Cavity Walls


11 Modern Cavity Walls - Parapets

Parapet walls require careful detailing. At the top of the wall it is good practice to provide a weathered coping. The coping can be once or twice weathered - in other words it can slope in one direction as shown in the picture (below left) or in both directions.  A once weathered coping normally directs the water onto the roof to avoid water running down the external face. In some designs brick copings are used although careful detailing may be required if the copings don't have drips (sometimes called throatings).
A good coping stone will overhang the wall either side and will incorporate small drips to prevent water running back under the coping. A full-width DPC should be bedded in mortar to prevent water penetrating the coping through the coping joints. Unlike the example on the left the DPC should be laid on a rigid support to prevent it sagging into the cavity and allowing water to pond where it may freeze and expand in cold weather. Any sagging may also form a trough and allow water to penetrate the cavity where the DPC is lapped.
Because parapet walls are exposed on both sides a cavity tray is required to prevent water running down the cavity face of the inner leaf and penetrating the building. The example on the left shows the tray stepping down to the outer face; water escapes through weepholes. Although this may cause minor staining of the wall it is sometimes preferable to sloping the tray inwards. This is because sloping the tray inwards may allow rainwater to run along the underside of the tray and reach the internal leaf. Click on the picture for an example. In moderate or sheltered exposures this is not normally an issue. However, if the cavity contains cavity batts the tray should always slope outwards to protect the top of the insulation. If it slopes inwards there is the risk of water running down the cavity face of the external leaf and crossing the cavity on top of the batts. 
Flashings should be in the same joint as the cavity tray (DPC) and always under them to prevent the risk of water ingress.
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