Cavity trays and water ingress

Most damp ingress occurs around either window/door openings or roof abutments. The correct installation of cavity trays reduces this risk. Understanding the different installations of cavity trays and best practice reduces the chance of water ingress.

Cavity trays – roll damp course material

When using roll damp course (dpc) material, the profile must rise a minimum of 140mm within the cavity and should be shaped to promote the easy removal of mortar droppings. The tray should be positioned immediately above the opening and not several courses above.

Trays formed in this way, using roll dpc material, must have stop ends formed by folding the ends into the ‘perp joint’ of the outer leaf. Building into the inner leaf is required to provide support, and the width of the material must be adequate to reach any supporting course that is not at a suitable height. The tray must extend beyond the lintel ends, and the finishing point must not be less than 25mm beyond any vertical closer forming the reveal.

Preformed cavity trays

Preformed cavity trays can either be built into the inner leaf, or alternatively terminate against it and are therefore equally suitable for both timber frame and masonry construction. Preformed cavity trays can be used on all stepped roof abutments, stop ends must be used.

Whichever cavity tray construction is used, weep holes are required and preformed ‘weeps’ should always be used at maximum 900mm centres and always be positioned at stop ends.

Cavity tray best practice

  1. Cavity trays should be installed over all external door and window openings including bay windows and at roof abutments, both horizontal and pitched.
  2. The cavity tray should be correctly located directly over the window/door head.
  3. Purpose made stepped cavity trays are best utilised for all pitched roof abutments.
  4. Where natural stone/artificial stone heads are being used. It is advisable to double up the cavity trays, one below and one above the head and ensure that the cavity insulation continues to cover behind the stone head.
  5. Ensure weep holes are provided at each end of a horizontal cavity tray and at maximum 900mm centres.
  6. Ensure a weep hole is provided at the base of stepped cavity trays.
  7. Ensure the cavity trays are kept free of mortar droppings.

You can learn more about the correct use of cavity trays and stop ends in Chapter 7.1 of our technical manual.

You can also read our blog on weep holes in rendered walls.

By Craig Ross