Case Histories Problems with Stepped and Staggered Gable Abutments
Author/Issuer: Cavity Tray Standards Added:
Location: South Devon, UK.
Hourly mean wind speed category; 22 metres per second.
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.
National NHBC registered house builder engaged upon large housing development.
Stepped and staggered gable abutments throughout the site, constructed using a proprietary tray system with lead flashing attached.
Considerable dampness 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 thus obtained from a different (second) manufacturer.
Following the second installation, the problem persisted and the gables remained damp.
The house builder then contacted a third manufacturer – Cavity Trays Ltd of Yeovil.
Cavity Trays 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 yet again, but this time using Cavity Trays of Yeovil preformed warranted Type X cavitrays.
Installation of the Type X trays resulted in the dampness problem being eliminated.
What originally went wrong?
Analysis regarding the initial installation is not possible, as the problematic gables had already been replaced once before Cavity Trays was contacted. However, inspection of the removed trays originally incorporated suggested capillary action may have been one of several reasons for failure?
Features to interrupt the capillary path are recognised as essential features when executing construction details. Incorrect or inadequate bedding can act as a catalyst in saturated masonry walls, and water permitted to reservoir the bedding course can transgress adjoining masonry. 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. (See conclusion)
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 used meant the full width was not protected whenever the cavity width technically went over-size. It should not occur, but in reality it often does.
It was also noted the size of the lead flashing attached to each tray did not match the length of the tray but was far shorter. It was therefore queried to what extent flashings lapped and whether wind-driven rain had been driven between? When built in following the roofline pitch, small lead flashings offer less overlap protection against wind-driven rain than larger flashings.
Cavity Trays Ltd recommended use of warranted Type X cavitrays – which have high exposure rating. Supplied with attached code 4 lead flashings, the trays benefit 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. Importantly, the size of each attached lead flashing matches the length of the tray, and the lap (and subsequent barrier protection against wind-driven rain) is greater that that witnessed on the alternative products.
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 considerable loss of confidence with the purchasers of the properties affected.
Perhaps the first and second installations might not have proved troublesome on a less exposed site? How can one be sure? In reality one cannot – so it is always prudent to build in anticipation of severe weather conditions. Warranted high exposure rated trays are generally no more expensive than alternatives. The trays that stopped the damp penetration problems were Type X trays from Cavity Trays of Yeovil, and interestingly Cavity Trays is the only tray manufacturer awarded European Technical Approval. The Company also holds LABC Local Authority Building Control Product Type Approval.
Type X trays are also accompanied with product performance liability protection for the benefit of Architect, Builder and Client.
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 provides peace of mind.
Thorough product selection is of paramount importance. Adherence to that selected specification can enable one to get it right – first time.
This article was compiled using information provided by the contractor and Cavity Trays Ltd and was originally published in a revised format by Cavity Trays Ltd in 2002.