Sealed Laps

A sealed horizontal lap must be 25 mm wide and filled with one 6 mm or two 4 mm sealant beads placed at the centre of the lap, covering any previously drilled rivet holes. (see drawing 5.6.4.1.)

Sealant must be neutral cure silicone or modified sylal. (MS)

The beads must be laid to avoid any air entrapment so that they cover the drilled fastener holes in a continuous line to ensure that the sealant under compression positively seals the fastener.

Sealed rivets are the preferred fastener. The stem holes of normal blind rivets must be sealed with silicone.

 

 

Fill Laps with Sealant

Fill Laps with Sealant

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If a complete ring of sealant is placed around the fastener, an air pocket forms which compresses during tightening of fasteners. It can blow a channel, thus preventing an effective seal.

The width of sealed laps should not exceed 25 mm when compressed, as this may prevent complete curing of the sealant. The lap should be sealed top and bottom or both ends if the transverse metal lap exceeds 25 mm, as is usual with metal cladding.

The best method for lap fabrication is.

1. Assemble and drill the components.
2. Separate the components and remove any drilling swarf.
3. Remove any strippable film.
4. Clean the joint surfaces as described above.
5. Apply a continuous sealant bead as described above.
6. Relocate the components, align the holes and fasten.
7. Externally seal the centre of all rivets.
8. Remove surplus sealant.

When placing the overlapping sheet, care should be taken to avoid disturbing the sealant when fastening. However, compression of the sealant is essential to achieve a satisfactory weather seal.

The correct placement and quantity of sealant is required to make the joint, as any silicone outside of the joint is wasted and any cleanup and squeeze-out removal will be minimised. Excess sealant should be removed with a plastic spatula or purpose made plastic scoop as excess sealant collects dirt that causes corrosion and is unsightly.

All surplus sealant should be cleaned off with detergent and a damp rag before it cures and the neatness of a sealed joint displays evidence of "good trade practice". Any sealant that can be seen is wasted sealant and as unmodified silicone sealants cannot be painted this area cannot be protected.

The joint should be finished within five minutes after sealant extrusion to prevent premature curing causing poor bonding to the second surface.

Visible excess of sealant is not acceptable trade practice.