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What Is a DPC? — Damp Proof Course Explained
A plain-English guide to damp proof courses: types, installation height, materials, replacement methods, and UK Building Regulations requirements.
Definition
A DPC (Damp Proof Course) is a horizontal barrier built into the walls of a building to prevent moisture from rising up through the masonry by capillary action. This phenomenon is known as rising damp. The DPC is typically a strip of impervious material installed in the mortar course at least 150 mm above finished external ground level, as required by Building Regulations Approved Document C.
When Is It Used?
A DPC is required in all new masonry construction under the Building Regulations. It is also relevant when extending existing buildings, building garden walls, and when diagnosing and treating damp problems in existing properties. DPCs work in conjunction with DPMs (Damp Proof Membranes) in floors to create a complete barrier against ground moisture. In cavity wall construction, cavity trays act as DPCs above openings.
DPC Height: The 150mm Rule
Approved Document C requires a DPC in an external wall to sit at least 150 mm above the level of the finished adjoining ground. The document names bituminous material, polyethylene, engineering bricks or slates in cement mortar as acceptable materials, alongside “any other material that will prevent the passage of moisture.” The 150 mm clearance stops rain splashing up off paving and wet soil from bridging the barrier before it can shed water away from the wall.
Installing or replacing a DPC on its own does not usually need Building Regulations approval, because it is treated as a like-for-like repair. Approval is more likely where the work also alters the wall structure or affects thermal performance, for example where hacking off and replastering also changes the insulation. If ground levels outside have been raised above the DPC by paving, decking or a flower bed, the fix is to lower the ground or cut in a French drain (a gravel-filled channel against the wall) to restore the clearance, rather than add a second DPC higher up the wall.
Types of DPC: Physical vs Chemical
A DPC falls into one of two categories. A physical DPC is a continuous strip built into the wall, most often polyethylene sheet, though engineering brick courses, slate, bitumen felt and pitch polymer are also used. Physical DPCs in masonry walls are specified to BS 8215:1991, the UK code of practice for their design and installation in solid and cavity construction. In timber frame construction, a pre-shaped DPC sits under the sole plate (the first timber member) and turns up the inner face, keeping the structural timber at least 150 mm above ground level per NHBC Standards Chapter 6.2.
A chemical (injected) DPC is a remedial treatment for an existing wall where the original DPC has failed or was never installed. A silicone or siloxane cream is injected into holes drilled along a mortar course, where it spreads through the masonry to form a water-repellent zone. Chemical DPC installation is covered by a separate standard, BS 6576:2005, which BS 8215 explicitly excludes from its own scope. See the Rising Damp Treatment Calculator for injection hole spacing and cream quantities, or the UK DPC Specifications reference for physical DPC widths and materials.
DPC in Cavity Walls
A cavity wall needs its own horizontal DPC in each leaf, inner and outer, both set at the same 150 mm minimum height. Where the DPC has to change level, such as over a lower roof abutment or a stepped site, or wherever a window or door interrupts the wall, a cavity tray links the two leaves and carries water back out to the outer face through weep holes, rather than letting it track across to the inner leaf. Vertical DPCs are also fitted at window and door reveals to close the cavity at the jambs.
See the Cavity Tray Calculator for tray lengths and stop ends, and Weep Hole Spacing for drainage positions at DPC and cavity tray level.
DPC vs DPM: What Is the Difference?
A DPC is a horizontal barrier in a wall, stopping rising damp in the masonry. A DPM (damp proof membrane) does the same job in a floor: a sheet, typically 1200 gauge (300 micron) polyethylene to BS EN 13967, laid under or within a concrete slab to stop ground moisture rising through the floor. The two are not interchangeable, but they must meet: Approved Document C requires the floor DPM to be lapped and made continuous with the wall DPC, so there is no gap in the barrier where a wall meets a floor.
Use the Floor DPM Calculator for membrane roll quantities and lap allowances.
Signs of DPC Failure and Bridging
Most DPC problems are not a failure of the DPC material itself, which rarely perishes, but bridging: something forms an alternative path for moisture to cross above or around it. The common bridges are ground levels raised by landscaping, decking or new paving above the 150 mm clearance; render or pebbledash taken down over the DPC line instead of stopping above it on a bellcast bead; mortar snots or debris left in the cavity connecting the outer leaf to the inner leaf; and internal plaster or dry lining that runs below the DPC line and draws moisture up the inside face.
Watch for a tide mark of damp on internal walls, salt deposits (efflorescence), peeling paint or wallpaper at low level, and a musty smell at skirting height. Because condensation and penetrating damp produce similar symptoms but need different fixes, a proper diagnosis with a calibrated moisture meter matters more than assuming a failed DPC. See the Damp Survey Calculator for a structured survey approach.
Key Facts
- Must be minimum 150 mm above finished external ground level (Building Regs Part C)
- DPCs have been a legal requirement since the Public Health Act 1875
- Modern DPCs are polyethylene sheet, typically 450 mm or 600 mm wide
- Chemical injection DPCs use silicone cream or resin injected into drilled holes at 120 mm centres
- The DPC must be lapped at corners and joints by at least 100 mm
- Common causes of DPC failure: bridging by raised ground levels, render, or paths
- Electro-osmotic DPC systems are not recommended by most independent surveyors
- A DPC should never be covered by external render - a bellcast bead should terminate the render above the DPC
Related Calculators
Use the Mortar Calculator for below-DPC mortar mixes, or the Brick Calculator for walling quantities. The Rising Damp Treatment Calculator helps estimate chemical DPC injection treatment quantities.
For full DPC width and material specification tables, see UK DPC Specifications. For below-ground structures under hydrostatic pressure, such as basements, a DPC alone is not sufficient: see What Is Tanking? for BS 8102 waterproofing types.
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Frequently Asked Questions
Building Regulations Approved Document C requires the DPC to be at least 150 mm above finished external ground level. This prevents rainwater splashing up from the ground and bridging the DPC. If the ground level outside is too high, it should be lowered or a French drain installed to maintain the 150 mm clearance.
Common signs include a tide mark of damp on internal walls (typically up to about 1 metre high), salt deposits (efflorescence) on the wall surface, peeling wallpaper or paint at low level, musty smells, and black mould growth at skirting board level. A damp survey using a calibrated moisture meter can confirm whether rising damp is present or whether the moisture is from another source such as condensation.
Modern DPCs are typically made from polyethylene (polythene) sheet, bituminous felt, or pitch polymer. Older buildings may have slate DPCs, engineering brick courses, lead sheet, or bitumen-coated hessian. Some very old buildings (pre-1875) may have no DPC at all, as the requirement was not introduced until the Public Health Act 1875.
A chemical injection DPC for a typical 3-bedroom semi-detached house costs between £2,000 and £4,000 including replastering. The injection itself costs around £30-£50 per linear metre. Some companies offer 20 or 30-year guarantees. Internal replastering with salt-inhibiting plaster adds approximately £40-£60 per m² to the cost.
DIY DPC injection kits are available from builders merchants for around £40-£80 for a 5-litre kit (covering approximately 10 linear metres). The silicone-based cream is injected into holes drilled at 120 mm intervals, typically 150 mm above external ground level. However, correct diagnosis is essential - most damp problems in older buildings are caused by condensation or penetrating damp, not rising damp, and injecting a DPC will not solve these issues.
A DPC (damp proof course) is a horizontal barrier in a wall, usually a strip of polyethylene, that stops rising damp reaching the masonry above ground level. A DPM (damp proof membrane) does the same job for a floor: a polyethylene sheet, typically 1200 gauge (300 micron) to BS EN 13967, laid under or within the concrete slab. Approved Document C requires the DPM to be lapped and made continuous with the wall DPC where they meet, so there is no gap in the barrier.
Yes. Each leaf of a cavity wall, inner and outer, needs its own horizontal DPC at the same 150 mm minimum height above ground. Where the DPC steps or the wall is interrupted by an opening, a cavity tray links the two leaves and directs any moisture in the cavity back out through weep holes in the outer leaf, rather than letting it bridge across to the inner leaf.
Installing or replacing a DPC on its own is normally treated as a like-for-like repair and does not usually need Building Regulations approval. Approval is more likely to be needed if the work also alters the wall structure or affects thermal performance, for instance where hacking off and replastering also changes the insulation build-up. New masonry construction always requires a DPC to satisfy Approved Document C as part of the wider Building Regulations application.
A physical DPC is a continuous strip, most commonly polyethylene sheet, built into the wall during construction and specified to BS 8215:1991. A chemical DPC is a remedial treatment for an existing wall with no working DPC: a silicone or siloxane cream is injected into drilled holes along a mortar course to form a water-repellent zone, specified to a separate standard, BS 6576:2005, which BS 8215 does not cover.
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Last updated: July 2026
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