The Self Build Self Help Site Keeping Water Out

Cavity Trays

Cavity trays are devices that ensure that any water that has penetrated the outer leaf of a cavity wall is returned back to the outside - hence preventing it from crossing the cavity and penetrating the inner leaf. Of course, water should not penetrate the outer leaf but it could if there is a crack in the render, over absorbent facing bricks etc.

The main areas where they are used are where one roof abuts another and beneath opening for windows and doors.

Roof cavity trays

These are bought as trays that sit within the cavity and lead fused to one side acts as the flashing. Normally they are sized so that they are built within brickettes, so that they are stepped down 75mm at a time. Each tray drains into the one below, so that water is carried down and will exit from the lowest via a weep hole left in the blockwork.

These will invariably fill with cement as they are built into the wall which, if left, may prevent the water cascading down and which may then cross the cavity on wall ties. So ensure that they are cleared of cement at the end of every day of their preparation. You will need to do this by directing a jet of water from a hose down the exposed cavity.

Photo of cavity tray

Photo of cavity trays built in
Cavity trays being built in, to a wooden former
to ensure their correct placement above the finished roof

Click on photos for a larger view

Window cavity trays

These are normally made from 12" or 15" DPC (for a 4" cavity) which is retained underneath the external cill and under a brick course layed on the inner leaf. Again, ensure these are cleared of cement - they are easily accessible and easy to clean out.

Wall ties

These are made with drip kinks such that water will not cross the cavity - unless cement remains attached to them. So ensure these are cleaned off. You may need to knock the cement off at the end of the day by poking a length of batten down the cavity onto each contaminated tie. Tedious, but if the ties are not meant to conduct water then stop them doing it!

Flashing

It seems impossible, but it can happen that rain can rise up behind flashing, over the soakers and hence penetrate into the roof space below and down into the rooms below. This may be due to a combination of driving rain and capillary action - pushing the water up and over the soakers. If this does happen, it can be remedied by just trimming the bottom off the flashing so that it doesn't touch the roof slates/tiles. But ensure the flashing is so trimmed in the first place!

For more advice on different types of flashing, see the Lead Sheet Association site.

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