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6.1.17 DPCs 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:
- provision of DPCs and cavity trays
- stepped cavity trays
- parapet details.
Provision of DPCs and cavity trays#
DPCs and flexible cavity trays should be of the correct dimensions to suit the detailed design.
High Bond DPC can be used for applications including parapet walls, beneath copings and cappings.
High load DPC should be used where it will be subject to a full storey of masonry.
The following materials are acceptable for use as DPCs:
| Bitumen based materials | BS 6398, BS EN 14967 |
| Polyethylene (except as cavity trays in walls, below copings and in parapets) | BS 6515, BS EN 14909 |
| Proprietary materials | Proprietary materials Technical Requirement R3 |
Table 8: Positions where DPCs and cavity trays are generally required
| Location | Provision 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 DPM | The 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 |
| Parapets | Beneath 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 walls | A 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 walls | Preformed 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 |
| Doorsteps | A DPC should be provided behind a doorstep where it is higher than a wall DPC |
| Sills | Where 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 cavity | The 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 |
| Stone heads | Where precast concrete or similar stone heads incorporate joints or are made of a permeable material, a DPC should be provided beneath them for the full length and built into or fixed to the inner leaf wall |
| Above gas membranes bridging the cavity | Where gas membranes bridge the cavity a cavity tray should be provided. Cavity trays should be sealed to the gas membrane in accordance with manufacturers instructions to prevent capillary damp ingress at the joint |
Cavity trays
Cavity trays should be provided at all interruptions to the cavity (eg window and door openings and air bricks) unless otherwise protected (eg by overhanging eaves). Cavity trays should:
- meet the requirements of BS EN 14909 and hold certification from an appropriate independent technical approvals authority accepted by NHBC confirming suitability for use as a cavity tray
- provide an impervious barrier and ensure that water drains outwards
- always be provided with stop ends where discontinuous
- project sufficiently beyond lintel ends and cavity face of the cavity closer or vertical DPC; forming a stop end in the nearest naturally occurring perpend joint
- be laid on a wet, even bed of mortar, free from projections which could puncture or adversely affect the DPC material; with masonry above bedded on wet mortar to ensure the DPC material is ‘sandwiched’
- 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
- not be low-density polyethylene (LDPE) to BS 6515
- 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 (eg by a roof at an appropriate level), be provided over meter boxes
- be in one continuous piece or where necessary have sealed or welded joints which are provided with rigid support and lapped at least 100mm
- be preformed where used at complicated junctions
- be used in accordance with the manufacturer's recommendation.
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 fair faced masonry is supported by lintels:
- weepholes should be provided at a maximum of 450mm intervals
- each opening should have at least two weepholes
- 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.
Weepholes
Weepholes in cavity walls should be the equivalent of a full brick perpend joint, eg 65mm x 10mm where exposed within the cavity. The size of the discharge opening in proprietary weepholes may be smaller, provided it is designed to discharge any water collected, safely. The end of the weephole within the cavity should be kept clear of mortar droppings.
Weepholes to cavity walls should be provided:
- to cavity trays above openings and penetrations through the wall (such as sub-floor vents, ducts, or flues), at least two per opening at not more than 450mm centres
- at least one to the bottom tray in a series of stepped cavity trays, eg at pitched roof abutments
- on cavity trays in parapet walls or at horizontal roof abutments at not more than 1m centres
- to cavity trays above inset gas meter boxes, a minimum of 180mm from the edge of the meter box at not more than 1m centres. See figure 30.
For guidance on weepholes to rendered walls see Chapter 6.11 ‘Render’.
Where masonry cladding is installed to timber framed structures, the guidance on design of weep vents in Chapter 6.2 ‘External timber framed walls’ should be followed.
Complicated junctions
Changes of direction of a cavity tray or interfaces with other elements in the cavity are more complicated than simple joints and would involve complex bending, folding, or cutting and sealing if fabricated on site. At complicated junctions, clear drawings and the design should be provided, and preformed cavity trays used.
Complicated junctions include:
- pitched roof abutments (ie, stepped trays)
- steps in horizontal level
- internal corners
- external corners
- t-junctions (ie, intersection of parapet wall)
- door thresholds (in conjunction with gas membranes, flat roofing detailing etc)
- penetrations in horizontal cavity tray arrangements (ie, telescopic vents, services etc)
- interfaces with windposts, balcony supports, balustrading or guarding.
Where preformed cavity trays are used, the joint with the flexible DPC cavity tray should be sealed in accordance with the manufacturer’s recommendations.
Horizontal DPCs
DPCs should:
- be the correct width
- lap the DPM where appropriate
- be laid on a wet, even bed of mortar, free from projections which could puncture or adversely affect the DPC material; with masonry above bedded on wet mortar to ensure the DPC material is ‘sandwiched’
- 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 considered in the design of masonry wall panel
- 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.
Cavity trays and insulation material
Where full fill or partial fill insulation is installed, particular care needs to be taken to ensure the insulation continuity is maintained around the cavity tray.
Connections with flashings
Where flashings link with DPCs, (eg 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.
Joints between the masonry and flashing should be pointed with cement mortar or suitable exterior grade sealant (polysulfide or neutral-cured silicone) in accordance with the manufacturer’s recommendations.
Arches
At openings with an arched head or circular windows it is not practical to shape a flexible DPC and achieve the correct protection and a preformed cavity tray should be used.
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, eg an attached garage
- should have two stop ends at the lowest cavity tray and a weephole to allow water to drain from the cavity
- are not necessary where the roof is not over an enclosed area, eg 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 30mm clear of the brickwork
- copings should comply with BS 5642 Parts 1 & 2.
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.
Figure 35: Parapet walls
Figure 36: Copings/cappings to parapet walls
Figure 37: Parapet to external wall junction - DPC/cavity tray arrangement
Last updated: 2nd January 2024