The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
While capillary action is both a durability and a weather tightness issue, it is considered here because it affects durability more often than water tightness, although the two are inter-related.
Capillary action is the ability of a liquid to flow into narrow spaces without the assistance of, and in opposition to, external forces such as gravity. It is caused by the combination of intermolecular forces of surface tension in the water and adhesive forces between the liquid and surrounding surfaces.
The effect can be seen in the drawing up of liquids between the bristles of a paintbrush, in absorbent materials such as blotting paper or a sponge, in a burning candle, a fountain pen, or the cells of a tree. The effect can occur in a tube, but also between two closely spaced mating surfaces.
Gravity will affect the degree of capillary action; a low sloping pair of surfaces will attract liquid more by capillary action than a vertical surface, and a narrow tube will draw a liquid column higher than a wider tube.
Capillary action is an important consideration in cladding installation and design, and can be considered in four main areas:
- Closely stacked sheets of trapezoidal or corrugate profiles, or flat sheet that has the sheet ends exposed to rain, will draw water between the surfaces which can infiltrate a long way into the stack of material. After a short time in the absence of air, it can form volatile corrosive products which are unsightly and detrimental to product life.
While metallic coatings have temporary surface protection against wet storage stain and organic coatings also give some protection, there is no hard and fast rule as to how long this will last; it is up to the roofing contractor to take appropriate measures. Packs of close stacked sheets exposed to water must be fillet or cross stacked to allow natural air movement and drying, before the onset of wet storage stain.
- Capillary action can take place in the side lap of roofing sheets, or between a longitudinal flashing and the adjacent rib. For this reason, side laps should be designed with a capillary break, and when calculating the water carrying capacity of a profile, the allowable water depth is taken as being to the bottom of the capillary bead, not the rib height.
Even corrugate profiles are designed to have an asymmetrical shape between under and over crest. However, this is not normally as effective as the capillary break on a rib profile. This is one of the reasons why the minimum pitch for standard corrugate is 80, although it has been proven to perform at lower pitches in short runs if the dimension of the overlap is not too generous and it is not extending into the water table.
With longitudinal flashings, such as barges, the downturn into the pan should not be tight against the rib, but have a gap to avoid capillary action from occurring.
- Capillary action is also common between the end laps of sheets. When short run sheets were the norm and end laps common, the onset of corrosion normally occurred around the lap despite primer being applied to the surfaces. It was most often concentrated on the upper end of the lap and was caused, not by rain water, but by condensation on the underside of the sheet entering the lap.
End laps on roofing should be avoided where possible. Where they do occur, end laps must be sealed at both ends to avoid ingress of moisture from both internal condensation and external rainfall, and end laps in vertical sheets must be sealed at the top end of the lap.
- The adhesive nature of water that causes capillary action can help drive water up the underside of the sheets at the eaves, rather than discharging into the gutter. Therefore, the ends of all sheets laid to a fall less than 80 require a drip edge; and the minimum roof pitch of standard corrugate is 80, as it is difficult to form a drip edge in that profile.The lower edge of all roofing sheets and flashings laid to a fall of less than 80 must be drip formed into the gutter to prevent capillary action.