Using roll DPC with a conventional top-hat shaped metal lintel necessitates the roll DPC is built into the inner skin to provide support. The rise required within the cavity by the NHBC is 140mm, whilst other bodies require 150mm. Inner skin blockwork is generally at 225mm coursing, so this arrangement usually dictates a far wider roll of DPC is required, to accommodate building into the inner skin, dropping from the 225mm height whilst transversing the cavity, before turning horizontally at lintel base level through the outer skin. Exactly where inner skin support will occur and the need for more DPC is often-overlooked.
Lintel bearings are commonly 100mm or 150mm. Therefore, with an outer skin constructed in (225mm long) brickwork, if face brickwork bonding is to be maintained the brick end perp will occur beyond the end of the lintel in 50 % of instances. Where DPC terminates in relation to the lintel end is an important consideration if stopends are to be incorporated within an appropriate perp and function as intended, but the arrangement is often compromised?
Where openings have lintels other than metal styles, roll DPC is often witnessed built into the inner skin and hanging down, suggesting stopends do not exist at all? No stopends exist in the arrangement shown in the photograph.
The extent of DPC hanging below the head in the photograph also suggests it to be insufficient to terminate forward of the frame head? Again, the width of DPC required has not been fully considered. Where might penetrating water end up if it is unable to evacuate out of the building envelope as per best practice?
Where an opening is not horizontal but shaped, the task of using roll DPC becomes exceedingly challenging. It is not possible for roll DPC to multi-directionally directionally stretch to accommodate and thus protect, such openings through cavity walls.
The use of preformed cavity trays offers the architect and contractor an alternative way forward that eliminates the above variables/concerns. Cavity trays can be scheduled, so exactly the right profile at the right length is provided for each opening. Additionally trays such as Type C cavity trays are self-supporting and can accommodate the as-built cavity width as opposed to the as-intended. (There is commonly a difference). Self-supporting trays do not require an inner skin course support - unless you specifically request a build-in profile.
Where the specification demands an external turn-down drip-lip (NHBC 6.1 D6. B) to provide drip protection to the door or window head, preformed trays can also incorporate this facility, providing a uniform finish, not possible using roll DPC.
Not every opening has a horizontal head, and preformed cavity trays are available in more than 24 different arch styles including Semi-Circular, Two-Radius, Gothic and Moorish. Type BA Barrier arches are ready-moulded DPC’s shaped to harmonise and protect openings in cavity walls and can be used with traditional centring, or curved metal or concrete lintels. The top of each Type BA can terminate in three ways - self-supporting, built into the inner skin or attached using an accompanying Cavistrap.
When raising an external cavity wall, opportunity is not a lengthy visitor. The contractor has one chance to ensure important inclusions such as damp protection measures are correctly shaped and correctly dimensioned. Preformed trays transform cavity wall opening protection into a scheduled undertaking, alongside the window and lintel schedules. The contractor enjoys the elimination of on-site labour variance and gains the added benefit of stock and cost control. Importantly, preformed means it can be right, first time – every time. Cavity trays for all styles of wall openings are manufactured by Yeovil-based Cavity Trays Ltd, the longest established specialist and the only UK tray manufacturer awarded European Technical Approval.