Type U Cavitray (Cavity Trays Ltd)

   designers'
   comments

    BS5628 stipulates that all
    previous or jointed sills or
    sub-sills should be
    provided with a DPC for
    the full length and width
    of the sill bed. There is
    also a secondary
    requirement for the DPC
    to be turned up at the
    back and ends for the
    full depth of the sill,
    where
    it is in contact with the
    backing (internal skin). A
    brick sill subjected to a
    20°C temperature rise will
    expand by
    0.10-0.16mm/m
    (millimetres per metre).
    Such thermal movement
    often results in small
    fissures or fractures
    within a sill mortar
    bonding joint. The
    benefit of the sill DPC
    arresting any rainwater
    which enters the sill detail
    is paramount.
    Permits easy compliance
    of NHBC 61 - S4(d) and
    6.2 - D4(a) (Moisture and
    control insulation).
    Exposed site
    classification.
    The preformed rigid
    shape also avoids the
    common site problem of
    misplacement of
    conventional DPC
    material.

use
For use under brick or tile sills, providing a DPC and integral guide for sill formation.

introduction
The formation of a brick or tile sill is easily accomplished with the use of a Type U undersill tray. Trays are supplied in a variety of profiles to suit the shape of sill required. Once bedded in position, the bricklayer has automatically established an undersill DPC. The sill bricks or tiles can then be laid using the profiled tray as integral guide shuttering.

type U cavitray
The Type U cavitray is a preformed DPC unit. It is profiled to also form integral shuttering to receive sills constructed of masonry or tile. The Type U cavitray is enveloped within the building structure providing the ideal purpose-shaped DPC. An upturned external edge lip is supplied to the front of the Type U cavitray. This acts as a guide to provide accurate tile or brick sill alignment.

The entire front section of the Type U which projects forward of the masonry line, is fully detachable. An integral separating link, permits this front portion of the tray to be removed, once the mortar has cured. The front bedding

area of the Type U undersill cavitray which is incorporated within the external skin features apertures which aid positioning and permit transit drainage.

insulation option
Type U undersill trays are now available with an optional 25mm polystyrene insulation barrier, bonded to the vertical upstand (subject to sill design or space permitting). Specify clearly if this option is required.

design
considerations
The Type U Undersill Cavitray may be used with timber frame build construction. The extent of differential movement is usually more pronounced with timber frame and allowance is required for the direction of shrinkage, as per NHBC Performance Standards. This is easily accomplished using the Type U Undersill Cavitray.

An expansion gap should be pointed in with Silicon bond to provide a weatherproof detail. See NHBC Design Performance Standards, 6.2D6(c). (An expansion gap is applicable to all forms of construction but when timber-frames are employed the movement factor is normally greater). At all times the designer should ensure that the proposed sill design is sufficient to create a strong mass of construction in terms of strength, proportions and service.

sizes
Illustrated is the standard profile in which the Type U undersill cavitray is available. Variations to the profile raised rear upstand permit a variety of sill sloped and material thicknesses to be accommodated. Our design department will be pleased to advise upon your requirements and submit recommendations.

Patent restrictions often dictate our undertakings and we will advise the most appropriate Type U style in which your proposed sill detail can be accommodated. Maximum length 2400mm. Glove-lap to make up longer lengths. Minimum 200mm. Stopends for use with Type U undersill cavitrays are supplied separately to permit the installer to incorporate them on site in the most appropriate position. If the sill feature is of a greater width than the opening dimensions, specify sufficient tray length to service the requirement.

type U undersill
(alternative style)
Type U undersill cavitrays are also available for use with solid concrete, reconstructed and natural stone sills. In the example shown, the Type U is acting as an arresting barrier whilst providing DPC integrity between the precast sill and the internal skin.

Cavity Trays Ltd

Type U Cavitrayfor undersills

Type U Cavitray
for undersills