Glossary
Damp-proof courses in London's older brick housing stock
A barrier preventing rising damp. New extensions must tie into existing DPC level.
A barrier preventing rising damp. New extensions must tie into existing DPC level.
A damp-proof course is a continuous impervious layer built into a wall to stop ground moisture rising through the masonry. Approved Document C requires it to sit at least 150 mm above finished external ground level and to link continuously with the damp-proof membrane in the floor.
At least 150mm above the finished external ground level. That single dimension decides more London extension details than any other in Approved Document C, because a new patio or a raised threshold can bridge an existing course and produce damp where none existed.
| Requirement | Dimension | Source |
|---|---|---|
| Damp-proof course above external ground | at least 150mm | Approved Document C |
| Cavity tray over an opening or abutment | required wherever the cavity is bridged | Approved Document C |
| Ground-supported floor damp-proof membrane | continuous with the wall DPC | Approved Document C |
| Ground floor, new thermal element | 0.18 W/m²K | Approved Document L, Volume 1 |
| Sub-floor ventilation, suspended timber floor | cross-ventilated void | Approved Document C |
| Full plans decision period | 5 weeks, or 2 months by agreement | Building Regulations 2010, reg 14 |
Sources: Approved Document C (site preparation and resistance to contaminants and moisture, 2004 edition incorporating later amendments); Approved Document L, Volume 1 (2021 edition); the Building Regulations 2010 (as amended). Read at 1 September 2026. On London’s Victorian stock the original course is often slate, and a level-threshold extension is the point at which it is most commonly bridged.
Part C of the Building Regulations requires walls to resist moisture from the ground. The Approved Document route is a damp-proof course of bituminous felt, polyethylene or engineering brick, positioned a minimum of 150 mm above the adjoining external ground, continuous with the floor's damp-proof membrane so there is no break at the wall-to-floor junction.
Where an extension floor is lower than the existing, or where a patio has been laid over the original DPC, the level is the first thing an inspector checks. Bridging is the single most common cause of damp in a London extension.
Raised external ground and paving, render carried down past the DPC line, mortar droppings in the cavity, a plaster coat run continuously from wall to floor, and an abutting extension slab poured above the existing DPC. Each provides a capillary path around the barrier.
On a solid-wall Victorian house with no original DPC, the modern approach is usually managing moisture rather than injecting a chemical course: improving external ground levels, removing cement render, and using a breathable internal finish.
The construction section should show the DPC position relative to external ground and to the floor DPM, the cavity tray over any opening, the weep holes at 900 mm centres above the tray, and the vertical DPC at reveals. These are the details building control inspects at damp-proof course stage.
Damp-proof course (DPC) in a Enfield context: London Borough of Enfield (Enfield Council) runs 22 conservation areas including the Forty Hill Conservation Area and the Enfield Town Conservation Area, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. That decides how damp-proof course (dpc) is handled: £548 and 8 weeks for a Enfield householder application, or £274 for the Lawful Development Certificate that records damp-proof course (dpc) as lawful. The law behind damp-proof course (dpc) sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Minimum height above ground | 150 mm |
|---|---|
| Governing document | Approved Document C |
| Common materials | Bitumen felt, polyethylene, engineering brick |
| Weep hole spacing | 900 mm centres above cavity trays |
| Must link to | Floor damp-proof membrane |
| Inspection stage | DPC and oversite |
Most often the DPC is bridged by external ground level, render or the new slab.
Sometimes a slate course. Many have none, and are managed by ventilation and breathable finishes instead.
It treats one symptom. Ground levels and impermeable finishes are usually the real cause.
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