What do you do with the DPM in a concrete shed base?
Discussion
I want to build a concrete base for a garden shed, size approx 5m x 3.5m (16'6 x 11'6). The shed will it on timber beams, and have an insulated wood floor, so no standing (or seeing) directly the concrete base.
From reading up in numerous places on the web, it seems that I can get away with digging a 10cm deep hole in my garden (turf) and pouring in concrete and tamp it down. This leaves me with a concrete base level with the garden, which is what I need rather than a raised base. I'm trying to avoid the time/hassle/cost of laying subbase first, as I reckon my ground is solid enough, and ultimately there will be no heavy weights on the base.
Many places also recommend laying a DPM. This is fine, but what I dont understand is that if I lay a DPM sheet, then it will fold up the sides of the concrete and stick up in the air. Presumably then any rain/water can just run down the side and ultimately pool between the DPM and the concrete slab. This surely defeats the point of the DPM?
How do you "finish" the DPM edge?
From reading up in numerous places on the web, it seems that I can get away with digging a 10cm deep hole in my garden (turf) and pouring in concrete and tamp it down. This leaves me with a concrete base level with the garden, which is what I need rather than a raised base. I'm trying to avoid the time/hassle/cost of laying subbase first, as I reckon my ground is solid enough, and ultimately there will be no heavy weights on the base.
Many places also recommend laying a DPM. This is fine, but what I dont understand is that if I lay a DPM sheet, then it will fold up the sides of the concrete and stick up in the air. Presumably then any rain/water can just run down the side and ultimately pool between the DPM and the concrete slab. This surely defeats the point of the DPM?
How do you "finish" the DPM edge?
You put plastic underneath concrete in these sort of situations to stop the ground sucking the water out of the cement mix causing it to go off to quickly and then probably crack or be crumbly. It doesn't provide damp proofing as such.
If you're bothered about it I'd just peel it back to the bottom level of the concrete base once it has set and trim it off with a Stanley knife.
Water is unlikely to pool between the DPM and concrete as several tonnes of the stuff on top of it will give a pretty tight seal. Under that it will just soak into the soil.
If you're bothered about it I'd just peel it back to the bottom level of the concrete base once it has set and trim it off with a Stanley knife.
Water is unlikely to pool between the DPM and concrete as several tonnes of the stuff on top of it will give a pretty tight seal. Under that it will just soak into the soil.
Edited by C0ffin D0dger on Wednesday 25th March 16:01
For your application I wouldn't bother with a dpm at all. Finish the concrete level with your lawn, then rest the timber beams of the hut on some blue engineering bricks: These won't suck up moisture and will allow a little airflow under the hut. A lot of times, a dpm is used under concrete slabs not to stop any moisture coming through it long term but rather to slow down its curing when it's being laid, by stopping the ground sucking all the water out of it. This helps it to reach maximum strength, which for a shed base is hardly critical.
kryten22uk said:
I presume that assuming I can dig a hole with stable vertical sides, then I dont need any wood plank shuttering either?
Yes, you could omit the shuttering doing it that way: If you rest the timbers on a blue brick, you can bed the bricks on mortar and use them to take out any discrepancy in the concrete (if you plan on slinging the latter in the hole and trowelling it up with the back of a shovel) 
Renovation said:
If you're not using the concrete as the floor - why bother with it at all ?
All my sheds have had timber battens either directly on the ground or on slabs / blocks simply placed on the topsoil.
No issues after 5-8yrs
Dunno really. Everywhere seems to tell you to lay concrete base.All my sheds have had timber battens either directly on the ground or on slabs / blocks simply placed on the topsoil.
No issues after 5-8yrs
kryten22uk said:
I presume that assuming I can dig a hole with stable vertical sides, then I dont need any wood plank shuttering either?
I did something similar. So yes you could do without, but I used the shuttering to get my levels and will ensure the slab is flat. I would have thought it would be easier to tamp down across the slab with a bit of '2 by 4' if you have the shuttering on each side.kryten22uk said:
Renovation said:
If you're not using the concrete as the floor - why bother with it at all ?
All my sheds have had timber battens either directly on the ground or on slabs / blocks simply placed on the topsoil.
No issues after 5-8yrs
Dunno really. Everywhere seems to tell you to lay concrete base.All my sheds have had timber battens either directly on the ground or on slabs / blocks simply placed on the topsoil.
No issues after 5-8yrs
I'd have placed them on blocks / slabs if the ground was wet.
I didnt "want" to use concrete, I just thought it was what I was meant to do.
So I looked into timber frames, and it looks sensible enough. I'm keen to maintain extra height over the topsoil though, as I'm looking at fitting celotex insulation foam between the floor joists, so need to raise the joists to get extra room for airflow over a wet soil base.
I noticed that there seems to be this option on the market, which involves putting a timber frame on adjustable supports/feet:
Anyone used this? Its quite cheap (if you buy it direct, not from the shed retailers), and would be an easy answer to ensuring a perfectly level base on which to build the logcabin.
Maybe actually I put the logcabin floor joists directly onto these feet, and not double up with a separate frame underneath.
So I looked into timber frames, and it looks sensible enough. I'm keen to maintain extra height over the topsoil though, as I'm looking at fitting celotex insulation foam between the floor joists, so need to raise the joists to get extra room for airflow over a wet soil base.
I noticed that there seems to be this option on the market, which involves putting a timber frame on adjustable supports/feet:
Anyone used this? Its quite cheap (if you buy it direct, not from the shed retailers), and would be an easy answer to ensuring a perfectly level base on which to build the logcabin.
Maybe actually I put the logcabin floor joists directly onto these feet, and not double up with a separate frame underneath.
Edited by kryten22uk on Thursday 26th March 08:52
kryten22uk said:
I didnt "want" to use concrete, I just thought it was what I was meant to do.
So I looked into timber frames, and it looks sensible enough. I'm keen to maintain extra height over the topsoil though, as I'm looking at fitting celotex insulation foam between the floor joists, so need to raise the joists to get extra room for airflow over a wet soil base.
I noticed that there seems to be this option on the market, which involves putting a timber frame on adjustable supports/feet:
Anyone used this? Its quite cheap (if you buy it direct, not from the shed retailers), and would be an easy answer to ensuring a perfectly level base on which to build the logcabin.
Maybe actually I put the logcabin floor joists directly onto these feet, and not double up with a separate frame underneath.
Oooh that looks easy..So I looked into timber frames, and it looks sensible enough. I'm keen to maintain extra height over the topsoil though, as I'm looking at fitting celotex insulation foam between the floor joists, so need to raise the joists to get extra room for airflow over a wet soil base.
I noticed that there seems to be this option on the market, which involves putting a timber frame on adjustable supports/feet:
Anyone used this? Its quite cheap (if you buy it direct, not from the shed retailers), and would be an easy answer to ensuring a perfectly level base on which to build the logcabin.
Maybe actually I put the logcabin floor joists directly onto these feet, and not double up with a separate frame underneath.
Edited by kryten22uk on Thursday 26th March 08:52
Any links to a supplier?
I've seen timber frames with feet on concrete slabs, and also seen the plastic mesh tile things.
Also concrete piles with timber bolted to it.
By far that looks like the easiest to install/DIY, keeps the timber frame well clear of the ground, and the plastic will last forever.
I've just made a timber shed base (although only 10'x6'). I bought treated 3"x2" for the frame, laid it onto a levelled bit of the garden and coach bolted some angle iron to it in each corner. The angle iron is postcreted in and I will have a dpc on top of the whole thing to stop moisture soaking into the shed itself.
Don't know if it's right or wrong, but it seems about right! Total cost about £60
Don't know if it's right or wrong, but it seems about right! Total cost about £60
ILoveMondeo said:
Oooh that looks easy..
Any links to a supplier?
There are loads of suppliers - just search "adjustable timber decking support". They only cost about £3 per foot. You can sit the feet on anything really, either directly on soil, on hardcore, on individual slabs or individual plastic mesh squares like in the picture. The benefit is that you dont need to prepare the whole base area, only the square on which the feet will sit.Any links to a supplier?
That picture is actually from DunsterHouse, who want £500 for the footing solution!
kryten22uk said:
There are loads of suppliers - just search "adjustable timber decking support". They only cost about £3 per foot. You can sit the feet on anything really, either directly on soil, on hardcore, on individual slabs or individual plastic mesh squares like in the picture. The benefit is that you dont need to prepare the whole base area, only the square on which the feet will sit.
That picture is actually from DunsterHouse, who want £500 for the footing solution!
Cool, thank you!That picture is actually from DunsterHouse, who want £500 for the footing solution!
Hope you don't mind me thread jacking OP but this thread came the day before I was about to have 3 ton of ballast and 25 bags of cement delivered to build a shed base for a 13' x 10' metal shed (and some other small projects).
Based on this thread, I'm thinking about flattening out the location for the shed, building a frame from 4x2's and creating a chipboard floor and pinning the shed to the base. DPM's are fairly inexpensive so I'll sit the frame on the DPM and I'm done.
To increase the security on the shed, I was thinking about lining it with chipboard or ply.
Thoughts?
Based on this thread, I'm thinking about flattening out the location for the shed, building a frame from 4x2's and creating a chipboard floor and pinning the shed to the base. DPM's are fairly inexpensive so I'll sit the frame on the DPM and I'm done.
To increase the security on the shed, I was thinking about lining it with chipboard or ply.
Thoughts?
I did a lot of research into this topic and eventually used 'mini plinths' from Swift Foundations (other manufacturers are available). These are formed of a concrete pad that sit on a 500x500mm plastic mesh base - the subframe fits on steel brackets that fit into the pads. They are not cheap - about £70 each - in fact I think in many situations it would be cheaper to do a concrete base, but they are a lot more convenient.
I was doing a building about the same dimension as the OPs, but it was pretty substantial.
If I was doing a lighter shed I think I would look at using the plastic 'megapads' mentioned above in combination with the plastic grids that come with the mini-plinths - the grids would be to spread the load and provide some stability. They are are available separately (the swift ones are called 'ecobase'). These megapads are really designed to suspend decking and paving slabs, but I don't see why they wouldn't work for a small shed - the heavy duty ones are claimed to have a bearing capacity of 800kg and at a tenner each you could use a few and not break the bank.
You can just put the subframe direct on the plastic grids, which in theory is better than laying direct onto concrete or slabs since it is less likely that water will pool underneath the timber, but I would rather raise the frame of the ground.
Someone will be along shortly to say that leaving a void under the building like this will make it a haven for rats, foxes, students, homeless people etc, but I still think it is the best approach.
I was doing a building about the same dimension as the OPs, but it was pretty substantial.
If I was doing a lighter shed I think I would look at using the plastic 'megapads' mentioned above in combination with the plastic grids that come with the mini-plinths - the grids would be to spread the load and provide some stability. They are are available separately (the swift ones are called 'ecobase'). These megapads are really designed to suspend decking and paving slabs, but I don't see why they wouldn't work for a small shed - the heavy duty ones are claimed to have a bearing capacity of 800kg and at a tenner each you could use a few and not break the bank.
You can just put the subframe direct on the plastic grids, which in theory is better than laying direct onto concrete or slabs since it is less likely that water will pool underneath the timber, but I would rather raise the frame of the ground.
Someone will be along shortly to say that leaving a void under the building like this will make it a haven for rats, foxes, students, homeless people etc, but I still think it is the best approach.
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