How do I best build a foundation for a new shed on an existing too large concrete pad?

[ChippyKlutz]

I have a large concrete pad left after demolishing a derelict chicken shed. I'm intending to construct a timber framed shed measuring 6.3 by 4.4 metres on part of this old pad. Given the size of the shed footprint, my intention is not to build a suspended wooden floor but to have a concrete floor.

I need to prevent damp ingress into the new build through the floor from rainfall on the larger pad tracking under and into the new build.

What's the usual practice in this situation?

 

[Lons]

If you want belt and braces then the same as if laying a floor in an extension to your house. A 2 brick high course around the footprint of the intended shed with a DPC then use visqueen DPM sheet to cover the floor and lap that up the inside of your brick then when pouring the concrete. If you have to lay the Visqueen in more than one piece make sure there's a good overlap, I'd recomment 300mm. What that gives is a sealed "envelope under and around your concrete floor so completely damp free. You could substitute blocks for the brick and whatever you use your timber shed is built up on top.

I'd recommend putting insulation in the floor but tbh I would seriously consider a wood floor with a central support wall, much easier, warmer and more comfortable on the feet

Just a warning. Unless you laid it you won't know what's under the exiting concrete, wheter it's an even thickness throughout, if reinforced or what the grond conditions are so if that cracks or moves it will affect your new structure and floor. The concrete may well have been deepened around the edges to support the chicken shed.

 

[Sachakins]

Are you trying to use the existing pad as your floor?
If so I would use a disk cutter and actually slice through the base, separating your shed area completely from the pad. Then add drainage channels in the gap.
But not knowing the base construction, I think you may still need a new DPM, insulation panels and a screed on top to prevent damp, just as per above post.

 

[Lons]

I've just read this again and think I've misunderstood as I thought you were going to lay new concrete on top of the old. It now looks as Sachakins says that you want to use the old floor in which case he's right. In my experience you have little or no chance of preventing damp through the old floor without laying a dpm and if you're doing that it's better to put in an insulated wood floor on top. You still need to lift the timber shed off the floor so a brick wall is desireable. Or....you just put up with damp and regret it later. You might get temporary protection by using a sealer on the existing concrete but not worth the effort and expense imo.

 

[ChaiLatte]

Buy one of the plastic shed base kits (plastic honeycomb tiles).

Two ways to use it:

Use it for the full area of the shed and DPM (better a breathable membrane) and T&G chipboard the inside. You would only need to fill the honeycomb where it projects outside the shed walls.

Use it only on the perimeter with the excess sticking outside and do your own thing on the inside (polystyrene, breathable membrane, chipboard). Fill any projecting parts of the honeycomb with pea gravel. The gravel will attenuate any rainwater. It will be a 'warmer' solution to the one above, with similar total overall build up. Putting the polystyrene on an unfilled grid woud not be a good idea and just robs headroom.

Gutters and downpipes to the shed would seem essential, making sure the outlet is piped off the slab.

It will be impractical to have a combination of these two factors: the inside shed floor at the same level as the existing concrete AND water resistance.

Spray some water onto the existing slab and see in which direction it falls. On the uphill side of the new shed, make an inverted V-shaped bund (might only need to be 20mm high) to direct any surface run off past the shed footprint.

 

[artie]

To my mind there's only two foolproof solutions.

Either lay another 3-4" concrete base onto the existing one or if you can, cut a drain track, 4-6" wide and either right through the existing concrete or deep enough that it won't ever be filled with surface water,

Second one is probably easier on the pocket.

 

[Jameshow]

Cheapest option lay treated 3x3 fence posts on the slap with dpm membrane under and across the floor with 2" concrete or insulation and boards on top.

Build shed off the fence posts making sure the cladding goes well down the fence post. Apply bitumen paint under.

 

[Jacob]

I've been looking into this myself. Plastic beams for sole plate say 100mm high, DPM over existing concrete dressed over the plastic sole plate, concrete in fill (with or without insulation?), build off the sole plate in the normal way

 

[ChippyKlutz]

Thanks everyone for taking the time to reply. I realise my question could have been better worded. I do indeed intend to float a fresh concrete floor on top of the existing pad, which has been in existence for at least 60 years. It was not expertly laid originally, not being perfectly flat and level, but was crudely overbuilt and has stood the test of time until now and I'm confident it will suffice for my project.

My query really centred around whether a waterproof membrane simply laid on the existing pad would be good enough or whether it would need elevating first in order to resist the wet. The first reply by Lons seems to have answered this and I can imagine the only tricky part is dealing with the corners where the DPC sheet would have to be folded in a sort of 'bed sheet corners'. Or maybe cut and patched?

My alternative thoughts had focussed on shuttering the entire new floor area to a minimum necessary to achieve a level rectangle - probably about 30 to 50 mm deep ( I don't yet know what amount of fall the existing pad has) then concreting to the top of this shuttering so as to easily be able to use a scraper bar to fill to level. Then a DPC covering the whole thing and build on a wooden sole plate as usual.

My hesitation with this idea is: would the loadbearing edges be strong enough if some parts of the 'table' were pretty thin?

Jameshow's suggestion attracts for not having to lay bricks but I'm not too keen on a wooden sole plate so low down - but combined with Jacob's plastic beams (which I was unaware of) might be the way to go.

Lots of food for thought and research. Thanks for the responses.

 

[Spectric]

Two questions, what are you going to use this shed for and what are you intending to put in it ?

No one has questioned the integrity of the existing base and if it was for chicken sheds then it might be lightweight and if this is so then you would be better breaking it up as hardcore and laying a new base.

 

[Jacob]

Thanks everyone for taking the time to reply. I realise my question could have been better worded. I do indeed intend to float a fresh concrete floor on top of the existing pad, which has been in existence for at least 60 years. It was not expertly laid originally, not being perfectly flat and level, but was crudely overbuilt and has stood the test of time until now and I'm confident it will suffice for my project.

My query really centred around whether a waterproof membrane simply laid on the existing pad would be good enough or whether it would need elevating first in order to resist the wet. The first reply by Lons seems to have answered this and I can imagine the only tricky part is dealing with the corners where the DPC sheet would have to be folded in a sort of 'bed sheet corners'. Or maybe cut and patched?

My alternative thoughts had focussed on shuttering the entire new floor area to a minimum necessary to achieve a level rectangle - probably about 30 to 50 mm deep ( I don't yet know what amount of fall the existing pad has) then concreting to the top of this shuttering so as to easily be able to use a scraper bar to fill to level. Then a DPC covering the whole thing and build on a wooden sole plate as usual.

My hesitation with this idea is: would the loadbearing edges be strong enough if some parts of the 'table' were pretty thin?

Jameshow's suggestion attracts for not having to lay bricks but I'm not too keen on a wooden sole plate so low down - but combined with Jacob's plastic beams (which I was unaware of) might be the way to go.

Lots of food for thought and research. Thanks for the responses.

Plastic beam idea not cheap but saves a lot of work and 100% impermeable.

 

[Lons]

I can imagine the only tricky part is dealing with the corners where the DPC sheet would have to be folded in a sort of 'bed sheet corners'. Or maybe cut and patched?

If you go down this route don't cut the DPM as it defeats the object, just fold in the corners as tight as you can, whatever you clad the insid eof the shed with will cover it.

 

[ChippyKlutz]

Two questions, what are you going to use this shed for and what are you intending to put in it ?

No one has questioned the integrity of the existing base and if it was for chicken sheds then it might be lightweight and if this is so then you would be better breaking it up as hardcore and laying a new base.

Fair comment but the original chicken shed was a commercial enterprise and much larger than my current use. I have had to break up large areas of it and I know from painful experience how hard it was so I have no doubt that it's up to my needs, which are just general storage and don't involve heavy stuff.

 

[ChippyKlutz]

Plastic beam idea not cheap but saves a lot of work and 100% impermeable.

Yes, I intend to look into this idea in due course, thanks.

 

[ChippyKlutz]

If you go down this route don't cut the DPM as it defeats the object, just fold in the corners as tight as you can, whatever you clad the insid eof the shed with will cover it.

OK got it, thanks