Horizontal Cavity Trays Over Horizontal Roof Abutments

This article is brought about in order to refresh our knowledge on a simple, yet vitally important, element of general building, in relation to horizontal roof abutments.

Within Chapter 7 of our Technical Manual the detail below can be found showing a horizontal roof abutment.

One of the critical details here is the correct linking of the cavity tray with the cover flashing, in that the cover flashing must sit directly underneath the cavity tray.

It is the incorrect installation of the horizontal cavity trays that is all too common on building sites.

In order to achieve this correct linking the cavity tray must be bedded on a thin bed of mortar, which is then raked out prior to the mortar curing, this is all in order to allow the cover flashing to be installed correctly underneath the cavity tray.

This raking out of the thin mortar bed will allow the horizontal cavity trays to be visible over any such roof abutments whilst walking around a site. Therefore, if no cavity trays can be witnessed over a completed roof abutment, then the construction practices on a site need to be discussed with the site management.

The image below shows an opening which is to receive a GRP flat roof canopy, where you will note the horizontal cavity tray has been lost, by fully pointing up the bed joint the tray sits in. When this occurs most commonly the bed joint is then ground out, using a mechanical grinder to remove the mortar and provide an opening for the cover flashing to dress into the bed joint.

The following photo shows a similar GRP flat roof canopy where the bed joint has been ground out using a mechanical grinder and where evidence can be seen of the cavity tray being damaged. It is this physical damage to the cavity tray that we need to prevent and furthermore ensure that the cavity tray and cover flashing are correctly dressed to shed water.