Sealants play an important part in cladding because AZ or paint coatings cannot be sealed using solder.
The purpose of sealants used in the joints between metal roof and wall cladding and flashings is to prevent the entry of rain or snow and to exclude dirt, debris, and moisture by capillary action. They may also have varying degrees of adhesive action.
Sealants are generally Neutral Cure silicone or Modified Silicone (MS) types. The latter is generally paintable. Proprietary self-adhesive tape sealants are also entering the market and give excellent performance in certain conditions. Acid cure sealant, designed to bond to inert surfaces such as glass, must not be used.
Designs using sealants to seal sheet components must always include a mechanical fastening such as a screw, rivet or a mechanical seam.
Generally, the function of a sealant is carried out by that sealant being sandwiched in the joint. In most cases, the post application of sealant to the outside of a joint is both futile and unacceptable.
The sealant does not require significant total adhesive strength, but it should bond positively to both surfaces as a continuous layer. To ensure complete sealant cure, the width of sealant in a lap should not exceed 25 mm when compressed
There are two main methods of sealing metal sheets — gun applied sealants and self-adhesive tapes.
Sealants should have a low resistance to compression so that metal sheets can be brought closely together by their fasteners and not held apart by the sealant.
Sealant should be gun-applied in a continuous flow to provide a joint without gaps or voids. This will prevent the entry of both condensation from above and water drawn up by capillary action, either of which can cause metal corrosion.
After placing the sealant cartridge in the gun, the tapered nozzle should be cut with a sharp knife at an angle of 45 ° at a position which would provide a bead of sealant approximately 6 mm in diameter.
Sealed rivets are the preferred fastener. The stem holes of unsealed blind rivets must be sealed with silicone, which can attract dust and look unsightly.
Sealant should be placed in a continuous bead close to the weather end, covering any pre-drilled rivet holes. If a complete ring of sealant is applied around the fastener, an air pocket forms which compresses during tightening of fasteners. It can blow a channel, thus preventing an effective seal.
The best method for lap fabrication is.
Assemble and drill the components. Rivets should be positioned at 50 mm spacings.
Separate the components and remove any drilling swarf.
Remove any strippable film.
Clean the joint.
Apply a continuous sealant bead as described above.
Relocate the components, align the holes and fasten.
Externally seal the centre of all rivets if required.
Remove surplus sealant after it has cured (not while uncured), to avoid smearing.
When placing the overlapping sheet, care should be taken to avoid disturbing the sealant when fastening.
Excess sealant should be removed with a plastic spatula or purpose made plastic scoop as excess sealant collects dirt that is unsightly.
Visible excess of sealant is not an acceptable trade practice.
