DPC in Brickwork: Types, Heights and Building Regulations Requirements

Summary

The damp-proof course is one of the most fundamental waterproofing details in UK masonry construction. A correctly installed DPC stops ground moisture rising through the masonry wall by capillary action — the mechanism known as rising damp. Pre-1920s buildings were often built without a DPC, which is why rising damp is such a common problem in Victorian and Edwardian properties.

Building Regulations Approved Document C requires a DPC in all new construction where walls are in contact with the ground. The standard residential DPC is a continuous impermeable membrane at least 150mm above external ground level — high enough to prevent rain splash-back from saturating the wall below the DPC. In cavity wall construction, both the inner and outer leaves require separate DPCs; a cavity tray is also needed above any openings.

This article covers DPC installation in new brickwork. For treatment of failed or absent DPCs in existing buildings, see dpc replacement.

Key Facts

• Building Regulations Approved Document C — DPC required at ≥150mm above external finished ground level
• BS 6515:1984 (confirmed) — specification for polyethylene DPC; still referenced in most specifications
• BS 743:1970 — specification for materials for DPCs (bitumen felt, slate, engineering brick); historical standard still referenced
• Standard polyethylene DPC — 300mm, 450mm, 600mm, 900mm, 1200mm widths available; match to wall width with 100mm laps at joints
• Minimum lap — 100mm at all joints and angles; no butting joints permitted
• Cavity tray — required over all openings and across stepped/angled sections of cavity; continuous from outer to inner leaf with stop ends and weep holes
• DPC at cavity — both inner and outer leaves require separate DPCs at the same level; a single DPC bridging the cavity is not acceptable (cold bridging)
• 150mm rule — 150mm minimum above external ground; increase to 225mm if any risk of ground heave or garden modification
• Internal floor DPC — linked to wall DPC via damp-proof membrane (DPM) in concrete floor slab; continuity essential
• DPC bridging — most common failure cause; render, soil, paving, or debris in cavity can bridge the DPC and allow moisture to bypass it
• Engineering brick alternative — two or three courses of Class A engineering brick can substitute for a DPC membrane where a membrane would be impractical; Class A absorption <4.5%; see brick types
• Slate DPC — traditional pre-war detail; still specified on heritage repairs; two overlapping courses of natural slate in lime mortar
• Lead DPC — used for complex details, flashings, stepped DPCs on sloping sites; Code 4 lead minimum

Quick Reference Table

Detailed Guidance

Standard DPC Installation in Cavity Walls

1. Set out DPC level — mark 150mm above external finished ground level on both inner and outer leaves
2. Install inner leaf DPC first — lay PE or bitumen DPC on a full mortar bed; ensure 100mm laps at all joints, especially at corners (butt-lap with 100mm return)
3. Install outer leaf DPC — at the same bed joint height; both DPCs at exactly the same level
4. Check continuity — the DPCs must be at the same course height; a step between inner and outer leaf DPCs creates a potential path for moisture to track around the step
5. Floor DPM connection — where a solid concrete ground floor is used, the DPM (typically 1200-gauge polythene) must be lapped up and over the wall DPC; this creates the continuous moisture barrier
6. Suspended timber floor — wall DPC must still be present; it prevents moisture rising from ground level to above the air ventilation space
7. Cavity continuity — ensure no mortar droppings or debris fall into the cavity during construction; mortar bridges on cavity ties can carry moisture from outer to inner leaf

Cavity Trays Over Openings

Cavity trays are required at:

• Window and door heads (above lintels)
• Points where the cavity is interrupted (movement joints, changes in wall thickness)
• Stepped DPC positions on sloping ground

A cavity tray must:

• Be continuous from outer leaf to inner leaf with upstand on both sides
• Have stop ends at each side of the opening to prevent water running sideways
• Have weep holes in the outer leaf perpendicular joints immediately above the tray (typically alternate perpends); at least two weep holes per lintel, spaced ≤450mm apart

Common failure: cavity trays installed without stop ends allow water to run along the tray and through the jamb masonry at the side of the window.

DPC on Sloping Ground

Where ground level changes, the DPC must step with it. Rules:

• The DPC must always be ≥150mm above the higher external ground level at every point
• Steps in the DPC must be in the inner leaf only (or coincide with the outer leaf at the same course)
• Lapped joints at steps: 100mm minimum overlap

For significant changes in level, consider:

• Tanked foundation walls with no cavity at the lower level, transitioning to cavity wall above
• An engineer's detail for split-level or sloping site construction

DPC Heights Near Paths and Landscaping

Many DPC failures are caused by landscaping changes after construction:

• A patio laid against the house wall that rises to within less than 150mm of the DPC
• Soil beds built up against the wall
• New turf level higher than the original finished ground level

For any client work near existing walls, check the DPC height above finished level. If the client insists on raised planters or paving that reduces the 150mm clearance, advise in writing. Consider installing a French drain or drainage channel to intercept surface water before it reaches the wall.

Frequently Asked Questions

What is the minimum height for a DPC?

Approved Document C requires 150mm above the finished external ground level. In practice, 150mm is the bare minimum — 225mm is better practice where there is any risk of paving or landscaping being raised in the future. The 150mm rule assumes that ground-level splash-back from rainwater cannot reach above this height; in exposed locations or against gravel-covered ground, higher is safer.

Does a DPC need to be installed in internal partition walls?

No. Internal partition walls that are not in contact with the ground do not require a DPC. However, internal walls immediately adjacent to an external cavity wall should not bridge the external wall's DPC — ensure the internal partition connects to the wall above DPC level.

The render on my wall goes below the DPC — is this a problem?

Yes. Render that extends below the DPC creates a bridge for moisture to travel from below the DPC to above it, bypassing the waterproofing entirely. This is one of the most common causes of rising damp in rendered properties. The render must be cut back to at least 25mm above the DPC level. If the render is flush with the DPC, a UV/moisture-resistant paint does not solve the problem — the render must be physically cut back.

Can I use expanding foam to fill the cavity at DPC level?

No. Expanding foam in the cavity is not an acceptable substitute for a cavity tray. Foam can crack, does not self-seal to the masonry, and may promote mould growth. Install a proper cavity tray system.

Regulations & Standards

• Approved Document C (Site preparation and resistance to contaminants and moisture) — requires DPC at 150mm minimum

• BS 6515:1984 (confirmed) — specification for polyethylene damp-proof courses for masonry

• BS 743:1970 — materials for damp-proof courses (bitumen felt, slate, etc.)

• BS EN 14909 — flexible sheets for waterproofing: plastic and rubber DPC sheets

• BS EN 1996-2 — Eurocode 6: design considerations and execution for masonry

• BRE GBG 12 — Building a new house: damp-proof courses and membranes

• Visqueen Building Products — DPC Technical Guide — polyethylene DPC product range and installation guidance

• Historic England — Damp-proof Courses in Old Buildings — guidance for repairing or adding DPCs in historic construction

• NHBC Standards Chapter 5.1 — DPC requirements for NHBC warranty

• dpc replacement — methods for installing DPCs in existing buildings (injection, physical)

• rising damp — diagnosing rising damp and DPC failure

• cavity wall ties — cavity construction and tie installation

• brick types — engineering brick as DPC alternative below ground