Get the moisture barriers wrong on a masonry wall and you’ll be back ripping it apart six months later. I’ve seen it happen on jobs that looked perfectly tidy at first fix, proper mortar, clean courses, nice brick. Then the owner rings up in winter asking why there’s a damp patch spreading across the internal plaster. Nine times out of ten, it comes back to the damp proof course being misunderstood, poorly lapped, or just missing entirely at a critical junction.
DPC, DPM and cavity trays each do a specific job. They’re not interchangeable, they don’t cover for each other’s absence, and Building Control inspectors know exactly where to look when any one of them has been bodged. Here’s how they all fit together and what you need to get right before the walls go up past first-floor level.
What each moisture barrier actually does
A damp proof course is a physical barrier built into the wall at a specific height, typically 150mm above finished ground level on external walls, to stop ground moisture wicking up through the masonry. It’s usually a strip of polythene or bitumen-impregnated felt, installed as a horizontal layer in the bed joint. Without it, rising damp climbs through the capillary structure of the brick and mortar like a wick. The relevant regulations for new builds and extensions sit within Part C of the Building Regulations, which covers site preparation and moisture resistance.
A damp proof membrane is a different animal. The DPM sits beneath a floor slab or screed, typically 1200-gauge polythene, and its job is to block ground moisture from coming up through the floor. The key thing tradesmen miss is that the DPM and the DPC must be linked. If you lap one over the other and seal the joint properly, you create a continuous barrier from floor to wall. If you don’t, moisture finds the gap. I’ve seen houses where the DPM was just tucked against the wall and left loose, with no connection to the DPC in the external leaf. That’s a damp patch waiting to happen.
Cavity trays are the third part of the system, and they tend to be the most misunderstood on site. A cavity tray is a damp proof layer installed at a stepped profile across the cavity, used wherever something bridges or interrupts the cavity wall, a roof abutment, a lintel over an opening, a change in construction type, or where a lean-to meets the main wall. Their job is to catch any water that runs down the cavity and direct it out through weep holes in the outer leaf. Without them at the right positions, water pools in the cavity and eventually saturates the inner leaf.
Where tradesmen get it wrong
The most common DPC mistake I see on domestic extensions is the wrong height above external ground level. The 150mm rule is well known, but on jobs with a finished patio or pathway close to the wall, the external level can end up creeping up after the DPC is already installed. If your landscaping brings the external ground within 75mm of the DPC, moisture can bridge it. Worth having that conversation with the groundworker before the wall goes up, not after.
Lapping is where DPMs catch people out. Laps should be a minimum of 300mm and taped with compatible jointing tape. If you’re using the DPM to link through to the DPC in the wall, the polythene needs to be turned up behind the cavity closer and butted tight to the DPC. Any gap, fold or tear in that junction is a moisture path. Same logic applies in older properties, where you might be working alongside an existing DPC, have a look at our guide to damp proof courses on UK period properties before you assume the existing one is still doing its job.
Cavity trays are where I see the most serious omissions. On a simple two-storey extension with a monopitch lean-to roof, the cavity tray at the abutment is essential. It needs to be stepped to follow the roof pitch, continuous across the full width of the wall, and fitted with stop ends so water can’t escape sideways back into the cavity. Weep holes, open perpend joints, need to be installed every other brick course in the outer leaf immediately above the tray. I’ve walked onto jobs where the roofer has flashed the abutment neatly but no one has installed the cavity tray behind the flashing. The flashing handles surface water, but the tray handles water running down the cavity itself. They’re not the same thing.
Lintels are another tray location that gets skipped. A standard pressed steel lintel incorporates a tray profile, but only if it’s the right specification for a cavity wall. Using a solid-section lintel designed for internal partitions in an external cavity wall is a mistake that shows up at inspection. If you’re ordering your lintels from a builders’ merchant, make sure the product is specified for cavity use with an integrated drip. The IG Lintels or Catnic datasheets will tell you exactly what you’re getting.
What Building Control looks for at DPC stage
Most inspectors will request a DPC stage inspection on new builds and extensions. They’re not just checking that the DPC is present, they want to see it’s at the right height relative to finished ground level, that it’s continuous with no breaks, and that the DPM is properly lapped and turned up. On cavity wall construction they’ll also want to see cavity trays over openings and at any roof abutments.
Paperwork helps. If you’re on a bigger plot or doing a full new build, having the trays and weep holes marked on your construction drawings before work starts means the inspector knows what to expect and you have a reference to work from on site. On a small single-storey extension it might be less formal, but the same standard applies. No inspector is going to sign off a wall where you’ve omitted a cavity tray over a lintel just because the job is small.
Getting moisture barriers right overlaps with other elements too. If you’re working on a cold roof over an extension, the junction between wall and roof is exactly where cavity trays and DPCs need to work together, something we’ve written about in detail in the article on cold roof vs warm roof flat roof construction. The same principle applies: get the moisture management sorted before the junction is closed in, because accessing it afterwards is expensive and disruptive.
A note on remedial work and hazardous materials
On older properties, cutting in a new DPC or installing cavity trays during refurbishment can open up walls that contain legacy materials. If you’re working on any pre-2000 structure, particularly in the East Midlands where a lot of post-war housing stock survives, it’s worth having materials surveyed before you start opening up cavities or grinding out bed joints. For work in that region, Asbestos Compliance Solutions handles surveys and removal if the works turn up anything that needs dealing with before your moisture barrier work can proceed safely.
Specifying the right materials
DPC strip comes in rolls of varying width, and the width needs to match the wall construction. On a standard 102mm outer leaf cavity wall, you need a DPC that fully covers the leaf with a slight overhang, 112mm or wider. For a full cavity wall where both leaves need DPC, you install them separately at the same course height, not one continuous strip bridging the cavity. That’s a common spec error that creates a cold bridge on insulated cavity walls, which is worth considering alongside the broader issue of cold bridging in UK new builds and extensions.
For cavity trays, proprietary formed systems (Cavity Trays Ltd manufacture a widely used range from their Somerset plant) are more reliable than site-cut polythene. The formed trays come with factory-made stop ends and step profiles. They cost more than a strip of DPC material folded on site, but they seat properly and the inspector can see at a glance that the detail has been thought about. On tight margins, it’s tempting to cut corners here. Don’t. A failed moisture inspection means rework and delay, and that always costs more than the tray did.
DPC, DPM and cavity tray installation isn’t glamorous work. It’s invisible once the job’s done. But it’s the difference between a wall that performs for 50 years and one that’s damp by its second winter. Get the sequence right, lap everything properly, install trays wherever the cavity is bridged, and make sure the DPM talks to the DPC at the floor-to-wall junction. Building Control will pass it first time and the client won’t be on the phone in January.
Frequently Asked Questions
What is the difference between a DPC and a DPM?
A DPC (damp proof course) is a horizontal barrier built into a wall, usually at 150mm above ground level, to stop moisture rising through the masonry. A DPM (damp proof membrane) is a sheet membrane laid beneath a floor slab or screed to prevent moisture coming up through the ground. Both are needed on most UK masonry builds, and they should be lapped and joined at the wall-floor junction to form a continuous barrier.
Where do cavity trays need to be installed on a masonry wall?
Cavity trays are required wherever the cavity is bridged or interrupted: above lintels over windows and doors, at roof abutments and lean-to junctions, where the wall steps down in height, and anywhere a structure crosses from the outer to the inner leaf. Each tray needs stop ends and weep holes in the outer leaf immediately above it to allow any water to drain out.
How high above ground level should a DPC be on an external wall?
The standard requirement under Part C of the Building Regulations is 150mm above finished external ground level. This gap must be maintained even after any external landscaping, paving or pathway is laid. If ground levels rise after the DPC is installed and reduce that gap to less than 75mm, there is a risk of moisture bridging the barrier.
What does a Building Control inspector check at DPC stage?
An inspector at DPC stage will check that the DPC is continuous with no breaks, at the correct height above external ground level, and correctly lapped with the DPM at the floor junction. On cavity walls they will also look for cavity trays over openings and at any roof abutments, and check that weep holes are open in the outer leaf above each tray. Missing or incorrectly installed trays are a common reason for a failed inspection.
Can I use polythene cut on site instead of a proprietary cavity tray?
It’s technically possible but not recommended. Site-cut polythene is difficult to form accurately to the stepped profile required at a roof pitch, and it’s hard to fit reliable stop ends without a factory-made product. Proprietary formed cavity trays from manufacturers like Cavity Trays Ltd come with integrated stop ends and step profiles, seat correctly in the cavity, and are much easier for an inspector to verify. The cost difference is small relative to the risk of failure.