Damp Proof Membrane Question

[Sappy]

Hi All
I’m after some advice if possible,
We are just starting to build a small extension and have an issue relating to ground levels on one side of the proposed extension because next doors drive slopes upwards which means the ground there is 3-4 courses higher than the internal floor level and the existing DPC level of the property.

We proposed to install the DPM from the base into the DPC as normal on the internal wall and on the inside of the external wall wrap DPM from the base of the first course of bricks up the inside and then link to DPC 150mm above the outside ground level and install a cavity tray from two courses above internally to a weep hole above the DPM externally.

We also wondered if it would be Appropriate to somehow prevent moisture rising from under the base of the internal leaf? How could we do this?

We have submitted our plan to building control and the inspector says we can’t use DPM to effectively ‘tank’ a vertical wall. He hasn’t yet offered an alternative and is now unavailable for several days which isn’t ideal as our builder is set to start laying bricks.

How can we easily resolve this?
Can anyone offer any advice or share experiences of a similar situation.
Thanks in advance
Lisa

 

[^woody^]

You need to install a cavity tray and double DPC. Your designer should have designed this, its not the building inspector's job to do your design or tell you what to do.

 

[wessex101]

Instead of DPM polythene use extra wide DPC, you can buy it up to a metre wide, it is much thicker and more durable than polythene. I would install it at the usual level to lap with the floor DPM then dress it up the cavity on the outer face of the inner skin then bring it back in to the inner skin at the same level as the external leaf DPC. Dont take it across the cavity to link with the outer leaf, it will allow damp to track across the cavity. That way you won't need a cavity tray as the cavity is clear.

 

[^woody^]

Instead of DPM polythene use extra wide DPC, you can buy it up to a metre wide, it is much thicker and more durable than polythene. I would install it at the usual level to lap with the floor DPM then dress it up the cavity on the outer face of the inner skin then bring it back in to the inner skin at the same level as the external leaf DPC. Dont take it across the cavity to link with the outer leaf, it will allow damp to track across the cavity. That way you won't need a cavity tray as the cavity is clear.

Got any Nurofen?

 

[Sappy73]

Instead of DPM polythene use extra wide DPC, you can buy it up to a metre wide, it is much thicker and more durable than polythene. I would install it at the usual level to lap with the floor DPM then dress it up the cavity on the outer face of the inner skin then bring it back in to the inner skin at the same level as the external leaf DPC. Dont take it across the cavity to link with the outer leaf, it will allow damp to track across the cavity. That way you won't need a cavity tray as the cavity is clear.

We considered this option but the inspector said it wasn’t suitable and wants the floor DPM to be tracked up the inside of the outer course fixed into place by using 100mm Quinn therm in the cavity to join the DPC on the outer leaf and also install a DPC cavity tray along the length above the highest point where they all come together..