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- 9 Sep 2025
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Hi there,
I would appreciate some advice regarding my situation with an old uninsulated garage shed base that is otherwise sound and looking to pour a new slab next to it in order to build or buy and put an insulated garden room for year round use. There is existing blockwork around the perimeter for a double cavity block wall but this stops below the concrete base level.
I was planning to join the new slab by drilling and connecting rebar rods with resin so the new slab and old slab would be joined. I have got contractors to dig down ~20cm so would have 10cm MOT type 1 under a weed membrane, and then pour the concrete over some mesh.
My question is: it always better/necessary to pour concrete on top of the DPM or can the DPM be done after? The old slab does not have DPM and is uninsulated. I would have considered breaking it all and then pouring a new slab but seems like a waste when the existing base is otherwise sound (except at the edges which I would have to address later on).
Contractor is saying we don't need a DPM till we have poured the concrete base as this is "below ground level". He does plan to leave an expansion joint so the concrete does not meet the brickwork but nothing else. If we did construct the building in future he would put dpm down and then lap over the raised blockwork and DPC which is ~2 engineering blocks above the existing perimeter blockwork.
In the pics the blue line shoes the blockwork and the red line the old base next to it
I would appreciate some advice regarding my situation with an old uninsulated garage shed base that is otherwise sound and looking to pour a new slab next to it in order to build or buy and put an insulated garden room for year round use. There is existing blockwork around the perimeter for a double cavity block wall but this stops below the concrete base level.
I was planning to join the new slab by drilling and connecting rebar rods with resin so the new slab and old slab would be joined. I have got contractors to dig down ~20cm so would have 10cm MOT type 1 under a weed membrane, and then pour the concrete over some mesh.
My question is: it always better/necessary to pour concrete on top of the DPM or can the DPM be done after? The old slab does not have DPM and is uninsulated. I would have considered breaking it all and then pouring a new slab but seems like a waste when the existing base is otherwise sound (except at the edges which I would have to address later on).
Contractor is saying we don't need a DPM till we have poured the concrete base as this is "below ground level". He does plan to leave an expansion joint so the concrete does not meet the brickwork but nothing else. If we did construct the building in future he would put dpm down and then lap over the raised blockwork and DPC which is ~2 engineering blocks above the existing perimeter blockwork.
In the pics the blue line shoes the blockwork and the red line the old base next to it
