How NOT to use cavity trays in flat roof wall abutments

Using horizontal cavity trays correctly in relation to flat roof abutments should be one of the more straightforward elements of a build. Yet we do find cases where cavity trays and cover flashings are not properly linked. We’ll explore a typical example of how NOT to install a cavity tray – and outline the correct steps to take.

Common mistakes we see in the installation of cavity trays in flat roof wall abutments

Take a look at the picture below, showing an opening ready to receive an abutted GRP flat-roof canopy. The horizontal cavity tray has been lost because the bed joint it sits in has been fully pointed up.

Lost cavity tray in horizontal abutment

Typically, this means that the bed joint will be ground out using a mechanical grinder to remove the mortar so that the cover flashing can be dressed into the bed joint.

In another case, shown below, a cavity tray has been installed but again the bed joint has been fully pointed up. When a mechanical grinder has been used to grind out the mortar to receive the flashing, the cavity tray has been damaged.

Damage to cavity tray in horizontal abutment

How to correctly install a cavity tray and flashing over a flat roof wall abutment

The diagram below, taken from our Technical Manual, shows how the cavity tray and cover flashing should be linked and layered so that water is shed from the building.

One of the critical details here is that the cover flashing must sit directly underneath the cavity tray. To do this, the cavity tray must be bedded on a thin bed of mortar, which is then raked out prior to the mortar curing to allow the cover flashing to be installed correctly underneath the cavity tray.

Correct installation of a cavity tray in horizontal abutment

Your surveyor will know if this procedure has been followed as the horizontal cavity tray should be visible, undamaged, over any roof abutments.

The LABC Warranty Technical Manual – free to download - also includes extensive details on installing cavity trays. Specific information can be found in Chapter 7: Superstructure, Page 133.

Are you installing a flat roof canopy to your home? This article discusses where you need to install cavity trays for canopies.


Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.


Was this post helpful? /