Calculating horizontal cavity tray quantities

Calculating the quantity of horizontal cavity trays required is reasonably straightforward:

• simply measure the overall width of the abutting roof, or width of the area where cavity tray protection is required
• divide this measurement by the effective length of the cavity tray component which you have chosen to use
• if the calculation doesn’t come to an exact figure, round up to the next whole number.

For example, if you have an abutment 2.5m in width and you are using a System 2000E horizontal cavity tray, divide 2.5m by the effective length of the tray, which in this case is 440mm or 0.44m. This gives a figure of 5.68, which would be rounded up to 6.

Important considerations and recommendations:

• allow for the run of cavity trays to be slightly longer than the overall width of the roof abutment or area requiring cavity tray protection
• a minimum projection of 150mm at each end of the cavity tray run is recommended
• when installing horizontal cavity trays over a roof abutment, the line of the cavity trays should be approximately 150mm above the top of the finished roof line (a minimum of 75mm above the top of the finished roofline
• this distance will vary slightly depending on how the roof line falls a minimum of 75mm above the top of the finished roofline
• the exposed area between the base of the cavity tray and the roof covering must then be fully protected by an appropriate external flashing

How should cavity trays be bedded?

Cavity trays must always be bedded onto fresh mortar, never dry bedded. The brickwork, which is then laid over the cavity trays, must also be bedded onto fresh mortar. The result should be that the base of the cavity tray is positioned approximately halfway through the mortar joint.

The cavity area must be kept clean and free from mortar droppings and debris. A build-up of mortar within the cavity tray could seriously affect the cavity tray’s performance.

Stop-ends

A stop-end must always be fitted at the start and finish of a run of cavity trays. The Inter-loc cavity tray has integrated stop-ends, but the System 2000 requires separate stop-end units. These stop-ends fit by means of a pre-applied butyl mastic seal.

In order for this to seal correctly, it is essential that the release paper is removed, the joint area is clean and dry and pressure is applied along the full length of the joint area once the stop-end and cavity tray are brought together.

It will usually be necessary to link a number of cavity tray sections in order to form a continuous cavity tray run of the required length. The Inter-loc range has an integrated capping system for joining units, whereas the System 2000 range has cavity tray sections that link by means of a lapped joint with a pre-applied butyl mastic seal.

In order for this to seal correctly, it is essential that the release paper is removed, the joint area is clean and dry and pressure is applied along the full length of the joint area once the two sections of cavity tray are brought together. If there is any doubt about the integrity of any joint, do not proceed with further installation until the joint is made good – apply additional sealant if necessary.

Weep holes

Weep holes must be provided in the face of the wall along the run of the cavity trays. These are essential as, without weep holes, the water collected by the cavity trays cannot discharge from the wall.

Weep holes can be formed by simply installing a Timloc purpose-made plastic wall weep unit (see Timloc 1143 or TW1 ranges).  Weep holes should be spaced every 450mm along the run of cavity trays over lintels and 900mm over horizontal cavity trays. Ensure one weep per Inter-loc cavity tray.

If it is necessary for the cavity trays to turn around a corner on the building, always use a purpose-made, one-piece corner unit.  Never try to site fabricate this critical detail.

High-quality workmanship, cleanliness and attention to detail are essential with all cavity tray installations. If there is any doubt regarding the correct method of installation, assistance must be sought from the Timloc Technical Services Team or your local Regional Technical Adviser.

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