Building Regs Part C (Site Preparation & Resistance to Moisture): A Trade's Guide (2026)

What Part C Actually Covers

Approved Document C covers two related things: the preparation of the site, and resistance to contaminants and moisture. In plain terms it is about making sure the ground a building sits on is fit to build on, and making sure water from the ground, rain and snow cannot get into the structure where it would cause damage or harm the people inside.

It breaks down into a handful of requirements that map onto the work you do in sequence on site:

• C1 — Preparation of site and resistance to contaminants: clearing the ground, dealing with contamination and ground gases.
• C2 — Resistance to moisture: stopping moisture from the ground reaching the inside (floors and walls) and stopping rain and snow penetrating walls and roofs.
• Subsoil drainage: dealing with groundwater so it does not pass into the building or undermine its stability.

Site Preparation

The first job under Part C happens before any foundations go in. The ground covered by the building, plus a margin around it, must have turf, topsoil and other vegetable matter removed. The reason is straightforward: organic material rots, settles and holds moisture, and you do not want any of that under a slab or oversite. Tree roots and any obviously deleterious material come out at the same time.

Where the site has a history that suggests contamination — former industrial use, made ground, infilled land, old tips — Part C expects the risk to be assessed and dealt with. That can mean removing contaminated material, capping it, or designing a barrier so contaminants cannot reach the building or its occupants. On most domestic extensions this is a non-issue, but on brownfield plots it is the first thing to check, because remediation drives both programme and cost.

Ground Gases — Radon and Methane

Two ground gases get specific attention. Radon is a naturally occurring radioactive gas that seeps up out of the ground in certain geological areas — large parts of the South West, the Pennines, parts of the Midlands and elsewhere. In affected areas Part C requires radon protective measures: at minimum a radon-resistant membrane (a gas barrier) laid across the building footprint and sealed at all junctions and service penetrations. In higher-risk areas you also need a means of extracting gas from below the slab — a sump and ventilated void — so it can be drawn off later if monitoring shows it is needed.

Methane and other ground gases are a concern on or near former landfill and made ground. Where there is a credible risk, a gas-resistant membrane and a ventilated sub-floor void are typically specified. The key point on site is that a gas membrane is only as good as its seals — laps, upstands at walls, and every pipe and duct that passes through it must be detailed and taped properly. A barrier with an open service penetration does nothing.

Subsoil Drainage

Where the water table is high, or where groundwater could pass into the building or affect the stability of the foundations, subsoil drainage is needed. In practice this means land drains laid in granular material to intercept and carry groundwater away to a suitable outfall, soakaway or watercourse. On sloping sites a cut-off drain on the uphill side keeps groundwater moving past the building rather than ponding against it. Getting this right also protects the work you do later — there is no point detailing a perfect DPM if the ground around it stays saturated.

Resistance to Moisture from the Ground

This is the heart of Part C for most jobs: keeping ground moisture out of walls and floors. Two elements do the work, and the detail that ties them together is what separates a compliant job from a callback.

The Damp-Proof Course (DPC) in Walls

The DPC is a continuous impermeable barrier built into the walls to stop moisture rising up through the masonry. The rule every brickie should know: the DPC must be at least 150mm above the finished external ground level. That 150mm gap stops rain splashing up off the ground and bridging over the top of the DPC, and gives a margin against ground build-up over the life of the building. Below that level the wall is allowed to get wet; above it must stay dry.

DPCs are usually a flexible polymer or pitch-polymer sheet bedded on mortar, or engineering bricks in some details. Vertical DPCs are needed at reveals around openings and wherever cavities are closed. The DPC must be lapped and continuous — a gap at a lap or a missed return at a doorway is a direct path for damp.

The Damp-Proof Membrane (DPM) in Floors

The DPM is the impermeable layer in a ground-bearing floor that stops moisture rising through the slab. It can sit above the slab (under the screed or insulation) or below it, depending on the build-up. A typical ground floor runs: compacted hardcore, sand blinding, DPM, then concrete slab, insulation and screed or a beam-and-block alternative — the exact order varies, but the principle is that the membrane forms a continuous tanked tray under the floor.

The single most important detail: the DPM must link to the DPC. The floor membrane is lapped and sealed to the wall DPC so the two form one continuous barrier around the bottom of the building. If they do not connect, moisture simply tracks around the gap between them and you get damp at the wall-to-floor junction — exactly the place it is hardest to fix once finishes are on.

Resistance to Rain and Snow on Walls

Part C also deals with weather hitting the outside of the building. The standard answer in UK construction is the cavity wall: an outer leaf that can get wet, a clear cavity, and an inner leaf that stays dry. Any water that crosses the outer leaf runs down the cavity face and needs a way back out.

That is the job of cavity trays and weep holes. A cavity tray is a DPC-style tray built into the cavity that catches water and directs it outwards through weep holes in the perpend joints of the outer leaf. Trays are essential above every opening (over lintels at windows and doors), at abutments where a roof meets a wall, at changes in cavity, and wherever the cavity is bridged. Weep holes go directly above each tray, typically at 450mm centres and at least two per opening. Leave them out and the tray fills up and discharges the water inwards instead.

Where walls are rendered or clad, the cladding or render system handles weather resistance, but the same logic applies — water must be shed outward and away, and any drained or ventilated cavity behind cladding needs its trays and weeps detailed properly. Solid walls and single-leaf construction rely on the render or an internal lining to do the job and need particular care to meet Part C.

Resistance to Moisture in Roofs

Roofs keep rain and snow out through the covering — tiles, slates or a membrane — but Part C is also concerned with moisture that forms inside the construction. Warm, moist air from inside the building can pass up into the roof and condense when it hits a cold surface. This is interstitial condensation, and it rots timbers and soaks insulation from the inside, invisibly.

The two ways to manage it are ventilation and the right membranes. A cold roof is ventilated so moist air is carried away before it condenses. Where a breathable (vapour-permeable) membrane is used under the tiles, water vapour can pass out through the roofline while liquid water is kept out — but a breathable membrane relies on the build-up being designed for it, including the air and vapour control layer at ceiling level. Mixing a breathable underlay with a sealed ceiling and no ventilation is a classic way to trap moisture. The principle to carry on site is the same as everywhere in Part C: keep liquid water out, and give any vapour that does get in a way back out.

Continuity — The Detail That Ties It All Together

If there is one idea to take from Part C, it is continuity. Every individual barrier — DPM, DPC, cavity tray, gas membrane — only works if it joins up with the ones next to it. Water and gas find the gap. The chain runs: DPM in the floor links to the DPC in the wall, the DPC links to the cavity trays above ground, and the cavity trays discharge through weep holes to the outside. A break anywhere in that chain is a damp path.

On site this means thinking about the laps, the seals and the junctions as much as the products themselves. The membrane is rarely the problem. The unsealed lap, the missed return, the tray that stops short of the cavity closer — that is where damp gets in.

Common On-Site Failures and How to Avoid Them

Most Part C failures are not exotic — they are the same handful of detailing mistakes repeated across thousands of jobs. Here are the ones building control sees again and again, and the fix for each.

When Building Control Checks Part C

Because so much of Part C is buried, building control wants to see it before it disappears. The key inspection stages are:

• Excavation / foundations: ground stripped of topsoil, contamination dealt with, foundation depth and bearing confirmed.
• Oversite / DPM before slab: hardcore, blinding, membranes and any radon/gas barrier inspected before the concrete goes down.
• DPC level: checked as the walls come out of the ground, confirming the 150mm rule and that the DPM links to the DPC.
• Damp-proofing and drainage: cavity trays, weep holes and subsoil drains seen before they are covered.

The practical lesson is to call inspections at the right moment. Cover the DPM before it is seen and you may be asked to break out the slab. Photographing each layer as you go — membranes, laps, trays, seals — protects you if a query comes up later, and it is good evidence for the customer and for your own records.

Frequently Asked Questions

How high above ground should the DPC be?

At least 150mm above finished external ground level. This stops rain splashing up off the ground and bridging over the top of the DPC. Watch out for later landscaping or a raised path that brings the ground up and effectively lowers your margin — that is a common way a compliant DPC ends up bridged.

Do I always need a radon barrier?

No — only in areas identified as affected by radon. Check the geographic radon data for the site at design stage. In basic protection areas a sealed radon-resistant membrane is usually enough; in full protection areas you also need a sub-slab depressurisation provision such as a sump and ventilated void so gas can be extracted if needed.

What is the difference between a DPC and a DPM?

The DPC (damp-proof course) is the horizontal and vertical barrier in walls that stops moisture rising up the masonry. The DPM (damp-proof membrane) is the barrier in the floor that stops moisture rising through the slab. They do the same job in different elements and must be lapped and sealed together so they form one continuous barrier.

Why do I need weep holes above cavity trays?

A cavity tray collects water that crosses the outer leaf and directs it outward. Without weep holes above the tray, that water has nowhere to go — the tray fills and eventually discharges the water onto the inner leaf, causing damp inside. Weep holes let the trapped water drain back out where it belongs.