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Dee J

Construction detail ... floor level below ground level

We've got a horrible 1970s blockwork porch on our house. I'd like to replace this with something more waterprooof and better insulated. To line up floor levels with the house, the floor of the porch needs to be below gound level at one side (sloping site). Is this allowed/possible with ordinary masonary cavity wall construction? All the helpful building guide books show the dpc above ground level then the floor above the dpc. I'm not talking about a fully tanked cellar, just an 8' run of wall (5' on one side and a further 3' round the corner) where the soil level is at or just above finished floor level. Can the inner dpc be lower than outer dpc? Any good books or websites for detailing?

Thanks

Dee
Nick

I reckon you'll need Gervase's opinion.
vegplot

You could dig a trench around the porch, where the soil level is close to floor level, fit a top perforated plastic drain tube (french drain?) and cover with gravel. This should ensure that any damp is allowed to run off before it gets near the DPC.
JB

Sounds like our last place. There the solution was to excavate the ground so that the floor was above the ground level and the ground behind that then sloped more steeply, although in out case we had a retaining wall in the garden some 4 metres from the back of the house.
Dee J

Thanks for the input so far... the porch adjoins our already narrow and awkward driveway - so I can really change the soil level. What I'm looking for is something which would allow the floor to be about 9" to 1' below ground.... Othe the other side of the porch it'll be about 10" above ground. (Sloping garden and sloping driveway). Of course quite a lot of the house floor is below ground, but thats not a problem for the house dpc 'cause there isn't one... Wink . Just 3' thick rubble stone walls.

Dee
Treacodactyl

I was also going to suggest putting in a drain/gulley at the side. Have you tried talking to your council's building regs department? Even if you don't need their approval they can be quite helpful and make suggestions, especially if you catch them at a good time.
Nick

I'm assuming you're going to remove and replace the block work. If so, could you put a DPC in the new blockwork at the right height, and continue it up the outside of the wall, where the soil rests against it. I'm guessing you'd need to continue it several inches above the soil level, and disguise it some how, but would it be a solution?
Dee J

Building regs folk is probably the way to go - just don't like waking the sleeping beast of bueaucracy when I can avoid it! Thats why I spent a wet bank holiday searching all the building guides in Borders bookstore Very Happy

The best idea I can come up with is - Internal skin and floor dpc below floor level. Outer skin dpc 6" above ground level. Keep cavity fill 6" below inner dpc (cavity normally filled to outer ground level). Of course that leaves the ground push on the outer skin unsupported - problem there perhaps. Any water ingress into cavity could drain from the downhill side. Bodge? legal? Alternatives?

Dee
Treacodactyl

Dee J wrote:
Building regs folk is probably the way to go - just don't like waking the sleeping beast of bueaucracy when I can avoid it!


Same here although when I've called they didn't seem interested to know my exact address and you'll probably get away with giving them a rough location so they put you through to the officer in charge of your area.

Another possible option I've come across, although I don't know if it was ever standard practice, is the use of engineering bricks to form the dpc. So you could have the other skin, up to say a foot above the ground level, made from suitable engineering bricks.
gnome

your house was built in the seventies and is still standing? that is impressive.
Dee J

No, our house was built about 1600 - and it's still standing. It's just the porch which was built in the 1970s (and is barely standing).

Dee
Gervase

The French drain or narrow gulley approach would be my suggestion. You should also have an interceptor drain uphill of the porch. As a belt and braces you could also tank the exterior from DPC to ground level with Vandex or something similar.
Take enormous care when digging out for the floor slab not to undermine the main wall of the house - rubble-stone doesn't have foundations and is just built on top of large boulders, Any excavation near such a wall should be at a 45 degree angle to preserve the compacted soil at the base.
If you don't do that, then this is likely to be the result:

This was a house where a builder tried to put in a small extension on the left, dug straight down next to a rubble-stone wall and brought most of the building down. I've now got to mend it!
frewen

Blimey Shocked
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