Where access into a building is required to be at the same level internally and externally, the designer must consider how damp and water transference is to be prevented.

At the opening threshold the internal floor must provide connection with the external approach but in a manner that will not support wet permeating inwardly. There are also the reveals to consider: At the corners of each opening the floor dpc membrane must integrate with the threshold dpc and in turn must integrate with the vertical reveal closing dpc. These three are joined by the horizontal inner and outer skin dpc’s that are often at a different level – because of the statutory requirement for them to be not less than 150mm off the external ground level.

Looking below floor level, BS 8102 can come into play. Section 6.3 states waterproofing protection falls into three classifications:

Barrier protection.
Structurally integral protection.
Drained protection.

Threshold designs must be able to drain conventionally yet be isolated against damp transference whilst permitting integration with the structure adjacent. An approach is required that is technically a cavity wall damp proof course capable of performing above and below ground level.

The Type LTT by Cavity Trays provides a standardised damp prevention solution for use on inner and outer skins supporting a level threshold. It isolates an area susceptible to damp transference, permitting flooring, insulation, dpc’s and membranes to merge with both skins that are themselves isolated from dampness at the point of convergence.

The Type LTT is manufactured in dimensions to accommodate all standard door openings and skin thicknesses, as well as being offered on a bespoke basis. Preformed from solid polypropylene dpc, it requires bedding in position only. (The exception being when used on the outside skin where it may be trimmed back following installation of the frame).

The wrap-around section at each end of the Type LTT provides a positive upstand against which a vertical cavicloser may locate.

These wraps also form protected areas into which dpc running horizontally in each skin can terminate. (such dpc’s are normally higher than the threshold level).

Regardless of whether the floor construction terminates on or against the inner skin or outer skin, damp transference through the masonry or rising within it cannot permeate inwardly as each skin is an encapsulated and isolated structural element. The threshold is protected.

Compatible compressible thermal cavistops are available for fitting between the Type LTT where thermal enhancement is required.

Type LTT trays are extensively used to provide a consistent preformed approach when constructing level thresholds in private, commercial, educational and industrial buildings. Type LTT Level Threshold Trays are popular with house builders seeking a standardised level threshold specification solution.

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