DPC in cavity wall

[Olivier1981]

Hi everyone,

I know there are many threads covering DPC but I could't find my answer. I am building an extension with an internal suspended timber floor. I poured a concrete cover on sand and Polyethylene DPM which I will overlap the DPC and then continue by sloping downward to the outer leaf helping evacuation of water through weep vents.

1. Can the DPC on the inner leaf of a cavity wall be at a different height than the DPC on the outer leaf ? That would allow me to fit my air bricks and timber floor lower

2. I have seen in some student books and on line that the air bricks have to be fitted below the DPC. But it sounds silly to me because the DPM has to join the inner leaf DPC creating a continuing vertical barrier from the concrete cover to the DPC level, then technically I would have to go through the vertical DPM with the air vents and it seems to me that the DPM should not be perforated at any point.

Can anybody help me on this ?

Other remarks and advice are welcome

Olivier

 

[^woody^]

Why have you fitted a DPM with a timber floor? There's no need

 

[Olivier1981]

Why have you fitted a DPM with a timber floor? There's no need

that's what it says in the regs for suspended timber floors :

"concrete cover at least 50mm thick laid on at least 300μm (1200 gauge) polyethylene sheet with sealed joints, and itself laid on a bed of material which will not damage the sheet."

I'm not a pro so I go by the books.

 

[r896neo]

I think you are mixing up concrete and suspended floor detailing or perhaps have a misleading book.

A concrete floor should be laid on a dpm which is lapped into the dpc in the inner leaf.

A suspended timber floor can have a dpm under the oversite concrete but it certainly wouldn't be lapped into the wall dpc. How would that be possible with many sleeper walls etc? All sleeper walls would have there own dpc along with the inner leaf of the cavity wall.

All wall plates then also laid on dpc.

 

[Olivier1981]

I am 100% sure for the polyethylene sheet under the concrete ground cover, it is in the building regs Part C but you are right, it is not a DPM per se, maybe it doesn't have to overlap the DPC as for a concrete floor.

 

[freddiemercurystwin]

This is not preventing the timber floor getting damp, that's what your dpc is doing, the dpm you are talking about is just preventing some moisture being soaked into the oversite from the ground.

Untitled

• freddiemercurystwin
• 12 Oct 2012
• 1

 

[Olivier1981]

I will squeeze it between 2 blocks at a lower level than the DPC then. It will act as a cavity tray and I will put my air blocks on the course above this.

 

[Olivier1981]

Thanks for the drawing