About Stepped Cavity Trays
There are three main types of stepped cavity trays:
The ridge tray sits at the top of the cavity tray run, above the apex of the new abutting stepped roofline. When installed, it should slightly overlap the cavity tray in the brick, block or stonework below.
Ridge trays are open-ended at both sides to allow collected water run off down to the intermediate trays below it.
The main body of the stepped cavity tray run is made up of intermediate trays. One intermediate tray is installed into each brick, block or stone block course that runs along the abutting roof.
Intermediate trays have one closed end and one open end. The closed end sits closest to the roofline, protecting it from water ingress. This is why there are right and left-handed intermediate trays.
It’s essential that the correct handed trays are specified and installed. The handing of the tray is determined by the position of the open end of the tray as you look at it face on. If it’s open on the left, it’s a left-handed tray. The open end points away from the roof to allow the collected water from throughout the cavity tray run to cascade down to the tray below until it reaches the stop-end starter.
Stop-end starter trays – either flush wall or corner trays
The installation of a stepped cavity tray run or system should always start at the bottom, with the installation of a stop-end starter tray at the lowest point of the new roof’s abutment.
Stop-end starter trays are double-sided, to prevent the water from going any further. A weep hole vent is positioned in the tray – at the furthest end away from the roof. It’s through here that the water is evacuated back to the outside. Stop-end starter trays are also handed, but only when they are supplied with lead pre-attached.
Every Timloc stepped cavity tray has a self-supporting back-up stand to protect the inside wall, by capturing any incoming water and delivering it safely back out through wall weeps that are installed throughout the cavity tray run.
Important specification and installation considerations and recommendations
When specifying or installing stepped cavity trays, it is important to consider the following recommendations:
• ensure that the run of cavity trays protects the abutment fully from eaves to ridge
• the first (lowest) cavity tray should ideally protect just beyond the eaves line of the roof
• further cavity trays must then be installed on every course for the full length of the abutting roof slope
• the cavity trays should be positioned on a line parallel to and 75mm above the top of the finished roofline
• in order to set out the cavity trays accurately, it is essential that some sort of guideline is set up for the bricklayer to work to
• allowance must be made for the thickness of the roof timbers, tiles and battens, so that the line represents the top of the finished roofline
• this is most easily achieved if an appropriate roof truss is available, which can be set up in the correct position and used to secure the guideline.
How should stepped cavity trays be bedded?
Stepped 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 base of the cavity tray should be positioned approximately half way 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.
Making a start
The first cavity tray installed at the lowest point on the abutment must always be a starter tray of some description. This type of tray is closed at both ends and collects the water that has been gathered by the rest of the cavity tray system.
If the side wall of the abutting building lies within the width of the main building, then a stop-end starter tray is used. If the side wall of the abutting building is in line with, or steps out beyond, the side wall of the main building, then a corner starter tray is used.
Positioning of intermediate trays
The main body of the cavity tray run is made up of a series of intermediate trays, with one positioned on every course for the full length of the abutment. Intermediate trays have one closed end and one open end, which results in them being either left or right-handed.
It is very important to ensure that the correct hand is used to suit the direction of the roof slope. The closed end of the intermediate tray must be positioned closest to the roofline, with the open end pointing away from the roof. The corner of the closed end of the tray should be positioned on the guideline, 75mm above the finished roofline.
Each cavity tray should overlap the next. If it is found that there is little or no overlap between the cavity trays, when they are correctly positioned on the guideline, then something is wrong. Please check with the Timloc Technical Services Team by calling 01405 765567.
Weep holes must be provided in the face of the wall. These are essential as, without weep holes, the water collected by the cavity trays cannot discharge from the wall.
In sheltered areas where the cavity trays are not required to cope with serious water penetration for long periods, then a single weep hole is satisfactory. This must be positioned at the bottom of the run in the stop-end starter tray or corner starter tray.
In more exposed locations, or if it is known that particularly porous facing brickwork is being used, then additional weep holes must be provided at other locations along the run of the cavity trays. Please consult the Timloc Technical Services Team if you have any questions.
Weep holes can be formed by installing purpose made plastic wall weep units (Timloc 1143 and TW1 ranges). Link to product page?
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.
Calculating quantities of stepped cavity trays
Timloc recommends that customers take advantage of our technical advisory service.
The Timloc Technical Services Team will be pleased to advise on the most suitable products to suit the application and will prepare a fully-itemised schedule of quantities if required.
Please provide as much information as possible, including a dimensioned drawing (if available). Please see our Upload Your Drawing facility at the top of the screen.
Stepped cavity trays are to be used where a pitched roof abuts a cavity wall.
To calculate the quantity of trays required to cover a section of roof abutment, one of three measurements must be determined, either the:
• vertical height
• sloping length of the abutment
• horizontal length of the abutment.
If the vertical height is measured, simply divide this distance by the coursing height of the material being used for construction. For example, if the vertical height is 1.5m and the wall is standard 75mm brick coursing (NB 75mm = 0.075m), the equation would be 1.5 ÷ 0.075 = 20. Therefore 20 cavity trays are required per sloping abutment.
If the sloping or horizontal distance has been measured, the tables shown below should be used to convert the distance (measured in metres) into the quantity of cavity trays. Take care to select the correct table and the appropriate column that relates to the coursing height and the pitch of the abutting roof.
For example, if the sloping measurement is 2.5m, at a pitch of 30°, with a 75mm brick coursing height, the equation would be 2.5 x 6.7 = 16.75. This would be rounded up, so 17 cavity trays are required.
If the horizontal measurement is 1.5m, at a pitch of 40° with a 150mm stone coursing height, the equation would be 1.5 x 4.3 = 6.45. This would be rounded down so 6 cavity trays are required.
Don’t forget that each stepped cavity tray run requires a ridge tray, intermediate trays and stop-end starter trays.