garden shed concrete slab DPC or not

[Matt@]

not for working in but for the mower, garden tools and a bit of garden storage, I'm about to lay a concrete slab for a 7ft by 7ft timber garden shed and am having some reservations about the slab construction!

Basically what I've done is dig down about 5" to compact soil, stuck 6" gravel boards around the edge all nice and level then part filled with hardcore consisting misc rubble to a rough depth of 3"to 4" , broken paving slabs all broken down to a max size of a cricket ball (many are smaller) . I've compacted this down as best I can by hand (sledge and tamper) but the pieces are quite big so lots of voids.

I am now intending to mix up concrete and pour in but my main worry is the water from the mix draining through the hardcore. I've read that people put in a DPC to stop precisely this but I havnt done so. I'm not worried about damp rising through the floor as shed will sit on tanalised bearers. Should I be re thinking this and getting some sand in there and a DPC? I really dont want to as I would have to re do the work and add expense.

So to summarise - without a DPC will the water from the mix drain through the hardcore and make the slab weak or am I worried about nothing?

thanks in advance

 

[RobinBHM]

I dont a dpm is needed, you would also need sand blinding.

Have you got enough space for about 100mm of concrete?

 

[morturn]

It’s not a problem, cement only needs a small amount of water to hydrate, the rest is to make the mix workable and compatible.
Once its set, then keep it damp for a time, ideally a month to cure. It’s quite strong after a week, so don’t be too worried.

I cannot quite work out from what you’re saying about the thickness, but aim for at least 5” thickness of concrete, otherwise it may distort. Also try to get it down, laid and tamped reasonably quickly. Trowel it off once it has gone into its initial stiffness stage.

 

[Matt@]

@ Robin, yes I would need sand as well as you say but only if using a DPC?

re. depth of concrete - theres alot of voids so I'm basing materials on 100 to 125mm of concrete as an average once it runs into the gaps. ie in some places 125mm in others 50mm.

I'm a bit worried I'm getting this wrong! got a ton of ballast arriving on the lawn tomorrow then off to get 9 bags of cement!

 

[Robbo3]

Matt, I don't know where you got your info from but you seem to be laying a base suitable for vehicle traffic.
Even the poorest slab in concrete is going to be more than adequate for a storage shed. All it needs to be is flat.
I've laid a few slabs in my time & I've never heard of preventing the water soaking away in fact probably the reverse. When laying foundations in trenches the ready mixed concrete lorry driver will often offer to add extra water so that you get a runny mix that travels on its own & self levels. Not saying that's the right way to do it for every circumstance but it worked for me.

 

[John15]

For higher spec slabs it is good practice to prevent the loss of cement and free water into the sub-base voids before the mix sets by placing a plastic membrane or concrete blinding but for a garden shed base it wouldn't be necessary.

John

 

[Matt@]

some great replies thanks looks like I'm on the right track then after all!

 

[Lons]

Overkill imo for a small 7 x 7 shed to keep a mower and garden tools. You could park a car on 100 concrete, in fact many post war semis had driveways of 2 lines of 3 x 2 conc flags laid on hardcore and a rough sand / cement base.

Many thousands of garden sheds are sitting quite happily on paving slabs, including the one I keep my garden tractor in and will do for very many years to come.

BTW a dpm isn't necessary in this case as the shed is off the ground though wouldn't hurt however, it shouldn't be laid straight on top of the harcore without blinding first or it will quite likely puncture which rather defeats the object.

Bob

 

[Beau]

For higher spec slabs it is good practice to prevent the loss of cement and free water into the sub-base voids before the mix sets by placing a plastic membrane or concrete blinding but for a garden shed base it wouldn't be necessary.

John

Yep this is what a building control officer told me. His view was good concrete was pretty much waterproof on it's own but the DPM would let it dry slowly and so stronger. For a shed I would just use some paving slabs and timber bearers.

 

[Lons]

Yep this is what a building control officer told me. His view was good concrete was pretty much waterproof on it's own but the DPM would let it dry slowly and so stronger. For a shed I would just use some paving slabs and timber bearers.

It surprises me that a B.I. would say that as it isn't the case and is why it's part of building regs to provide a DPM in order to stop rising damp. Depends I guess what he means by "good" concrete. :roll: You can specify waterproof concrete however that has additives and is a very different material.
A dpm does of course hold the water longer and therefore does help it dry out more slowly but that is definitely not the main purpose.

Normal concrete as previously said contains excess water without which it would be very difficult to lay and as it cures the water evaporates which leaves a kind of open cell structure in the slab. These are very fine but allow moisture to draw up through the slab. It's a bit like a non compressible sponge really!


In my former life as a builder I came across a few dim B.I.s.. Taught a few as well.

Bob

 

[Beau]

Yep this is what a building control officer told me. His view was good concrete was pretty much waterproof on it's own but the DPM would let it dry slowly and so stronger. For a shed I would just use some paving slabs and timber bearers.

It surprises me that a B.I. would say that as it isn't the case and is why it's part of building regs to provide a DPM in order to stop rising damp. Depends I guess what he means by "good" concrete. :roll: You can specify waterproof concrete however that has additives and is a very different material.
A dpm does of course hold the water longer and therefore does help it dry out more slowly but that is definitely not the main purpose.

Normal concrete as previously said contains excess water without which it would be very difficult to lay and as it cures the water evaporates which leaves a kind of open cell structure in the slab. These are very fine but allow moisture to draw up through the slab. It's a bit like a non compressible sponge really!


In my former life as a builder I came across a few dim B.I.s.. Taught a few as well.

Bob

Admit to being sceptical when I was told this but it was for a slab that had a second DPM laid over the top but not sure he knew that at the time.

 

[flying haggis]

keep the concrete moist for a couple of days ie water it if it looks dry and this will stop it drying too quickly, when I built my workshop I put lengths of dpm under the bearers only fastened at each end so as not to puncture the dpm under the bearer

 

[RobinBHM]

Paving slabs actually make for a good base because although they are often just laid straight onto top soil, or a layer of sand, they float on the surface and the bearers of a shed only create a small load to each slab.

If you are going to the effort of laying a concrete slab it does need to be of reasonable thickness or it is likely to crack. I dont know what the min thickness but Id guess 2" is not enough and more like 3" plus is good. I guess a concrete base is less likely to move over time.

 

[HappyHacker]

About 35 years ago I laid about 3-4 inches of concrete on top of a sandy subsoil after I had taken the topsoil off the top. It has now cracked but only where I have had 20 ton trailers and cement lories on it, it has taken cars, 7.5 ton lorries and tractors without any problems.

Without a DPC the slab will take up water from the soil but this would only be a problem if you were using it as a floor of a building. Use DPC strips under the bearers and you should be ok for many years.

 

[Matt@]

thanks for all replies and advice ... I'm doing this job tomorrow and not looking forward to it! wish I'd just done slabs but the shed is going near to the side of the house and the ground is raised (not by me) almost to the DPC and a slab works better as I can allow for a trench with chippings between it and the house wall. Also I tend to do most things belt and braces! First time ever with a cement mixer tomorrow too so got my work cut out :roll:

 

[angelboy]

How did you get on with the slab?

Takes a lot of effort to mix up and get it down but once it's done it's usually pretty satisfying that you've created a solid base that should last for decades!

 

[Matt@]

How did you get on with the slab?

Takes a lot of effort to mix up and get it down but once it's done it's usually pretty satisfying that you've created a solid base that should last for decades!

it went great thanks backbreaking work! 850kg ballast, 6 x 25kg bags of cement, approx 10 mixes - slab down and rock hard. Hired a mixer for £17. My first ever big concreting job and first time using a mixer and no issues at all. The only thing I would say is the ballast is not sold as weighing a specific weight and its often known as "a ton" yet its actually 850kg, I used the last bits of it and just about had enough.

 

[Lons]

How did you get on with the slab?

Takes a lot of effort to mix up and get it down but once it's done it's usually pretty satisfying that you've created a solid base that should last for decades!

it went great thanks backbreaking work! 850kg ballast, 6 x 25kg bags of cement, approx 10 mixes - slab down and rock hard. Hired a mixer for £17. My first ever big concreting job and first time using a mixer and no issues at all. The only thing I would say is the ballast is not sold as weighing a specific weight and its often known as "a ton" yet its actually 850kg, I used the last bits of it and just about had enough.

Merchants shouldn't these days refer to bags as "tonnes" as they are now sold as bulk bags and different materiels have different weights for the same volume. old habits die hard though!

Bet you're very pleased with your efforts and is hard work if no-one to help though to a seasoned ex builder like me 10 mixes is just a little job. I used 3 sand / cement mixes the other day building a couple of pillars :lol: That reminds me, I really must sell one of the mixers, certainly don't need 2 in retirement. #-o

Bob

 

[John15]

Merchants shouldn't these days refer to bags as "tonnes" as they are now sold as bulk bags and different materiels have different weights for the same volume. old habits die hard though!
Bob[/quote]

Variable amounts of water also.

John

 

[Matt@]

Lons thanks, I'm just a lightweight I know! my excuse I'm late 50s. Another thing is I measured it all into buckets so double handled it. Had another slab down a few yrs back using contractors and it measured 20ft x 10 ft and they mixed it on the drive and barrowed down the side of the house to rear garden - young fit and motivated though....