Flat Raft Slab Damp Proofing at Wall

[vbear]

Hi All,

Bit of a DPC/DPM conundrum I've got here.

Long story short, I'm self-designing an extension which is required to go on a raft slab. I envisioned this would end up on a raft with a boot/stepped edge detail, however my structural engineer has recommended (and designed) a flat raft slab owing to the small size and footprint of the extension. This removes the need for rebar fixing on site as it's just top and bottom A393 mesh so much simpler from that perspective - happy days.

However, I'm now a little confused over the damp proofing. I've proposed the below, similar to what I believe a strip footing with the cavity filled 225mm below internal level would be. This has the DPM over the raft, lapped into the DPC in the wall. Separate DPC externally.

However, I came across a detail from the NHBC which shows a cavity tray in the wall just above the raft with a 225mm step. I'm struggling to understand what this actually achieves as surely the insulation/floor internally will be protected by the DPM going up the internal wall? Also, I didn't think you could have weeps at ground level - would have thought that would increase chance of water entering the cavity.

I'd also rather not do this as the existing ground levels around the house are 150mm below internal FFL throughout, so stepping down an extra 75mm around the extension wouldn't be ideal.

I also found this in the LABC website although this is for a stepped slab, however it does show the DPM going under the inner leaf, around the first block and lapping with the internal DPC. Just worried this may introduce a slipping risk as a smooth material but assume not?

Given I've found two details which seem to protect the inner block, I'm concerned my detail isn't actually suitable. Should I be wrapping the DPM under the internal block or using a cavity tray? Of the two I'd probably go for the LABC detail which would at least allow me to bury the lowest facing brick and maintain a consistent level around the house.

Thanks in advance,

Rob

 

[freddiemercurystwin]

LABC

 

[^woody^]

Cavities can hold water at the bottom, so in terms of design, risk and buildability a drained cavity and tray is a better option.

Weep holes are fine at ground or slightly below ground level. They also let people know in the future that there is a non standard construction, which is useful.

You should design to principles not absolutes, so it's possible to use concepts of several typical design details into a final design.

A slab can be lower than the surface, which deals with uneven ground.

 

[vbear]

Cavities can hold water at the bottom, so in terms of design, risk and buildability a drained cavity and tray is a better option.

Weep holes are fine at ground or slightly below ground level. They also let people know in the future that there is a non standard construction, which is useful.

You should design to principles not absolutes, so it's possible to use concepts of several typical design details into a final design.

A slab can be lower than the surface, which deals with uneven ground.

Thanks Woody,

I suppose my confusion was why a conventional cavity on strip footing isn't drained/require weeps 225mm below DPC when it is often filled 225mm below DPC with concrete (so would have the same void potentially retaining water).

So in the below sketch, I've got a 225mm cavity tray with weepholes sitting in the bottom course of brick - the top of this brick would be below the external ground level so the weep wouldn't be visible - is that alright? I'm proposing to single size decorative stone around the extension for ~100mm so the water will have somewhere to go . I may include a geotextile to prevent any crud from blocking these over time.

The PIR board would then sit on the top of this cavity tray - is that right or do I need a small gap? Do I need to break the board at DPC level using the DPM to prevent a possible hydraulic path?

I can see the benefit of the tray as if any water does get into the cavity, it will at least have somewhere to drain rather than filling up... I just can't have/don't want a lower ground level so if these can be below this then all is dandy.

Thanks for you help - much appreciated!