Cavity Tray Standards – DPC placement and positioning
Author/Issuer: Cavity Tray Standards Added:
Design Considerations – Identification Report
When installing damp courses and trays in cavity construction, functionality is dependent on correct placement.
Any roll dpc or preformed cavity tray incorporated within a cavity wall will only provide the requisite protection and functionality if the installer incorporates it in an appropriate position and height to suit the design of the wall. In the United Kingdom construction work must be executed in accordance with the Building Regulations. This identification report draws attention to the relevant Codes of Practice and British Standards. It also refers to the build quality demanded by the NHBC and Zurich Insurances.
How should a dpc be positioned within the thickness of a masonry skin?
For conventional dpc laid at the base of a wall to function, it should be within a fresh mortar bed to provide protection (and thus functionality) throughout the thickness of the skin in which it is incorporated. Failure of the dpc to finish flush with or project slightly forward of the external masonry face can result in dampness bypassing the dpc. Do not cover edges of the dpc with mortar. Within the cavity do not permit the dpc to project and create ledges upon which mortar can collect. At this level dpc’s are commonly installed a minimum of 150mm above ground level – always check site requirements.
Base of wall dpc’s should cover the full width of the masonry leaf if they are to prevent the passage of moisture effectively.
See report DC/06/18/3 for guidance regarding the bedding of dpc’s and cavity trays
One can apply this understanding when dpc is incorporated at a higher level. Window and door openings within a cavity wall benefit structural support from lintels but provision must also be made for protection against damp. The dpc must be appropriately shaped and extend through the thickness of the exterior skin, after which it must rise and cross within the cavity to a height of 150mm minimum, of which not less than 100mm is as shown. (After which if it is roll dpc requiring support to hold it in shape, it is built into the first available horizontal course of the inner skin). Again, functionality will not be assured unless the dpc travels through the thickness of the exterior skin.
Dpc’s and trays for use with lintels are commonly installed at the lintel bedding course level and extend sufficiently past the lintel ends to afford cover to the reveal arrangement. (British Standards suggest 25mm clear is the minimum dimension).
An often-overlooked detail is the forming of an external drip over the window or door opening. Whilst this British Standard and NHBC observation is not practical using conventional DPC, it is achievable using a preformed cavity tray which is our recommendation if this benefit is sought.
Where the dpc is required to protect a change of level, such as a lower roof intersecting the cavity wall, an accompanying flashing needs to be incorporated. The relationship between dpc and flashing is of paramount importance as is the positioning and finish of the installation within the masonry parameters:
British Standard 5628-3:2005 stipulates the flashing should be located below an associated dpc and/or cavity tray. The flashing is required to extend into the bedding course a minimum of 25mm. The Lead Sheet Association goes further and provides guidelines on how theenveloped edge of separate flashings can be bent over to aid location and promote a satisfactory relationship.
Visually when a flashing is present, the dpc should not project externally beyond that flashing. Wind-driven rain must be prevented from accessing or flooding any layered relationship between the two materials, hence the flashing is always under the dpc and the flashing only is visible, emerging from the (preferably) flush-jointed masonry joint. Thus the fundamental difference when flashings are used with a dpc is that the dpc is rarely visible.
Unacceptable Installation: The DPC extends past the exterior face exposing the flashing / DPC join. Flashings should emerge from a struck mortar bed and be dressed vertically downwards against the masonry face. In this example the flashing travels horizontally, presenting a table upon which wind-driven rain collects to attack where flashing and DPC join.
Horizontal cavity trays with attached flashings
There is an exception to both the above approaches. One can use a preformed cavity tray with a flashing already bonded on. Trays with attached flashings follow much the same rules, but with the link between the two materials already established, the union is not in doubt and the installer achieves dpc and flashing placement in one operation. Any tray must be enveloped within a mortar bed and it is the flashing only that should emerge from the struck mortar. Horizontal intersection trays are commonly installed 150mm above the abutting roof surface – always check specific site requirements.
Visually one should not see the horizontal base of the tray. It should be hidden (and thus protected) from the weather. If it is visible, the installation is probably unacceptable. (Some tray types have features to prevent the tray being pushed (and thus built-in) too far back or too far forward in the masonry skin).
Acceptable Installation: Using cavity trays with attached lead flashings, the flashings emerge from the bedding course and the tray base is not visible. Weepholes in central perp joints provide the evacuation route for water arrested by the trays. All flashings are dressed vertically and tightly against the masonry.
Cavity trays with attached flashings for sloping abutments.
Preformed trays are easier to use on sloping abutments compared with fabricating using roll materials on site. Again some manufacturers provide trays with features to prevent positioning the tray too far forward or too far back in the masonry skin. Where flashings are already attached, they should emerge from the bedding course and the tray base should not be visible. Gable abutment trays are commonly installed with the inboard end of the tray 75mm off the finished roofline when measured as shown. It is recommended you always check the specific site requirements.
Important: If the flashings are not already attached to the tray but are being cut and offered up later in the build programme, it is most important the mortar underneath the dpc/tray is raked out a minimum of 25mm whilst green.
Correct relationship with flashing under dpc tray
Later flashings can be inserted underneath the dpc medium in the wall, and the two materials effectively lapped 25mm. Flashings are then wedged and pointed-in.
Wrong relationship with flashing positioned above dpc tray
Prior to building in any tray or dpc, always read the installation instructions and the site instructions. Ensure you observe those relevant to the specific circumstances in which you propose to build.
Trays, dpc’s and masonry should always be laid on fresh mortar.
Dpc’s at the base of a wall should protect the entire thickness of the masonry leaf but should not protrude into the cavity.
Generally trays and dpc’s at intermediate levels installed without an accompanying flashing, finish flush with the external wall face unless the style used incorporates an external lip, as identified within the British Standard 5628.
Generally trays and dpc’s at horizontal abutments are used with an accompanying flashing and it is the flashing only that should emerge from the bedding course – the tray/dpc should not be visible.
Flush jointing as opposed to pointing in afterwards provides superior weathering and protective qualities and should be employed on masonry around flashings if possible.
Emerging flashings should immediately drop vertically against the masonry face – avoid dressing flashings in any other plane.
Flashings exposed to high winds can be secured using specialist fixings or anchoring caulking, so determine whether such restraints are desirable early in the build programme.
If using conventional dpc or open-ended trays, individual stopends can be introduced to prevent cavity discharge to comply with construction requirements. (Many cavity trays have integral stopends).
Remember to incorporate weeps that promote immediate discharge of water arrested in trays or dpc arrangements.
Lead-attached systems promote an assured relationship between flashing and dpc so the join is of a consistent quality.
If introducing separate flashings, observe the Lead Sheet Association guidelines regarding maximum dimensions of individual pieces of lead. Failure due to expansion and contraction can be avoided by keeping the sizes of flashings within the recommended parameters.
If in doubt, request direction.
Suggested reading: BS 5628. BS 8215. Part L and M of the Building Regulations.