Internal floor level below ground level

[jamse]

Hello,

I am in the process of building a block-built shed / workshop. It will measure about 7m x 4m, so is below the size at which Building Regs are needed, and I will be keeping the height below 2.5m so Planning Permission is not needed.

Although it's not intended to be habitable space, I want the space to be nice enough that I can usefully use it in the winter, so it is pretty over-specced for a shed. I am planning on building it with a double skin of block work walls, so am digging trench footings 600mm wide, and deep enough to hit our gravel sub-soil (about 1.5m deep). I also have quite a lot of stuff that I want to store in it, so I want to keep it nice and dry.

My question is, can I gain a bit of extra storage height by having the internal floor a step or two down from the ground level outside? I am not talking about full cellar territory here, I just thought it would be handy to have it maybe 300mm or 500mm lower inside. The site is quite level.

I have had a good trawl of the web, and I haven't been able to find a reason why I shouldn't do this, but then again I haven't found anyone saying that it can be done! All of the helpful diagrams showing you how to arrange your layers (like the one halfway down this link) seem to assume your inside floor level will be about the same as outside ground, if not a bit higher.

I am happy to make a pucker job of this, so for the internal floor I intend to put in a layer of hardcore, blind it with sand, then a DPM, then some insulation, then the concrete floor slab. So, I just wanted to get a view from the knowledgeable folks on here about whether I am likely to come a cropper with any part of this plan.

Thanks in advance,
Jamse

PS - I have put this in the building section, as I figured it's more like building work than putting up a wooden shed, but if the moderators feel that this belongs in the garden section of the forum, please feel free to move it.

 

[stuart45]

It's normally best to have your inside floor level higher than the external ground level.
You can get problems if the water table gets close to the surface and starts pushing up against the floor.
I've just put a French drain in for someone with this problem.

 

[pinenot]

You need to establish the ground water level for your property, once this is done the build method can be established.
If all is above ground water level reasonable waterproofing is all that should be required. On the other hand, if you are hitting, or below ground water level, then you are into some serious tanking and not only that you will require some form of damn to ensure water free foundation workings... choices, choices.

Good luck...pinenot

 

[jamse]

Thanks for your quick responses. I have established the depth of the water table, if that's the same as the ground water level Pinenot refers to - it's about 1.7m down (below the depth where I hit solid surface most of the way round the trench. Even allowing for a bit of variation in the height of the water table with the seasons, I suspect that the sort of drop that I am considering will be well clear of that. If the finished floor level is, say, 0.5m below ground, and all of the concrete slab, insulation, DPM etc. comes to around 0.3m, then the bottom of all of those layers is still going to be getting on for a metre above where the water table is today.

So, from Pinenot's description it sounds like I should be safe to just use "reasonable waterproofing". What would that look like? I'm going to have a DPM under the main concrete floor slab - should I be bringing that up the inside of the wall to go into the same course where the DPC is? Should I be using a particular type of blocks that has low water permeability?

Any tips or further thoughts very gratefully received!

 

[pinenot]

Use a good quality dpm, ensure there are no tears or un-taped joints. Take it up the wall far enough to allow it to be dressed over the dpc of the outer wall skin - below the dpc (that's if it's cavity) this will effectively form a bund/tank. everything else is as normal build...seemples.

Good luck...pinenot

 

[jamse]

Thanks pinenot - that's great. Apologies for asking very basic questions, but I want to get this right first time, so here goes:

How do I spot a good quality DPM? Is there particular brands that have a good reputation, or a particular thickness, or some other specification?

Yes, I am planning on a cavity wall. From the picture I linked to earlier, it looks like the cavity should be filled with weak concrete to ground level, and that image only shows the DPM going as far as the inner skin.

When you say "dressed over the dpc of the outer wall skin", do you mean that the DPM should come up the inner wall to DPC level, across the inner skin, across the cavity, and into the DPC level of the outer skin?

You say "dressed over the dpc" and "below the dpc" - sorry for being particularly dense on this - should I be aiming for the DPM to be on top of or underneath the DPC?

Thanks again for all your help with this.

 

[stuart45]

Thanks pinenot - that's great. Apologies for asking very basic questions, but I want to get this right first time, so here goes:



When you say "dressed over the dpc of the outer wall skin", do you mean that the DPM should come up the inner wall to DPC level, across the inner skin, across the cavity, and into the DPC level of the outer skin?



Thanks again for all your help with this.

I would guess he meant that. You should also put in a tray above the DPC on the external wall to stop any moisture coming down the cavity.

 

[jamse]

Thanks for that. I've not come across trays before. From looking at them online, can I form one by getting a wide strip of DPC material and putting it in one or two courses of blocks higher on the inner wall than the outer wall?

 

[stuart45]

Thanks for that. I've not come across trays before. From looking at them online, can I form one by getting a wide strip of DPC material and putting it in one or two courses of blocks higher on the inner wall than the outer wall?

Yes

 

[pinenot]

Good dpm - 1200 micron or thicker polythene (the thicker the less flexible in corners and the like) Don't think you need cavity trays in this instance, just fold the dpm over both block leaves, which should be mortor/concrete filled. After that lay your dpc and start your cavity build.

Are you intending to insulate the cavity?...pinenot

 

[stuart45]

Don't think you need cavity trays in this instance, pinenot

The tray protects from any moisture penetrating the external skin higher up and running down the cavity and entering inside where the inside wall is below ground level.

Untitled

• stuart45
• 5 Mar 2013
• 1

 

[jamse]

I had vaguely intended to use insulation in the cavity, but had not really got as far as working out the implications of that - does that affect the design of the damp proofing at the earlier stages?

I can see logically why a cavity tray might be helpful here - if I go with a DPM that is extending across the cavity then any moisture that somehow gets into the cavity will hit the DPM and will have to flow either to the inner wall or the outer wall.

I have tried to make a sketch of an idea I have to get the effect of a cavity tray, by running the DPM one course higher on the inner wall than the outer wall:

Untitled

• jamse
• 5 Mar 2013
• 1

Is that a completely daft idea?!

 

[stuart45]

What finish are you putting on the inside wall?

 

[jamse]

Good point, Stuart - I had been thinking about the same issue. I had been intending to just keep the bare blocks on the internal wall - this is intended as a shed / storage space, not as habitable space, so I don't want to go to the extra expense of plastering or similar.

So, going with how I have it in that diagram would not be ideal - it wouldn't be great to have the DPM flapping around on the bottom foot of the wall. I guess having the DPM there, even if I did plaster it, would mean that anything going through (e.g. a fixing for a shelf) would pierce the tank and create a potential point of ingress.

I would very much appreciate any other ideas people might have about how I could do this. Having thought some more about how we're likely to use this, I think it is quite important that it's dry, so we're able to store all sorts of stuff in there without risk of it getting damp and rotting.

 

[stuart45]

Your best bet may be to go with pinenot's idea and run the DPM through the inside wall and up the inside of the external wall and under the external DPC.
This is a similar idea to the traditional method of using vertical Asphalt up the wall and joining it up with an asphalt layer on the floor, which is covered with a screed.