External masonry walls

6.1.17DPCs and cavity trays

DPCs and related components shall be provided to prevent moisture rising or entering the building. Issues to be taken into account include:

  1. provision of DPCs and cavity trays
  2. stepped cavity trays
  3. parapet details.

Provision of DPCs and cavity trays

DPCs and flexible cavity trays should be of the correct dimensions to suit the detailed design.

At complicated junctions, clear drawings and the design should be provided, and preformed cavity trays used.

The following materials are acceptable for use as DPCs:

  • Bitumen to BS 6398
  • Polyethylene to BS 6515 (except below copings and in parapets)
  • Proprietary materials assessed in accordance with Technical Requirement R3.

Table 9: Positions where DPCs and cavity trays are generally required

LocationProvision of DPCs and cavity trays
Base of walls, piers, etc.A DPC should be provided a minimum 150mm above adjoining surfaces and linked with the DPM in solid floors.
Base of partitions built off oversite where there is no integral DPMThe DPC should be the full width of the partition.
Base of wall built off beam, slab, etc.Detail to prevent entry of damp by driving rain.
ParapetsBeneath coping, and 150mm above adjoining roof surface to link with the roof upstand.
In cavity walls over openings, air bricks, etc.A cavity tray should be provided to direct any water that enters the cavity to the outside. The cavity tray should fully protect the opening.
At the horizontal abutment of all roofs over enclosed areas and balconies to wallsA cavity tray should be provided 150mm above any adjoining roof or balcony surface. The tray should be lapped over any roof upstand or flashing to ensure water penetrating into the cavity does not enter the enclosed area.
At sloping abutments of all roofs over enclosed areas to cavity wallsPreformed stepped cavity trays should be provided above the roof surface and linked to the roof upstand or flashing to ensure any water penetrating into the cavity does not enter the enclosed area.
DoorstepsA DPC should be provided behind a doorstep where it is higher than a wall DPC.
SillsWhere precast concrete or similar sills incorporate joints or are of a permeable material, a DPC should be provided beneath them for the full length and be turned up at the back and the end of the sill.
Jambs in cavityThe reveal should be protected throughout its width by a continuous DPC. The width of the DPC should be sufficient to be fixed to, or overlap, the frame and fully protect the reveal. For very severe exposure conditions, rebated reveal construction or a proprietary closer, suitable for the conditions, should be used.

Cavity trays

Cavity trays should be provided at all interruptions to the cavity (e.g. window and door openings and air bricks) unless otherwise protected (e.g. by overhanging eaves). Cavity trays should:

  • provide an impervious barrier and ensure that water drains outwards
  • cover the end of the lintel and project at least 25mm beyond the outer face of the cavity closer or, where a combined cavity tray and lintel is acceptable, give complete protection to the top of the reveal and vertical DPC
  • provide drip protection to door and window heads
  • have a 140mm minimum upstand from the inside face of the outer leaf to the outside of the inner leaf
  • be shaped to provide 100mm minimum vertical protection above points where mortar droppings could collect
  • be provided where the cavity is bridged by air bricks, etc. and the DPC should extend 150mm beyond each side of the bridge
  • where not otherwise protected (e.g. by a roof at an appropriate level), be provided over meter boxes
  • be in one continuous piece or have sealed or welded joints.

The upstand part of the cavity tray should be returned into the inner leaf unless it is stiff enough to stand against the inner leaf without support. In Scotland, Northern Ireland, the Isle of Man and areas of very severe exposure to driving rain, the upstand part of the damp proof protection should be returned into the inner leaf of masonry (this does not apply at sloping abutments).

Where fairfaced masonry is supported by lintels:

  • weep holes should be provided at a maximum of 450mm intervals
  • each opening should have at least two weep holes
  • cavity trays or combined lintels should have stop ends.

Where the lintel does not require a DPC, it should:

  • have a suitable profile and durability
  • give complete protection to the top of the reveal and vertical DPC, where provided.

Where the cavity has full-fill insulation, a cavity tray should be used above the highest insulation level, unless the insulation is taken to the top of the wall and is in accordance with the manufacturer’s recommendations.

Horizontal DPCs

DPCs should:

  • be the correct width
  • lap the DPM where appropriate
  • be laid on a surface free from projections which could puncture or adversely affect the DPC material
  • be fully bedded on mortar where required by the design, or where the building is over three storeys in height
  • at ground level, generally be a minimum of 150mm above finished ground or paving level
  • where intended to prevent rising damp, joints should have 100mm lapping, or be sealed or welded
  • be used in accordance with the manufacturer’s recommendations.

The concrete fill in a cavity wall should stop at least 225mm below the base DPC. This may be reduced to 150mm where special foundations, such as rafts, are used.

Where a jointed or permeable sill is used, a DPC should be:

  • placed between the sill and the outer leaf
  • turned up at the back and ends of the sill.

At sills where there is:

  • a DPC, it should be lapped with the reveal DPC
  • no DPC, the vertical DPC should be continued 150mm below the sill level.

Special DPC detailing may be required at accessible thresholds.

Vertical DPCs

A separate vertical DPC should be provided around openings, extend to the underside of the lintel, and:

  • be of a proprietary material assessed in accordance with Technical Requirement R3, or
  • 150mm wide DPC material, nailed to the full height of the frame and protrude 25mm into the cavity.

A fillet joint of sealant should not be considered a substitute for good workmanship or DPCs. However, a bead of mastic should be used around openings.

Connections with flashings

Where flashings link with DPCs, (e.g. horizontal or preformed stepped cavity trays), 25mm of mortar below the DPC should also be raked out as the work proceeds to allow for the flashing to be tucked in.

Stepped cavity trays

Where the roof abuts at an angle with the wall, preformed stepped cavity trays should be provided.

To minimise the risk of water ingress below the abutment, preformed stepped cavity trays:

  • should be provided where a roof abuts a cavity wall above an enclosed area, e.g. an attached garage
  • should have two stop ends at the lowest cavity tray and a weep hole to allow water to drain from the cavity
  • are not necessary where the roof is not over an enclosed area, e.g. open car ports and open porches.

Preformed stepped cavity trays should be installed in accordance with the manufacturer’s recommendations and positioned:

  • to suit the dimension of the flashing (which should be in accordance with the manufacturer’s recommendations or a minimum width of 65mm)
  • so that the stepped cavity tray cannot discharge behind flashing (where it is necessary to cut bricks or blocks, the bond should be maintained in the following joint).

Parapet details

Parapet walls should have:

  • a DPC under the coping, and a DPC tray starting 150mm minimum above the roof
  • coping throating which is 40mm clear of the brickwork.

DPCs in parapet walls should be:

  • supported over the cavity to prevent sagging below copings
  • fully bedded in mortar
  • specified to achieve a good key with the mortar
  • sealed to prevent water seeping through the joints.