Cladding should not fit tightly onto the horizontal surface of any apron flashing; as it could retain moisture and collect dirt and debris. It should have a minimum clearance of 25 mm to allow for cleaning and maintenance.
Apron flashings should be in position before any cladding is installed. Where the finish above the apron is plaster or the durability of the cladding is 50 years, the hidden apron flashing is also required to have a 50-year durability. This durability can be achieved by using a non-ferrous material for the flashing. Alternatively, a two-piece apron should be used where the over flashing has 50-year durability, and the apron flashing can be renewed independently. (See 8.4.4F Two-piece Apron.)
When the parapet walls will be plastered or when other trades are likely to follow the roof cladding installation, the roof cladding and flashings should be protected from damage. Provision should also be made for working and walking on the roof cladding during this time.
Where an apron abuts a block, brick, or concrete wall without any cladding above it, the apron flashing can be weathered by a step or a continuous chased flashing. These flashings can be made from aluminium, stainless steel, or zinc when used with metallic-coated steels and should extend 25 mm into the wall. They should be hooked or mechanically wedged and should be sealed using a compatible, flexible mortar or sealant. Over-clad flashings give better weather protection than chase-cut flashings.
8.4.4A Chased Apron and 8.4.4C Angle Diverter are not the preferred installation methods for apron flashings. 8.4.4B Vertical Cladding (Parapet Flashing and Detail)shows a better method that provides more positive weathering by covering the wall with vertical metal cladding up to the capping.
It is not possible to ensure that all the water discharged from the apron flashing will be collected by the spouting, unless special provisions are made including:
Spouting should be fitted after the wall has been finished and should be clear of the wall cladding.
Aprons should be stop-ended and turned down to weather and bird-proof the end of the apron at this junction.
When flashing a cavity parapet, the apron should be in place before the cavity batten as it is not possible to retrofit the apron. The apron material should have a 50-year durability unless the parapet cladding is easy to replace.
When a chase or rebate (8.4.4G Chase-fixed Flashing) is not provided, a pressure bar flashing can be used to weather an apron flashing into a concrete tilt-up slab wall.
Over-clad apron flashings are always preferred to chase-cut alternatives in structures such as residences where the occurrence of chase-cut weatherproofing failure could have severe consequences.