4m x 3m outbuilding solid floor dpm dpc lapping question

[wavetrain]

I know how to lap a dpm into a cavity wall on a new build, but is there a chance of damp proofing failure with a single leaf wall?

The idea is to build an non habital dampproof single story outbuilding for storage purposes.

I would plan to lay the edges of the dpm over the top of the brick dpc level but trim the dpm a little short of the full brick so only the black dpc edge would be visible from outside. But this cosmetic trimming could allow water to creep round and apear inside where the wall meets the floor concrete finish.

Is this likely to occur, and if so is there another way to lap the dpm in single leaf construction?

Also, on London clay here, do you think 1 meter depth strip footings would be Ok, and if so what depth and width strip would be suitable? Don't really fancy going for trench fill footings as the concrete cost will be significant.

 

[^woody^]

There is no issue with your method. But you should also lay a proper DPC and not rely on the DPM membrane for this function as well.

1m should be OK for clay, as long as there is nothing to cause ground water content to vary dramatically over the seasons - like trees

However, London clays have a high/very high potential for volume changes when wet/dry, so it may be worth checking to see what others have done with extensions locally, or have a word with the BCO for your area who will know of any issues

 

[wavetrain]

Thanks for that info.

This will be an *interesting* task in putting the foundations down as currently contained within the site is a mature full height apple tree and all the ground has been worked for decades as a large garden bed.

I have thought of a raft conrete foundation going down to the required depth at the edges perhaps 1 metre. The top of the raft will be well below final ground level. It could be renforced with steel, but I will need to check if steel is OK in concrete that is below ground level...

Once brickwork to DPC is layed, layer of suitable sand then DPM then 50mm of concrete, then bring the height up by bedding in a load of used thermolite blocks which may be available if a neighbour demolishes a block made structure this Spring. Then finsh off with another 100mm of concrete. The thermolight blocks save on infill costs since they would be free and also would provide some insualtion.

This way the floor is linked to the footings.

BTW do BCOs provide free advice when a proposal is not subject to building control?