What Should a DPC Look Like? Spotting Damp Proof Course Problems
A DPC is a black polythene strip at least 150mm above ground level. Learn how to spot bridged, missing, or failing damp proof courses on your property.
What should a DPC look like?
A damp proof course (DPC) is a continuous strip of black polythene (typically 1200 gauge) or bituminous felt built into the mortar bed of external walls. It must sit at least 150mm above the finished external ground level. The DPC should be visible as a thin dark line running horizontally between courses of brickwork, and it must connect to the damp proof membrane (DPM) in the floor slab.
What this means for your project
The DPC is one of the most important moisture barriers in your home, and one of the easiest to get wrong. Its job is simple: stop ground moisture from wicking up through the brickwork by capillary action. Without it, moisture travels upward through the porous masonry like water being drawn up a sponge, eventually soaking internal walls, rotting timber, and creating the conditions for mould growth.
In a new build or extension, the DPC is laid into a mortar bed joint during bricklaying. It's a strip of material - typically black polyethylene at least 1200 gauge (0.3mm thick) or bituminous felt - that spans the full width of the wall. In a cavity wall, each leaf has its own DPC, and a cavity tray above links them to prevent moisture crossing the cavity at vulnerable points like lintels and openings.
The critical measurement is the 150mm height above finished external ground level. This means the final ground level - after paths, patios, driveways, and landscaping are complete. Not the ground level at the time of bricklaying. This catches people out constantly. The DPC gets laid at the right height, then a landscaper comes along six months later and lays a patio that sits 20mm below the DPC. Suddenly you've got a moisture problem.
The DPC must also connect to the DPM in the floor slab. These two barriers form a continuous moisture envelope around the base of the building. The connection is made by lapping the DPC over the DPM by at least 100mm and taping the joint. If this lap is missed or poorly executed, moisture finds the gap - every time.
Walls must have a damp proof course to prevent moisture from the ground reaching the inside of the building. The DPC should be at a height not less than 150mm above the adjoining ground level. It must be continuous and connected to the damp proof membrane in the floor to form a complete moisture barrier.
Walk around the outside of the building and look for the DPC line - a thin dark strip in the mortar joints. Measure from the finished ground level to the DPC with a tape measure. It should be at least 150mm. Check that no soil, paving, render, or debris bridges the DPC line. Photograph any areas where the ground level is close to or above the DPC.
Common mistakes
Building up ground levels above the DPC. This is the most common DPC failure and it's usually caused by landscaping done after the building work. Paths, patios, raised planters, and even just accumulated soil from gardening can raise the external ground level above the DPC. Once the ground is above the DPC, moisture bypasses it entirely. If you're planning landscaping, check the DPC height first and design accordingly.
Render carried down over the DPC. When external render is taken below the DPC line, it creates a continuous path for moisture to travel from below the DPC to above it, completely defeating the purpose. The render should stop above the DPC, with a bellcast bead (a small drip moulding) to throw water away from the wall face. If your builder is rendering and you can't see the DPC line afterwards, ask questions.
Missing or broken DPC-to-DPM connection. The lap between the DPC in the wall and the DPM in the floor is frequently missed or poorly executed, especially at corners and junctions. This creates a gap in the moisture barrier right at the most vulnerable point - where wall meets floor. Insist on seeing this connection before the screed goes down, and photograph it.
DPC Inspection Checklist
A printable checklist for verifying DPC height, continuity, and connection to the floor DPM.