Type of works: supply and installation of Type X Cavitrays by Cavity Trays Ltd.
Hourly mean wind speed category: 22m/s.
Category of exposure to wind driven rain / spell indices: 100+.
Topography category: shallow.
External skin: clay brick to BS 3921, frogs up.
Cavity width: 50mm.
Mortar 1:1:5.5 by volume, weathered-tooled joints.
A national NHBC registered house builder engaged upon a large housing development had used proprietary trays with attached flashings on all the stepped and staggered gable abutments. The trays were not of Cavity Trays Ltd manufacture.
Considerable dampness was witnessed in party walls below stepped tray arrangement to some abutments.
Initially the house builder contacted the manufacturer, but the dampness problem was not solved.
The seriousness of the situation prompted urgent attention, and a decision was made by the house builder to remove the existing trays from the troublesome gables and replace them with a different proprietary make. Replacement trays were obtained from another manufacturer.
Following the second installation, the problem persisted and the gables remained damp.
The house builder then contacted the Help Desk at Cavity Trays Ltd of Yeovil. Cavity Trays Ltd carried out a survey of all the problem gables on site and advised the contractor of its findings. The contractor accepted the submissions and made the decision to execute the work a third time, but this time using Cavity Trays Ltd of Yeovil preformed warranted Type X Cavitrays. Installation of the product resulted in the dampness problem being eliminated.
Full analysis regarding the initial installation is not possible, as Cavity Trays Ltd was not contacted before the gables were taken down. However, inspection of the trays used suggested capillary action may have been one of several factors. Features to interrupt the capillary path are recognised as essential in tray design. Incorrect or inadequate bedding can act as a catalyst in saturated masonry walls, and water permitted to reservoir the bedding course can transmit to adjoining masonry.
This is one of the reasons the British Standard stipulates damp-courses be bedded on mortar. In any stepped abutment, a saturated brick within a tray must be prevented from contacting (and therefore soaking into) the inboard masonry. The end upstand of the tray must provide isolation. If the upstand height is insufficient or is bridged by mortar, dampness will track inwardly.
It is possible the second installation might have been more successful had the cavity upstands (the part of the tray that should rise in the cavity by 150mm minimum) suited the actual cavity width to a greater extent. The fixed dimension trays that had been used meant the full width was not protected whenever the cavity width went over-size. Cavity Trays Ltd also questioned the adequacy of the dimensions of the flashings, and to what extent flashings lapped.
The company recommended removal of the existing products and installation of Cavity Trays Ltd warranted Type X Cavitrays that have high exposure rating. Supplied with attached code 4 lead flashings, the trays have the benefit of anti-capillary tracking measures to their base, plus a flexible isolation integrity barrier to the end upstand, to prevent dampness being inboard transmitted. The main cavity upstands also adjust to take up the maximum cavity width encountered, thus addressing variances. The length and the extent of flashing overlap protection was also greater.
To achieve dry status, the contractor had, in total, purchased three different types of product from three different manufacturers. Thus three costs for materials, three costs for operatives' labour, and three costs for use of scaffolding, plant etc. Because the gables failed to achieve dry status at the first and second installations, the contractor also experienced some loss of confidence with the purchasers of the properties affected. Neighbours whose properties were not suffering dampness voiced concern. They questioned whether they might experience similar problems in the future.
One prices to execute any job just once, and this was clearly a costly exercise for the contractor concerned. On any site, identical properties will behave in different ways. Use of high exposure-rated products with integral features provide peace of mind. Possibly the first and second installations might not have proved troublesome on a less exposed site, or the installations were not perfect.
Cavity Trays Ltd advises it is always prudent to build in anticipation of severe weather conditions and warranted high-exposure rated trays are generally no more expensive than alternatives. Thorough product selection is of paramount importance. Adherence to that selected specification can enable the project to go smoothly, first time. Type X Cavitrays are accompanied with product liability, for the protection and benefit of architect, builder and client. Type X Cavitrays are also the subject of European Technical Approval.
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