Sealants
Sealants are an important component in making a pre-finished steel clad building weather-tight. They are used whenever the pitch or laps of a roof sheet are inadequate to prevent ingress of water or snow through a lapped joint. They are also used to prevent the passage of moisture vapour from within the building through lapped joints, due to the influence of the vapour pressure differential. Seals must provide a continuous, effective barrier to water for the design life of the roof. Sealant manufacturers should be consulted with regard to the suitable choice of sealant for a given application and environment. Further information can also be found in the MCRMA publication, ‘Guidance for the Effective Sealing of End Lap Details in Metal Roofing Constructions’.
Sealants for use in pre-finished steel cladding
Sealants for use on pre-finished steel cladding have special requirements which include:
- Seals need good adhesion. This is required for the hard, non-porous metal surfaces, which are frequently damp, dusty or greasy under external conditions.
- They must have low resistance to compression. It is important that pre-finished steel sheets can be held tightly together by their fasteners, not held apart by the sealant. This compression must be achieved, even with large percentage variations in the gap thicknesses.
- Seals require good flexibility. The sealant in end laps must accommodate extreme movement relative to its thickness, whether in expansion or fixed joints. This sealant shear continues to take place over the full life of the building at all external temperatures.
- They must have resistance to high temperaturevariations. Due to surface radiation, metal sheets reach much higher and lower temperatures than many other building materials.
- Sealants should be capable of being drilled and fastened without displacement or twisting, as fasteners may be passed through the sealant.
- End joints of sealants, especially strip types, must be watertight, with no gaps forming after installation caused by contraction of the sealant.
- The life of the sealant chosen should be compatible with the life of the pre-finished steel cladding system.
Sealant manufacturers can provide specific information about the performance of their products in these conditions.
- Gun-grade sealants. The sealant must be non-setting and must not harden significantly with time. These sealants tend to flow better into corners and allow a closer fit, but depend entirely upon the fixer for integrity. Low modulus and non-setting varieties are best.
- Pre-formed sealant ribbon. Soft types must always be chosen, many are too hard and will not compress to the thin joint required, producing bulges between fasteners.
- Foam sealant tapes, such as Poly Vinyl Chloride (PVC) or Ethylene Propylene Diene Monomer (EPDM) require a certain compression (typically 30%) to be watertight. Strip butyl sealants are thin and good for flashing shapes that nest well and for bridging 90° corners.
- The adhesive qualities of some sealants can make life difficult if a flashing has to be opened up for inspection. Bead butyl strips can be more effective because the bead compresses to fill gaps. To flow, these sealants should be above 5°C at installation, otherwise they become too stiff.
- Pre-compressed expanding foam sealants fill gaps and are good where flashings do not nest well, although they should not be used universally for flashing applications. They exert a pressure onto the flashing and require fixings at closer centres to avoid bulging. These sealants require compression to about 30% of their uncompressed depth to remain watertight. The sealant must be positioned on the weatherside of a fixing if a single line of sealant is used.
Surfaces must be clean and dry and at temperatures above 5°C before sealants are applied. Sealants, particularly ribbon or tapes, should be placed so that they cannot work their way out of the joint. Pre-formed sealants have the advantage of uniformity and continuity of sealant.
It should be noted that sealants must be neutrally curing materials. Sealants using acetic acid to cure must not be used because the acetic acid can ermeate through coatings and attack the zinc coating on the substrate, leaving the pre-finished steel vulnerable to corrosion.
