Rendering over DPC?

[virginblue4]

Hi all,

We are having an extension built and the builders seem to be doing a good job. I just have a concern regarding bridging the DPC when we come to render. I’ve raised it with the builder and he doesn’t think it’s an issue but I wanted a 2nd opinion and everything I’ve seen online says not to bridge the DPC with render.

My builders plan is to render the extension including below the DPC. He said that due to it being the external leaf, even if it does rise up it won’t cause any issues internally. We have a partial filled cavity with Celotex boards held against the the internal leaf. I suppose technically that’s correct, unless there is cement on the wall ties which I can’t be sure of. What’s the best thing to do here?

A second issue, our suspended timber floor is roughly inline with the DPC. They’ve boarded the floor today and left a gap round the edges. I assume they will lap DPM around this? Also, with the insulation, will the air not come through the air bricks and blow both on top and below the insulation, meaning it isn’t fully performing it’s duty?

Maybe I’m over worrying about this, I’m just spending a lot of money and want to make sure it’s right. They’ve generally been great right the way through, so I’m not doubting them, but this could cause problems if done incorrectly.

Any advice would be welcome!

 

[jeds]

Rendering over the dpc is laziness. Correct method is to apply a hard thin coat render to the lower part up to the dpc > then fix a bell cast bead (this is the bit they don't want to do) at the dpc level > and then render above the bell cast bead.
The thin coat below the dpc is susceptible to rain splashing and will discolour much quicker than the main render. To avoid this it is normally painted with a bituminous paint, which can then be occasionally re-painted to keep it looking fresh.

The boards should have a gap all around. It is very important that the boards do not touch the blocks. Normal gap is about 10mm - yours looks a bit bigger than that. The gap should be sealed with compressible material.

The vents should not vent directly against the insulation. There should be a telescopic duct directing the air below the insulation. The insulation should be fitted tightly all round which should prevent draughts penetrating above.

 

[virginblue4]

Thanks for your response.

All sounds good, the only thing we don’t have is the telescopic ducting for the air bricks. I’ve had a search online and can’t seem to find anything that would suit the large clay air bricks that I’ve got. Will it cause issues that the air will be flowing directly to the insulation / joists?

 

[virginblue4]

Here is a photo from a few weeks back of the airbrick. There is no ducting, just a straight through gap. Is this ok?

 

[jeds]

The vent should be ducted below the level of the floor. This is normally done with a 65mm vent - see photo below.
The vents in your photo are 140mm vents. Off the top of my head, I can't recall ever seeing a 140mm telescopic? But I may be wrong. noseall will know, if he's about? If a tele vent isn't available the ducting can be made up with slate or similar.