workshop?) but it is still bloody hard.

I've never looked into the sums for the insulating of a concrete floor, but I would need to be convinced the heat loss is all that great in comparison to that through windows and, in a workshop's, case the walls and ceiling.

So - why the over-engineered concrete floor, and have you done the sums to see if the effort and cost of floor insulation is justified when the roof, wall and window heat losses are far more serious.

A little bit of realism needs to be applied about what a workshop is and what basic building techniques apply to it.

Rob

Reply to
robgraham

Ummm.... so far, 2 tons of vintage Holbrook toolroom lathe, planned for the future is a turret mill (Bridgeport or, ideally, Balding Beaver) of a ton plus, rest of the workshop will have pillar drill, bench space etc. - at present the lathe, drill and workbench are in a draughty and cold 8x10 shed which is a touch, er, snug? I've done a few layout drawings, and I admit the lathe and mill (and room to move around 'em) will take up about half the available space, but as there's another shed for "dirty work", this'll be the clean room, as much as anywhere with metalworking tools can be :)

The rubber mats sound a good idea - but can't go under the machines, of course, and might not last too long with coolant / cutting oil spills.

The cost of 2" of Jablite under the floor pales into insignificance really, when compared to the rest of the shell, wall and roof insulation, wiring, let alone the kit going into it...

The reality is that I plan to use it year-round, in comfort, as a fairly serious home metalwork shop (making parts for our bikes/trikes, mainly) and it has to support a few serious machines! Old Iron doesn't take kindly to condensation, so the shed will need to be heated above dew point during the winter months - low-cost insulation at the construction stage should pay off pretty quickly?

Thanks for the input, Dave H.

Reply to
Dave H.

Fair enough Dave - your workshop use will tend to exceed the norm which is what I was quoting for !!!! I did try to play a large workshop tool and you raised me considerably.

I would suggest you try the rubber mats I mentioned - you may well be right in thinking that they won't like cooling fluid, etc. but one pack for a tenner isn't much for a try and they are really very effective in the walk areas of the workshop. I can only manage a Myford 7 myself and, although my Dad did arrange a cooling pump for it, the hoses had perished by the time I got it and I don't do heavy enough work to justify trying to re-instate the system.

Rob

Reply to
robgraham

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