making base for block work shed and need advice particularly on membrane

happyhero

Hi I want to make a base for a block work shed that will house tools and a workbench and wanted to stop damp coming through concrete base I am making.
I was thinking of using MOT1 layer on top of my very hard soil, about 2" thick and then a membrane and then 4" of cement with one of those grid things inside.

Does that sound ok?

Do I need to put sand on the eventually compacted MOT1 first, blind it I think they call it, or can you put a membrane straight on top of MOT1?

My shuttering is 6" so crazy question maybe but you wouldn't put 6" of cement on top of a membrane on top of the soil would you? And therefore not use any MOT1?

Any advice appreciated.

stuart45

Are you going to build the blockwork off the slab?

tacpot12

You shouldn't put a damp proof membrane (DPM) directly on top of MOT1. The material in MOT1 can have some sharp edges or points that would pierce the membrane. You will need a layer of blinding. Ordinary building sand (not sharp sand) is good for this, and you only need enough to guarantee that after its compacted nothing in the MOT1 will touch the DPM. You don't need to compact the sand, the concrete will do this for you. 

The MOT1 is a load bearing layer, and ideally will be laid slightly larger that the concrete it supports, to better distribute the load. 

Adding some A142 steel mesh will give you a very strong base, but if you are going to have something really heavy in the workshop, like a Myford lathe, A193 mesh would be advisible. You will need to support the mesh so that it is 40-50mm off the DPM. eBay is a good source of the small quantities of reinforcing mesh supports you will need.  

FreeBear

happyhero said: I was thinking of using MOT1 layer on top of my very hard soil, about 2" thick and then a membrane and then 4" of cement with one of those grid things inside.

I'd go 100-150mm of hardcore (MOT1 or crushed brick.concrete), topped off with a thin layer of concrete, say 50mm thick. Smooth off, cover with a DPM, and then another 50mm of concrete. Alternatively, put a layer of compacted sand over the top of the hardcore, followed by the DPM, and then the concrete.

Don't go building the shed off the slab - Dig foundations and do a proper job from the outset. Lay the hardcore & floor slab once the blockwork is above ground level, and you don't have to worry about shuttering.