The example in steel shown on the preceding page combines
structural support with a damp proof course, without the need for
temporary support from a centre. Where traditional centring is
retained, a lightweight preformed polypropylene cavity tray/dpc
can be used. These factory made plastic trays are produced in
various thicknesses of 1.5 to 3 mm relative to spans up to about
2 m. Arch centres are made to match the tray profile and with
care can be reused several times.
An alternative material is code 4 lead sheet*. Lead is an adaptable
material but relatively heavy. Therefore, its suitability is limited to
small spans particularly with non-standard profiles.
*BS EN 12588: Lead and lead alloys. Rolled lead sheet for building
purposes. Lead sheet is coded numerically from 3 to 8 (1.25 to
3.50 mm † see page 452), which closely relates to the traditional
specification in lbs./sq. ft.
Ref. BS 5628-3: Code of practice for the use of masonry. Materials
and components, design and workmanship.
Alternative Arch Cavity Tray