Workshop flooring question

[memzey]

Hi gang,

At long last I am having my workshop built at the bottom of my garden. It will be of flat roofed, block and render construction with a reinforced concrete slab containing EPS insulation in the slab. As the maximum external hight of the roof can be no more than 2.5m I have to be very stingy with the the thickness of the flooring so as not to lose too much headroom. I wondered if I could get away with a DPM layer on top of the slab and just lay ship lapped OSB or something over it and call it quits at that? I also intend to have sound insulating rubber matting under the bench and my machines to help keep the noise down. Does that sound ok to those that have done this before or am I missing a trick?

 

[katellwood]

Shouldn't the DPM go under the concrete slab prior to casting and fold up to connect with the DPC in the blockwork

 

[Phil Pascoe]

Yes. If you wanted to gain a few inches for underfloor insulation, couldn't the slab be sunk? It wouldn't matter so long as the DPC was put in correctly?

 

[memzey]

Yes the slab will have dpm as described and be no higher than flush with the outside ground (needed for sound proofing as much as anything else). I thought people often put another layer over the slab and beneath the flooring though? Also does anyone have a recommendation as to what a suitable flooring layer would be given my circumstance? Not too expensive but suitably had wearing and stable. I have a mix of fixed machines (band saw, dimension saw, drill press - all British and heavy) and am looking to add a PT to that cohort in the not too distant future.

 

[Beau]

Yep we have a second membrane as you describe for our house. The lower DPM is more to stop the concrete drying too fast than to to stop water ingress. Just used T&G 18mm chip on my workshop and this is floating on 25mm Jablight. It proved hard waring and copes with the weight of some good lumps of cast.

 

[katellwood]

My workshop originally had a uninsulated concrete slab. on it I placed 2 x 2 timbers 16" centres on top of a DPM insulation in between then 3/4 shuttering ply on top, all screwed down then painted with garage floor paint.

it safely carries all of my machines some weighing over a ton with no issues whatsoever

it has made the working environment much more pleasurable

 

[skipdiver]

Not really advisable to lay OSB or similar straight on top of concrete (even with a membrane) as you need some kind of underlay to even out the floor. I went with 1" jablite and flooring grade chipboard in my last workshop and will go with that again in my new one when i eventually have the time to sort it. All my machines are on braked rubber wheels and moving them is a breeze.

 

[memzey]

Perfect peeps thanks for the advice. Is there a kind of chipboard flooring I should be on the lookout for? I'll have 30m square to cover and I have that old conundrum of not wanting to either break the bank or be left with something rubbishy that I regret.

 

[Lons]

Perfect peeps thanks for the advice. Is there a kind of chipboard flooring I should be on the lookout for? I'll have 30m square to cover and I have that old conundrum of not wanting to either break the bank or be left with something rubbishy that I regret.

V313 or "green" type as it has some moisture resistance.

No harm in fitting a second DPM on top of the slab but I strongly disagree with Beau who says " The lower DPM is more to stop the concrete drying too fast than to to stop water ingress ". It's primary purpose is very definitely to prevent water ingress into the slab from below which is why specific gauge and guideline on prevention of puncturing the DPM during construction are outlined in building regs.

Ok maybe not so important in your workshop but certainly in a habitable building.

cheers
Bob

(retired builder)

edit: here's a link though this is for 22mm thick it's also available in 18mm. https://www.roofingsuperstore.co.uk/pro ... GwodtoMIyw

 

[Beau]

No harm in fitting a second DPM on top of the slab but I strongly disagree with Beau who says " The lower DPM is more to stop the concrete drying too fast than to to stop water ingress ". It's primary purpose is very definitely to prevent water ingress into the slab from below which is why specific gauge and guideline on prevention of puncturing the DPM during construction are outlined in building regs.

Ok maybe not so important in your workshop but certainly in a habitable building.

cheers
Bob

(retired builder)

edit: here's a link though this is for 22mm thick it's also available in 18mm. https://www.roofingsuperstore.co.uk/pro ... GwodtoMIyw

When designing our house saw designs with the key membrane above and below the slab. Ours has the key one above but it is an unusual build but it's defiantly that way around as I built it. The bit about membranes stoping the concrete slab drying too fast was from the BCO but no other sources on that.

Edit. A quick search shows many systems with the DPM above the slab. This is one and pretty much how ours is done

 

[Lons]

Wouldn't get away with that in our part of the country beau, always put the DPM under the slab though often a second one under the rigid insulation. There is nothing wrong with yours though IMO having seen the drawing as it's linked through to the DPC so I see where you're coming from

DPM is never put there to retain moisture though, it's just a side benefit that it does.

 

[skipdiver]

You need 1000 gauge DPM on a sand blinding to keep the damp at bay from your concrete. I would let the concrete thoroughly dry for a few weeks or months if you are not in a hurry, then add a second DPM with the insulation and flooring on top. 18mm flooring grade chipboard in 2400mm x 600mm sheets, laid floating and glued at the joints makes a good hard wearing surface. You could go to 25mm thickness if you like but really no need. Chipboard flooring isn't expensive.

 

[memzey]

Thanks folks. I don't have the luxury of being able to wait months or even weeks after the block has set unfortunately. I'll be putting the flooring down tops a week and a half after the slab has set up. Will this be a problem? I do intend to keep a dehumidifier running in there to keep the rust gremlins at bay. Not sure if that will make a difference.

 

[skipdiver]

It's just good practice to let a concrete floor dry thoroughly but in truth it very rarely happens.

 

[memzey]

Cheers Steve. What was the flooring grade chipboard you used in your shop? I'm thinking about the best way to do this with minimum outlay. I could get 8 x 4 sheets, rip them into something more manageable and mill shiplaps along the ends if I could settle on the right raw material. I also think I'm going to paint it with the garage floor stuff.

 

[skipdiver]

Cheers Steve. What was the flooring grade chipboard you used in your shop? I'm thinking about the best way to do this with minimum outlay. I could get 8 x 4 sheets, rip them into something more manageable and mill shiplaps along the ends if I could settle on the right raw material. I also think I'm going to paint it with the garage floor stuff.

Complete waste of time Memzy. Flooring chipboard is rated v313, which is moisture resistant and comes in 8ft x 2ft sheets. It is tongued and grooved all round and just needs gluing on the joins. It's not expensive and i think it was about £7 a sheet last time i bought some. Wickes sell it as P5 and if you buy 5 sheets or more, they do it for £6.99 a sheet, although i got mine from my usual timber supplier.

 

[Lons]

Cheers Steve. What was the flooring grade chipboard you used in your shop? I'm thinking about the best way to do this with minimum outlay. I could get 8 x 4 sheets, rip them into something more manageable and mill shiplaps along the ends if I could settle on the right raw material. I also think I'm going to paint it with the garage floor stuff.

Complete waste of time Memzy. Flooring chipboard is rated v313, which is moisture resistant and comes in 8ft x 2ft sheets. It is tongued and grooved all round and just needs gluing on the joins. It's not expensive and i think it was about £7 a sheet last time i bought some. Wickes sell it as P5 and if you buy 5 sheets or more, they do it for £6.99 a sheet, although i got mine from my usual timber supplier.

+1 .......and what I suggested in an earlier post though the link I posted was expensive stuff.
It's many years since I've used standard chipboard as the price difference is small enough to warrant V313 / P5 grade. They originally fitted it only in kitchens and bathrooms.

Bob

 

[memzey]

Thanks again guys. I can't believe I'm about to say this but it looks like Wickes are actually the cheapest for this product (when was the last time anyone said that). 18mm P5 at £6.99 a sheet - I need 21 so will qualify for the bulk discount. £30 for delivery though - I will see if I can wangle a van from somewhere. Do we know anyone that does it cheaper?

 

[skipdiver]

Doubt you will get it cheaper. I got mine from Howarth Timber who i have an account with and paid about £7 a sheet i think, can't remember. Wickes do some stuff quite cheap if you bulk buy. Their 3" CLS is only £2 for an 8ft length for instance if you bulk buy. Most of their stuff is cack though but you should be alright with the chipboard.

 

[memzey]

And here it is all laid on top of 1" jablite:

Decided against a DPM under the jablite as my builder said it would create a sweat trap, what with the DPM under the slab. Thanks again for all the help.