joining damp proof membranes

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OK, another conservatory question:

Due to the shape of the conservatory, I need a dpm which is bigger than the sizes any of the DIY places stock. So what I want to know is - can I join two sections of dpm together, to enable me to cover the full floor area? Or is this going to let damp through?

Also, where the dpm comes up against the house wall, does it have to be physically chased into the house wall so it joins or overlaps the house dpc?
 
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Generally, DPM is used to provide a waterproof barrier beneath a concrete slab. In this case, the DPM sheets should have a lap of at least 150mm. When putting sheets together, you normally find that you have enough to provide more than 150mm lap - 300mm is best. Having said that, the construction of the sub base to go under the slab means that water will drain away very well and the material is doing what it says in the tin - its a DAMP proof membrane. (If you want to "tank" a structure then DPM is not recommended and you will need to ask your architect and or the builder or plasterer).

You will need to stick the sheets together otherwise the concrete will get between the sheets and bang goes the barrier against water ingress.

The DPM is brought up and over the blocks of the inner leaf. The DPC (100mm wide) is then laid over the top. The two membranes do not need sticking together here - the weight of the blocks will ensure a good contact and prevent water from seeping above the Damp Proof Course.
Surfaces must be clean and dry. Adjacent sheets must be overlapped by a minimum of 150mm. Visqueen Double Sided Jointing Tape should be used to join adjacent DPM sheets. The sheets should then be sealed by using 100mm wide Visqueen Girth Jointing Tape. Where the sheets have been perforated they should be patched with sheets of identical thickness lapped at least 150mm beyond the limits of the puncture and sealed with Visqueen Double Sided Jointing Tape and Visqueen Girth Jointing Tape as per standard lapping instructions above.
 
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