Summary

The damp proof course (DPC) is a horizontal waterproof layer built into the masonry of walls, typically one brick course above finished external ground level at the time of construction. Its function is to prevent moisture from rising up through the masonry (rising damp) and from bridging between an external wet surface and the internal wall.

When a driveway, patio, or hard standing is laid at the wrong level — too high relative to the DPC — it creates a damp pathway. Water from rain, hosing down the surface, or general splashing collects against the wall above the external paving and, if the paving bridges or nearly bridges the DPC, that moisture can pass across into the wall structure. The result is internal damp: typically appearing as tide marks on internal plaster low down on the wall, peeling wallpaper or paint, and eventually mould growth.

This is one of the most common causes of avoidable damp in UK homes. Many homeowners (and some contractors) raise the level of a patio or driveway without realising the DPC consequences. An extension, new driveway, or garden re-level can easily bring the external ground level above or close to the DPC if finished levels are not checked at the design stage.

Key Facts

  • Minimum 150mm gap — External finished paving level must be minimum 150mm (two courses of standard UK brick at 75mm per course) below the DPC line
  • Building Regulations Approved Document C — Site preparation and resistance to contaminants and moisture; confirms the 150mm ground-to-DPC requirement
  • What is DPC? — Horizontal waterproof membrane (modern: 250 micron polyethylene; historic: slate, bitumen felt, or engineering bricks) built into the mortar bed of the wall at one course above original external ground level
  • Where to find the DPC — The DPC is typically visible as a thin dark layer in the mortar between two brick courses, usually 150–200mm above original ground level. In rendered walls it may be hidden but is usually at the same relative level
  • Cavity wall DPC — In cavity wall construction, the DPC in the outer leaf must be kept clear; if the outer leaf is wet and DPC is bridged, moisture can also bridge across cavity if debris or mortar in the cavity connects the two leaves
  • Threshold level — Door thresholds are often at or slightly above DPC level; the 150mm rule applies from the threshold downward as well as from the DPC
  • Extensions — When adding an extension, confirm the new extension's ground-floor DPC level and ensure any new external paving around the extension is at least 150mm below it
  • Stepped DPC — On sloping sites, the DPC often steps in the masonry to follow the slope; paving must be 150mm below the DPC at every point around the building
  • Rainwater splash — Even where the gap is maintained, ensure drainage falls away from the building to prevent splash from adjacent hard surfaces bouncing up the wall
  • Air bricks — Air bricks providing ventilation to timber suspended floors must remain above the finished external paving level and not be blocked or buried

Quick Reference Table

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Situation Requirement
New driveway adjacent to house Finished level ≥150mm below DPC
New patio against wall Finished level ≥150mm below DPC
Air brick in wall Air brick top must remain above finished paving level
Door threshold Threshold ≥150mm above finished external paving
Existing DPC too low Tanking, cavity injected DPC, or re-siting DPC required
New extension with new paving Confirm extension DPC level; match 150mm rule

Detailed Guidance

How to Locate the DPC

Standard brick construction — Look along the mortar joints near the base of the external wall. The DPC is usually visible as a slightly darker, sometimes thicker, or more protruding mortar joint, or as the edge of a membrane visible in the joint face. It is typically one or two brick courses above the original external ground level. If the original ground was 75mm below the first brick course, the DPC will be at the second or third joint up (150–225mm above old ground level).

Rendered walls — The DPC is hidden behind render. Use a moisture meter probe to detect the damp/dry boundary, or check the internal plaster for tide marks which often indicate the DPC height.

Modern houses — DPC is typically a 250-micron (1000-gauge) polyethylene membrane installed during construction. It is at one course above finished floor level externally.

Historic buildings — DPCs were often: slate layers (very durable), engineering brick courses (impervious), or bitumen felt (variable longevity). If no DPC can be located in an old building, treat the situation conservatively — assume the DPC is at or near original ground level and keep new paving well below it.

Calculating the Correct Finished Paving Level

Step 1: Measure the DPC height above the current adjacent ground level.

Step 2: The finished paving level must be DPC height minus 150mm from the DPC, so:

  • If DPC is at 300mm above existing ground: finished paving = 300 - 150 = 150mm above existing ground → you can raise the ground 150mm
  • If DPC is at 100mm above existing ground: finished paving must be at or below existing ground level → raising the paving would bridge the DPC

Step 3: If the required finished level of the paving is above what the 150mm rule allows, the customer must either:

  • Accept paving at the lower level (step down at the building)
  • Have the DPC raised or supplemented (tanking, chemical injection)

Consequences of Bridging the DPC

When external paving is at or above DPC level:

Rising damp pathway — Moisture migrates through the masonry from the external wet surface upward, bypassing the DPC. The DPC cannot prevent moisture that enters above it.

Splash-back damp — Even with the 150mm gap maintained, if water ponds adjacent to the wall and the paving level is high relative to the wall cavity, splash from adjacent traffic or hose cleaning can drive moisture into the wall face.

Timber floor damage — In suspended timber floor construction, the DPC also protects the floor structure. If bridged externally, joist ends and plate timbers in the cavity can absorb moisture, leading to rot. This is a significant structural failure mode that can take years to manifest but is expensive to remediate.

Mould and internal damp — The visible signs inside: tide marks on low internal plaster (typically to 1m height), peeling wallpaper and paint, black mould growth in corners, musty smell. These often get misdiagnosed as condensation; in adjacent-paving situations, check DPC level first.

Remediation When DPC Is Already Bridged

If a customer has an existing driveway or patio where the DPC has already been bridged:

Option 1: Lower the paving level — The correct solution but involves lifting and re-laying paving, removing sub-base material, and sometimes significant groundworks. Expensive.

Option 2: Chemical injection DPC — Specialist damp-proofing contractors drill horizontal holes at the required level and inject a chemical waterproofing fluid that creates a new DPC above the old one. BWPDA (British Wood Preserving and Damp-Proofing Association) member contractors. This addresses rising damp from below the injection level.

Option 3: Tanking — Apply a cementitious or resin-based waterproof coating to the external face of the wall from paving level up to above the existing DPC. Prevents water penetrating the wall face at the bridged section.

Option 4: Drainage channel — Install a drainage channel at the wall base to intercept and remove water before it builds up against the wall. Reduces moisture exposure but does not eliminate the DPC bridge.

In practice, a combination of option 2 (chemical injection DPC) on the internal wall face and option 4 (drainage channel externally) is a common remediation used by damp-proofing contractors.

Air Bricks

Air bricks provide ventilation to suspended timber floors in older houses. They are typically at or just above original ground level. When new paving is laid adjacent to these walls, air bricks can be buried or blocked.

What happens: Without ventilation, the sub-floor space becomes damp and stagnant. Timber joists and floor boards rot. This is a serious structural problem.

What to do:

  • Check for air bricks in any wall adjacent to new paving
  • Ensure finished paving level is below the bottom of each air brick
  • If paving level must be above the air brick, extend the air brick through the new paving with a duct (terracotta duct pipe or a purpose-made paving duct block)
  • Never block or cover an air brick with paving material

Steps and Level Changes at Buildings

When a driveway or patio must meet a doorway threshold at a higher level than the 150mm rule would allow for the main surface area:

Design the junction carefully:

  • Install a step at the building with the riser set to create the required level difference
  • The step riser face should be minimum 150mm above the adjacent paving
  • The tread of the step at the door threshold is then at the correct level for the threshold
  • Drainage away from the building is still required from the step tread

Frequently Asked Questions

My customer's existing patio is above DPC level — do I have to lower it?

As a contractor taking on a patio replacement or resurfacing job, you are not legally obligated to correct a pre-existing DPC bridge that you did not create. However, you should advise the customer in writing that the existing level is above DPC and may be causing damp. Documenting this protects you if the customer later claims damp problems resulted from your work.

The DPC seems very high in this wall — can I raise the driveway level to just 100mm below it?

The 150mm requirement is from Approved Document C and represents the minimum safe clearance based on splash height from rain falling on horizontal paving. It is not a guideline — do not reduce it. If the DPC is high enough to accommodate a driveway at the level you need, 150mm clearance still applies.

Is there a DPC around extensions and outbuildings?

Yes — any new construction must include a DPC under Building Regulations Part C. Extensions must have their DPC at least 150mm above external finished ground level. When adding an extension and new paving in the same project, confirm the extension DPC height before finalising paving levels.

Regulations & Standards

  • Building Regulations Approved Document C (2004+) — Site preparation and resistance to contaminants and moisture; explicitly requires external ground level to be at least 150mm below DPC

  • BS 8215:1991 — Design and installation of damp-proof courses in masonry construction; the British Standard governing DPC specification and position

  • BS 5250:2021 — Management of moisture in buildings; condensation and damp control; contextual reference

  • Building Regulations Approved Document A — Structure; relevant when ground level changes affect wall loading or stability

  • GOV.UK: Approved Document C — official Building Regulations guidance on damp proof course clearances

  • BWPDA: Rising Damp guidance — British Wood Preserving and Damp-Proofing Association technical guidance

  • Property Care Association — damp diagnosis and DPC remediation guidance

  • block paving installation — installation methodology; level setting relative to DPC is part of the design

  • tarmac driveway installation — finished level requirements for tarmac adjacent to buildings

  • driveway drainage channels — drainage falls away from buildings to prevent water ponding at walls

  • concrete driveway slab — concrete slab level design and DPC clearance