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Workshop floor construction

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Martin Whittle14/01/2013 22:24:10
101 forum posts
12 photos

I am converting one side of a detached double garage for use as a workshop, and would appreciate some advice on floor construction.

The anticipated floor construction sequence is concrete subfloor / polythene damp proof membrane / polyisocyanurate foam insulation (probably 45 - 70mm) / polythene vapour control membrane / 22mm flooring chipboard / top floor surface layer.

Do I use treated timber battens to support the floor between the subfloor and chipboard, to take the load off the insulation? This seems often recommended, but I really don't like the idea of sandwiching wood between 2 damp proof membranes: if (when?) it gets damp, it cannot dry out! The compressive strength of the insulation appears adequate, for example Kingspan Thermafloor TF70 is rated 140 kPa at 10% compression and 125 kPa at 5%, or approximately 14 / 12.5 tonnes per square metre. I therefore assume a maximum practical load of say 5 tonnes/sq m.

A typical relatively heavy bench having 6 legs, and of 300kg laden weight (100kg bench, 100kg machine, 100kg miscellaneous material ) then requires each leg to have the load spread over a minimum 10cm square of insulation. This spreading should be adequately provided by the 22mm chipboard layer.

So is a fully floating chipboard layer resting on the insulation OK in respect of load bearing including machines on benches? This would be at variance to other recommendations seen for direct rigid mounting of machines to the concrete subfloor, but I am not anticipating machines weighing much over 100kg, or swinging very heavy unbalanced workpieces.

Any recommendations for floor surface? I have seen laminate flooring at the local DIY shed priced as low as £5/sq m, for a tolerably acceptable appearance, smooth surface, heavy domestic / light commercial rating. I wonder also about use of a heavy industrial grade of PVC flooring, in sheet or tile form.

Any opinions gratefully received, especially if they coincide with my own smiley


Gray6214/01/2013 22:46:57
1058 forum posts
16 photos

As an example, the raised floor in one side of my workshop is constructed as follows:

On top of the existing concrete floor (which already has a DP membrane underneath) I laid 75x50 treated timber bearers on 400 centres. Inbetween these is a combination of polestyrene and Celotex as this is what was available at the time. On top of this is a layer of 18mm flooring chipboard, on top of this, 18mm exterior grade ply (again, this what was available at the time).

The walls and ceiling are similarly insulated and clad with 11mm OSB and plasterboard. (This was originally used as a drum practice room for my son!)

I have no problems with damp in this part of the workshop. The lathe weighs in the region of 600kg, plus various other machine tools and benches, none of which move in use.



NJH14/01/2013 23:33:28
2314 forum posts
139 photos

Hi Martin

One thing to remember, if this is an existing garage, is that it probably slopes from back to front ( to allow water from wet cars to drain towards the door) It won't be much but may fool you a bit when you come to level things! (For example a long bench needed extension pieces added to legs at one end)

Like you I converted half of my double garage. The existing concrete floor had a damp proof membrane and was perfectly dry. As a belt & braces measure I covered the concrete with a polythene sheet and then 2in x 2in (untreated) battens between which I placed glassfibre insulation. On top of the battens went waterproof flooring grade chipboard sheets. The floor was then covered with " Marley Tiles" ( and yes I did polish them- but only once!)

This set-up has proved to be very satisfactory and very comfortable. Not a sign of any damp either.

You can see a couple of pictures in the "Workshop" folder in my ' photos.



Edited By NJH on 14/01/2013 23:38:31

Bazyle15/01/2013 00:19:20
6379 forum posts
222 photos

Following advice on a building related forum I am planning hardcore/sand blinding/insulation/DPC/4inch reinforced concrete. The insulation is closed cell so waterstable but not damp proof. The concrete will spread the weight over the insulation and provides a stable thermal mass as the walls will be lightweight.

You can drill cores down to the concrete under each leg. Then you can leave out the battens and the chipboard will spread minor loads out over the insulation..

Terryd15/01/2013 08:08:08
1936 forum posts
179 photos
Posted by NJH on 14/01/2013 23:33:28:

Hi Martin

One thing to remember, if this is an existing garage, is that it probably slopes from back to front ( to allow water from wet cars to drain towards the door) It won't be much but may fool you a bit when you come to level things!




Edited By NJH on 14/01/2013 23:38:31

Hi Norman,

The slope in garage floors towards the door is to ensure that any petrol fumes are not allowed to accumulate. Under current building regs there must also be a step up of at last 100mm into any occupied part of the property if the garage is attached with a connecting door (30 min. fire resistant, self closing) into the house for the same reason.

Petrol fumes are very heavy and difficult to disperse, and of course have a low flash point - very nasty stuff,

Best regards


EtheAv8r15/01/2013 09:26:13
111 forum posts
3 photos

My workshop was constructed on a reinforced concrete base that had a DPC wrapped outside it. Internally there was laid DPC plastic sheeting and then insulation foam slabs on top of that and then the final concrete floor was laid over that - this is an area of 30 square meters. there was no supported battons between the concrete raft and the concrete top floor - just the foam insulation.

So far (nearly 3 years after construction) no issues, problems or cracks.

Cornish Jack15/01/2013 11:18:55
1219 forum posts
171 photos


Set up two different workshops, one built from scratch, concrete slab panels, t'other a converted garage. Both floors had DP layer then double layers 50mm poly foam slabs criss-cross (no battens) and loft type interlocking floorboards on top. No problems with any machinery - ML7, Dore-Westbury on heavy stand, Fobco Star etc. Would probably use metal plate load spreaders if I installed a Harrison or (I wish!) a DSG!



NJH15/01/2013 11:48:14
2314 forum posts
139 photos

Hi Terry

"The slope in garage floors towards the door is to ensure that any petrol fumes are not allowed to accumulate"

That, of course, makes complete sense - I didn't question the guy who told me about the water although i suppose that would be an additional advantage! In my case I've never put a car in any of the garages I've owned on the principle that cars are waterproof and there are far more useful things to do with a garage!


As well as your treatment to the floor it is worthwhile spending some time insulating both the ceiling and the walls. Any heat you put into the workshop will be retained for longer and rapid changes in temperature will be reduced. If you can keep the temperature / humidity above the dew point the dreaded rust monster will be kept at bay! I track these variables daily and provide low level heat throughout the colder weather (by means of a thermostatically controlled oil filled radiator) The cost of keeping above the dew point during 2012 was just £15!



Edited By NJH on 15/01/2013 11:49:36

Martin Whittle15/01/2013 17:01:29
101 forum posts
12 photos

Hi Gents

Thanks for a number of interesting and helpful replies.

I now feel confident in using foam insulation without battens on the floor. Not yet sure about thickness: 45mm would give a height close to the door threshold allowing for the chipboard and the floor surface layer. 70mm would give better insulation, but would mean the floor was above the door threshold.

As a matter of interest: the reason I was concerned about damp was the possible use of timber below the vapour control membrane. One could imagine a problem with damp due to thermal cycling and condensation, in just the same way that a double glazed windows mists up if there is an air leak. I don’t anticipate a damp problem with the workshop itself, after it is insulated and background heated. Nevertheless it is interesting that the detached garage, of standard brick (part 110mm, part 230mm thick, no cavities) and tile construction, is significantly more damp in the winter than my current draughty wooden workshed.

The garage floor has been levelled on the side intended for the workshop. The slope, varying from nothing up to approximately 1 brick height over the garage length, required approximately 0.8 tonnes of dry material for filling: around 24 bags of sharp sand and 6 of cement for the screed mix, and finally 4 bags of self-levelling compound to give a smoother finish. It has had a large domestic fan and a dehumidifier operating most of the last 7 weeks to help dry it out.

The workshop area will be boxed in using stud and plasterboard construction. The studs are 45 x 70mm, I expect to fill the space with 70mm insulation. I regret losing the floor area this takes, but this should give a good standard of insulation. I intend to use a minimal 25-30mm cavity between the outer brick wall, and the inner studding and insulation: I have already glued short pieces of batten to the brickwork at various heights behind the proposed stud locations, to hold the insulation in location. The roofspace framing is also 70mm depth, so I can use 70mm insulation between ceiling joists, and partially board the space above for light storage.

I did wonder about using 11mm OSB for cladding the walls. This would be stronger than plasterboard, and therefore easier to attach fixings for hooks, shelves, etc. It is however more expensive, has a rougher surface, and does not offer the fire protection of plasterboard. With plasterboard, shelving uprights can still be easily screwed into the studs to give extensive shelf storage.

Thanks again for the advice


Edited By Martin Whittle on 15/01/2013 17:03:07

Stub Mandrel15/01/2013 19:35:57
4315 forum posts
291 photos
1 articles

I used OSB over 3x2 joists floating on top of strips of DPC over a plastic membrane over the concrete floor of a garage. It seems to be a fine, warm, slightly resilient floor for standing on, but if I was doing it again I'd put wooden blocks to support under the benches. they haven't sagged, but i think they might if I had bigger machines.


Jon15/01/2013 19:38:56
1001 forum posts
49 photos

Could use that interlocking chipboard floorboard long lengths laid vertically over the battens with insulation. Cuts the noise down as well.

Done the apex roof boarded from underside and on top with loft chip board interlocking sections with 4" insulation. Throw empty boxes up there keeps even more noise down. Screw the boards down it will carry a lot of weight.

Bare concrete is always going to feel cold, travels through feet up to knees. Last two lathes have dug in to the concrete after levelling. Could put some matting down around the fixed devices, even 3mm works wonders but a pain to broom and shovel up.

steamdave15/01/2013 22:39:10
521 forum posts
45 photos

My workshop was a detached garage, built with insulated cavity walls and a rough screeded cement floor. The workshop floor was constructed as follows:

Damp-proof membrane, on which was laid 2” thick polystyrene insulation in 4’ x 4’ slabs. The perimeter of the floor and boundaries of the slabs are 2” x 2”. The whole is covered with T & G flooring grade chipboard and the top surface is industrial grade vinyl flooring.

Over the last 10 years this has proved to be very easy to stand on and with the insulated walls and roof is quite warm, but not draft proof (the car door is not perfectly sealed). It is easy to keep clean, either by sweeping or now and again with a damp mop.

Machinery that the floor has to bear is a Harrison M300 weighing some 750 kgs with accessories, a Thiel 158 at 1.25mt, 2 x work benches and several other smaller machines. There is no sign of sinkage to the floor.


The Emerald Isle

Springbok16/01/2013 04:29:22
879 forum posts
34 photos

Any opinions gratefully received, especially if they coincide with my own

Sounds like you have already made up your mind but any heavy equipment sitting on battens and ply
the heavy kit will compress the material over time. I used a good chippings base compressed with sand on top a thick membrane over that and about 2" of crete left to go off then painted with a decent floor sealand that was 30 years ago and still no problems. BUT even though the floor is swept the gremlins still hide odd bits that I drop.

Hope that you get your perfect workshop


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