Trades master copy
Last reviewed: March 2026
What Part C is
Part C is the damp and ground-conditions bit of Building Regulations. It's about making sure the site is suitable, keeping contaminants in the ground from harming people, and stopping moisture getting into floors, walls and roofs.
For most small jobs it boils down to: clear bad ground, deal with any contamination (including radon where relevant), and detail DPMs/DPCs, drainage and ventilation so you don't build in damp problems.
This guide is a summary to make Part C easier to use on site. It does NOT replace Approved Document C: Site preparation and resistance to contaminants and moisture (current edition).
You must read and follow the full Approved Document C and any radon/damp guidance relevant to your area.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
Part C affects almost every ground-touching bit of work:
- New builds and extensions - site strip, foundations, floor slabs, sub-floor ventilation, DPMs and DPCs.
- Changes to external ground levels - patios, paths, driveways that can bridge DPCs and cause "rising damp".
- Basement and semi-basement work - tanking, drainage and ventilation.
- Conversions and refurb on older stock - especially where you're replacing floors, re-rendering or dry-lining external walls.
- Projects in radon or contaminated-land areas - where you may need gas membranes, radon protection or special ground treatment.
Key "trigger points" - what catches you on site
Site preparation and ground conditions
- Before you build, the ground under the footprint must be free of topsoil, organic rubbish, old foundations and anything that could rot, shrink or compromise stability.
- You need to think about: soft spots, filled ground, old drains, trees and roots that might affect foundations and future movement.
- Check for invasive species (especially Japanese knotweed) before you start stripping the site. Spreading it is a criminal offence under the Wildlife and Countryside Act, and mortgage lenders will often refuse or delay lending on affected sites without a recognised management plan.
Resistance to ground moisture - floors and walls
- Ground-bearing slabs need a DPM (usually 1200 gauge polythene or equivalent) properly lapped and sealed, and linked to the DPC in the walls.
- Suspended floors (timber or concrete) need ventilation to the void and protection against moisture from below.
- External walls need a DPC at the right height (typically minimum 150 mm above finished ground) and detailing to stop water tracking or bridging.
External levels and bridging the DPC
- "Rising damp" in a lot of cases is actually bridged DPC - new patios, raised flower beds or paths built up to or above internal floor level.
- Part C expects you to maintain that 150 mm step where practicable, or detail drainage/flashings to keep water away from the wall.
Resistance to contaminants - radon and ground gases
- In many parts of the UK you need to consider radon or other gases (e.g. methane, landfill gas) as part of the design.
- Basic radon protection often means a 1200 gauge DPM across the whole footprint, sealed to the DPC/cavity trays, detailed carefully at joints and penetrations.
- Higher-risk radon areas may need additional measures like radon sumps and more robust barriers - check the UKHSA radon maps (ukradon.org) to find your site's risk band, follow BR 211 guidance, and coordinate with Building Control.
Rainwater and sub-soil drainage
- Sites that are prone to standing water or high water table may need drains or free-draining layers under/around the building to relieve water pressure.
- Poor sub-soil drainage + bad detailing is how you get damp patches, blown plaster and slab issues even when you've technically "put a DPM in".
Quick reference table - common jobs
New ground-bearing slab for single-storey extension
- Strip off topsoil and soft/organic material; compact suitable fill.
- Lay and lap DPM correctly, seal it, and link it to the wall DPC/cavity trays.
- Keep external finished ground at least 150 mm below DPC with falls away from the building.
New suspended timber floor replacing old solid floor
- Provide adequate cross-ventilation to the void (airbricks, vent ducts) in line with AD C guidance.
- Ensure ground beneath is not full of debris likely to rot and create moisture; consider a thin blinding/DPM to control ground moisture if specified.
- Maintain DPC position and avoid bridging with insulation, screeds or hardcore.
Raising patio levels to just below internal floor
- Don't bridge the DPC - aim to keep at least a 150 mm step down; if you can't, you need careful detailing (drain channel, tanking detail, cavity trays) to keep water off the wall.
- Make sure falls take water away from the building, not towards it.
New extension in a radon area
- Check radon maps and follow the radon guidance (e.g. BR 211) for basic or full radon protection as required.
- Provide and seal a membrane to act as both DPM and radon barrier, fully sealed at joints, edges and penetrations, linked to wall DPC/cavity trays.
- In higher-risk areas, allow for radon sump and pipework as per guidance.
Tanking a semi-basement
- Use a tanking or cavity-drain system designed for the water pressure and substrate.
- Provide drainage paths and sumps to relieve water - don't just "paint it with goo" and hope.
- Coordinate with structural design and Part A; water pressure can be significant.
Routes to compliance for trades
Use Approved Document C as your default reference
Follow the guidance for site strip, fill, DPM/DPC levels, cavity trays, floor build-ups and sub-floor ventilation.
For radon and contaminants, follow AD C and any referenced guidance (e.g. BR 211 for radon), and check with Building Control where you're unsure.
Coordinate early on site levels and drainage
Agree finished floor levels, external paving levels and drainage falls at design stage, not when the patio man rocks up.
Make sure there is a clear plan for keeping DPC unbridged and surface water away from the building.
Treat membranes as critical components, not "bin bags"
Use the specified gauge of DPM/radon membrane, laid on a suitable surface, with properly taped joints and careful detailing around pipes and penetrations.
Protect membranes during work - ripped torn polythene with random patches is not compliance.
For radon/contaminants, don't wing it
Check the radon potential band and any known contamination issues before pricing - allow for membranes, sumps or specialist advice where needed.
In higher-risk cases (landfill sites, heavy contamination), you may need a specialist designer and installer for gas protection systems.
Who is responsible for what
On a typical small job:
- The designer/engineer is responsible for choosing the right foundation/floor/wall build-ups and any radon/contamination strategy.
- The builder/main contractor is responsible for clearing and preparing the site properly, installing membranes, DPCs, drainage and ventilation as designed and as per AD C.
- Specialist damp/radon contractors (if used) are responsible for installing barrier systems to the required standard, including documentation.
- The client/owner lives with damp, mould or radon issues if it's wrong - and Building Control queries if details don't match the approved design.
Blunt version:
If you bridge the DPC with a patio, bodge the DPM, or ignore radon guidance because "we've always done it this way", you're building in problems. When the walls start blowing or radon readings come back high, it's your job everyone points at.
Simple rule to drum into your team
If you're touching the ground, floors, external walls or levels, treat it as a Part C job. Think: is this going to let water or gases in, or trap moisture where it shouldn't be?
On-site checklist (Part C)
Before you start
- Check the site investigation info (where available) for ground type, water table, contamination and radon.
- Agree finished floor level and external ground/paving levels to keep the DPC unbridged.
- Confirm floor type (slab or suspended) and what membranes/ventilation are specified.
While you're working
- Strip out topsoil and rubbish under any new building footprint, compact suitable fill only.
- Lay, lap and tape DPM/radon membranes as specified, and link them to wall DPC/cavity trays.
- Keep sub-floor vents clear and properly ducted through any paving or cladding.
When you finish
- Check there's a visible DPC line above external ground where possible (or agreed alternative detailing).
- Confirm all service penetrations through floors/walls are sealed around the membrane/barrier.
- Make sure surface water drainage falls away from the building and isn't pooling against walls.
Sources
Based on:
- Approved Document C: Site preparation and resistance to contaminants and moisture (current edition).
- Planning Portal and LABC summaries of Part C requirements for site preparation, damp-proofing and contaminants.
- Industry and local-authority guidance on radon protection, DPM/DPC detailing and moisture control under floors and at ground level.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
Know someone who needs this?
Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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