What makes weatherboards weathertight?

This Issue This is a part of the Claddings feature

By - , Build 109

A detailed study of water leakage through 14 walls clad with weatherboards sheds some light on the origin of leaks.

Figure 1: Comparison of leakage rates between preprimed and painted rusticated weatherboards.
Figure 2: Comparison of leakage rates between weatherboards with different gap widths.
Figure 3: Comparison of leakage rates between uPVC weatherboards and those without butt joiners.

It might appear that claddings have been forgotten in the debate about leaking buildings. While much of the discussion in recent years has centred on timber treatment, cavities and flashings, effective claddings are still regarded as a crucial part of weathertight buildings. In fact, there is a stand-alone performance requirement for the cladding on any cavity wall tested to Building Code Verification Method E2/VM1. Apart from breaking new ground internationally with this cladding test, the Verification Method makes a clear statement about the importance of the claddings.

Weatherboard claddings have a long and relatively trouble-free history in New Zealand, but aspects of their design, such as lap dimensions, weather grooves, drainage paths and air leakage paths, are often questioned.

BRANZ project

BRANZ has made a detailed study of water leakage through 14 weatherboard walls to understand the origin of leaks and how traditional claddings stack up against the test in E2/VM1. All specimens were 2.4 m by 2.4 m walls built by appropriate trades. Their leakage characteristics were measured in the pressure box used for all BRANZ E2/VM1 tests. Only one of the traditional weatherboard walls failed the test by allowing water to spatter across to the building wrap.

Figure 1: Comparison of leakage rates between preprimed and painted rusticated weatherboards.

Gravity leakage paths

Gravity leakage paths were more common than expected. Leaks through cracks around fixings and defects, such as knots, were common in timber weatherboard walls. Painting the wall dramatically reduced the leakage at these sites. Even after painting, these gravity leaks were more significant than leakage over the lap joint between boards at normal wind pressures.

Much of the discussion about the weathertightness of weatherboards centres on overlap dimensions and the role of the weather groove. These results may shift some emphasis to the defects that simply allow water to run in under gravity. Results for a rusticated weatherboard profile show the difference between a wall in pre-primed weatherboard and the same wall after it has been painted (see Figure 1). The scale for leakage rate is omitted on all graphs because the test applies extreme rainfall that would be unusual in practice. Nevertheless, the comparisons are still valid.

Water leaks through lap joints

Water travels through the lap joints in two ways – first, by wind pressures lifting water hydrostatically over the lap and, second, by air entraining water as it passes through the cladding. Both of these are driven by the same air pressure difference across the cladding which, it must be remembered, will be less than the total wind pressure across the wall because some pressure will be supported by the wall wrap and internal lining.

Figure 2: Comparison of leakage rates between weatherboards with different gap widths.

A series of leakage rate measurements show air-entrained water leakage increasing with gap width in a series of composite weatherboards where concealed nailing had not held the laps tightly together (Figure 2). These leaks occur at much lower pressures than for water lifted hydrostatically over the lap joint, which tends to start at 200–300 Pa, depending on the overlap dimension.

Weather grooves in lap joints

Weather grooves are used throughout buildings to prevent water in capillary joints from reaching sensitive materials, but their influence over wind-driven water leaks in the lap joint between weatherboards is less clear. Although it has been shown theoretically that weather grooves may help a little, this advantage failed to show up in a series of careful comparisons between timber bevel-backed weatherboard walls with and without weather grooves. This result might simplify the process of deciding where a weather groove is important and where it serves no useful purpose.

Non-timber weatherboards

Walls built with composite, metal and plastic weatherboards were largely free of the cracks and imperfections seen in timber weatherboards, but were sometimes let down by the quality of butt jointers. Figure 3 illustrates this weakness in one of the experimental uPVC weatherboard walls. In this case, the butt jointer allowed water to drain past the cladding under gravity. The data from this experiment helped the manufacturer to significantly improve the overall weathertightness of the cladding.

Figure 3: Comparison of leakage rates between uPVC weatherboards and those without butt joiners.

Weatherboard claddings are extremely clever in the way they deflect water from a building and cope with occasional water leaks with drainage and ventilation. Although many of these capabilities are only partly understood, the measurements described above offer a few pointers on how to minimise rain entry.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Comparison of leakage rates between preprimed and painted rusticated weatherboards.
Figure 2: Comparison of leakage rates between weatherboards with different gap widths.
Figure 3: Comparison of leakage rates between uPVC weatherboards and those without butt joiners.

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