Raised Patio that Would Swallow the Damp Proof Course
You are building the external leaf of a single-storey extension in a 300 mm cavity wall: 100 mm concrete block inner leaf, 150 mm clear cavity, 102.5 mm facing brick outer leaf. The DPC is set so that it sits 150 mm above the existing finished ground level on the front and sides, lapped to the DPM under the floor slab. The client's landscaper has now issued a revised layout showing a raised paved patio running the full 7 m of the rear elevation, built up 450 mm above existing ground, with the paving falling towards the house. That would place the finished paving 300 mm above the DPC. There is also a rear door with a level threshold and an internal finished floor level 150 mm above existing ground. The extension blockwork is already up to DPC and the patio build-up has started at one corner before anyone noticed.
1. Explain what is wrong with the proposed patio, and state what has to happen before you build the brickwork above DPC.
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The DPC is already in the right place; the patio is in the wrong place.
The DPC exists to stop groundwater and splash water from rising into the wall and into the floor. A normal requirement is that it sits at least 150 mm above finished external ground level, with the wall below it built in frost-resistant units and a frost-resistant mortar. If paving is brought up to 300 mm above the DPC, the DPC has no free edge to shed water off. Water will sit on top of the DPC, run sideways into the cavity, soak the insulation, and find the inner leaf - rising damp by another name, plus a permanently wet cavity and a hidden rot risk at the sole plate and floor build-up.
The falling-towards-the-house detail makes it worse: the run-off is directed at the wall, so even the 150 mm step to the door threshold is being asked to handle water that should never be there.
So the work above the DPC must not proceed to the rear until the detail is resolved. The realistic options are: lower the patio so its finished level is at least 150 mm below the DPC; or redesign with a drainage channel and a raised threshold detail agreed by the designer; or introduce a properly detailed guarded upstand/stepped DPC with a cavity tray and weep holes, designed and signed off, not improvised. This goes back to the designer and the client in writing, with a photograph of the started build-up.
2. The landscaper says a slot drain at the wall will solve it. What would you check before accepting that, and how does it affect your brickwork?
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A slot drain only helps if it removes water faster than the paving delivers it and if it cannot discharge into the wall build-up. So I would check four things:
- Level. A drain at the base of the wall does not raise the DPC. If paving is still above the DPC line, the DPC edge is still buried; the drain removes surface water, not the water sitting against the wall above the DPC.
- Fall and capacity. The 7 m run needs a continuous fall to an outlet, a fall direction away from the door threshold, and an outlet that is not below the DPC into the footing.
- Detail. The drain must not rely on the cavity as a soakaway, and the DPC must still be able to discharge freely above the paving level, over a drip or a projecting brick course.
- Sign-off. This is a design change, so it needs the designer's instruction in writing, not a site conversation.
For my brickwork, the practical consequences are: use the specified frost-resistant facing brick and mortar below and above the DPC, keep the cavity clean and the DPC unslumped and undamaged, and form a projecting cill/drip course or a clear drip at the base so water drips clear of the paving rather than tracking back. I would also make sure no mortar snots bridge the DPC and no cavity insulation gets saturated.