Changes In Roof Level
In cases where a roof changes level between 2 buildings or there is a step/stagger in a building layout, it can happen that what is an external wall at a high level is an internal wall at a low level. It is important that adequate DPCs are used in these cases. Using a proprietary cavity tray is recommended in situations like these.
In these situations, flashings and soakers are not sufficient because below roof level the external wall becomes an internal wall. Due to this, it is necessary to provide cavity trays to prevent penetrating moisture from running down into the building. This is very important in brickwork outer leaves.
As shown below where a cavity tray is not provided there is a significant risk of moisture gathering within the cavity. This situation is not permitted.
Diagram C49 - Cavity tray not provided
The purpose of a cavity tray is to collect water that penetrate the outer leaf and direct it back outside.
Diagram C50 - Typical cavity tray installation
Different Types Of Cavity Tray
The catchment tray is placed at eave level or slightly above; it has upstands at both ends in order to catch any water that runs from higher intermediate trays and stop water from entering the cavity. As well as having two at either side, some sort of discharge unit needs to be included to deal with the water running from above.
The purpose of the intermediate trays is to divert water from the roof down to the catchment tray where it is discharged. Where possible there should be 1 weephole per cavity; otherwise provide weepholes at 1 m max centres. There should be an overhang of at least 100 mm over the tray below.
The apex tray only differs from the catchment tray in that the lead flashing is different, as illustrated.
Consider the following points when using cavity trays:
Use purpose-made self-weeping trays
Build trays only into the outer leaf
Tray dimensions vary according to roof pitch
When built-in, each cavity tray unit discharges individually onto the roof slope. This avoids a build-up of water flow towards the bottom of the run.
Diagram C51 - Cavity tray types
Setting out is important. Ensure care is taken and that the manufacturer’s instructions are followed. In cases where bricks are cut to ensure equal spacing of trays, the lead flashing from the tray above should cover the cut brick.
Diagram C52 - Typical setting out details
The same trays as outlined can be used for rendered blockwork walls. Concrete bricks should be used in the region of the trays in order to ensure the spacing of the trays is correct.
Diagram C53 - Cavity trays in rendered walls
Flashing and render will hide the bricks and it is also permissible for the render to partly conceal the lead flashing. Render should never be applied directly to the lead however, as this would restrict movement causing the flashing to split or the rendering to break off. The correct method of rendering over flashings is in the next file. Expanded metal mesh should be fixed to the blockwork; stop bead should be placed at least 75 mm from the roof level. This allows movement of the flashings.
It is important that fixings do not pierce the lead. Weepholes in the intermediate trays will be blocked and so it is important that weepholes are provided to the catchment tray to allow the collected water to drain onto the roof.
Diagram C54 - Typical brickwork cavity trays in rendered walls
Diagram C55 - Damp proofing - key junctions
The above junctions require special care when being constructed. It is important that purpose-made products are used and the manufacturer of the product is consulted regarding proper installation.
1) First tray to be built-in; image showing catchment tray incorporating lead flashing.
2) Build the catchment tray into the outer leaf. Shown is the tray built in with mortar placed for next course of brick. A weephole is left in the middle.
3) Do not build any tray less than 75 mm from finished roofline.
4) Intermediate trays are constructed after the catchment tray. The overhang from tray to tray should be at least 100 mm. Cut bricks to be laid so as to be covered by flashings.