Damp Proof Course Level Against a Door Threshold
You are building the external leaf of a cavity wall to a garage conversion. The drawings show a dpc at 150 mm above external ground level, a door opening with a 100 mm deep pre-cast threshold, and a finished floor level 150 mm above external ground. The concrete slab has already been poured 20 mm higher than drawn. The threshold is on site and is 100 mm deep. If you keep the dpc at the drawn level, the top of the threshold will sit below the finished floor, and the door will not clear the floor. The site agent suggests raising the dpc by one course of bricks to fix it.
1. Identify the conflict and explain the damp-proofing principles that constrain the solution.
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The conflict is between three fixed levels: external ground, the dpc, and finished floor level, plus the physical depth of the threshold.
Principles that constrain the answer:
- The dpc must be at least 150 mm above external ground level to stop rain splash bridging it.
- The dpc must sit below the level of the internal floor so that any water on the dpc can escape outward, not inward.
- The threshold must sit so the door clears the finished floor and so water runs off outward, not into the building.
- The slab is already 20 mm high, which eats into the available upstand.
Raising the dpc by a full course (about 75 mm with a 65 mm brick and 10 mm joint) would lift the dpc and could put the threshold above the floor, but it also raises the dpc relative to the slab and may trap water. It is not automatically a solution; the levels must be re-checked as a system.
2. Evaluate the agent's proposal and set out a safer sequence of options.
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Evaluate the proposal: raising the dpc by one course does not change the slab level or the threshold depth. If the threshold top must be above the slab and the dpc below the slab, the available zone is fixed. Lifting the dpc by 75 mm may push it above the slab, so any water reaching the dpc cannot drain outward over it - it can track inward. It also reduces the 150 mm upstand above ground only if ground is raised to match, which is not proposed.
Safer options, in order of preference:
- Lower the threshold by forming a recess or using a thinner threshold, so the door clears the slab without moving the dpc.
- Reduce the slab locally at the door, or form a ramped upstand, so finished floor and threshold levels work.
- If levels must change, raise the external ground and dpc together, keeping the 150 mm upstand and the dpc below the internal floor, and get the designer to confirm.
- Only after the levels are agreed, set out the brickwork to suit.
Record the decision and get it in writing before laying the affected courses.