COP v3.0:flashings; sealants

7.9 Sealants 

 

Sealants play an important part in cladding because AZ and organic coatings on metal roof and wall cladding 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;
  • end laps in flashings;
  • side lapped joints between profiled sheets where the pitch of the roof is less than that permitted for the profile (curved roofs);
  • end lapped joints between metal and plastic profiled sheets if the length of the roof cladding is less than the rafter length;
  • side laps of plastic and metal roof cladding;
  • any overlap that constitutes a joint designed to allow relative movement between flashings;
  • penetration flashings where the back curb acts as a gutter;
  • the passage of moisture vapour from within the building under the influence of vapour pressure differential; and
  • joints or outlets in spouting and gutters;

This COP uses "sealant" to describe polymeric sealants used in roof and wall cladding, but which act differently to other sealants used in the building industry. The main use of sealants in the building industry is to accommodate thermal movement in a building without any through-fastening. Fasteners are always used in conjunction with sealants in the installation of roof and wall cladding because the sealant thickness sandwiched between two pieces of metal is insufficient to obtain the necessary shear strength for durability.

When sealants are designed to seal sheet components, a mechanical fastening such as a screw, rivet or a mechanical seam must also be used.
Although sealants are not adhesives they can act as such, and their main purpose is to exclude moisture from joints or mating surfaces that would not drain and dry naturally.

Polymeric sealants are formulated to provide a continuous bonding layer to both surfaces and to fill and seal a gap. They are cured either by polymerisation initiated by curing agents or by the evaporation of solvents. Polymeric sealants include natural rubber and generic types of synthetic elastomers of nitrile, neoprene, silicone, and butyl.

There are many sealants available and the designer and contractor should recognise that it is necessary to use the correct sealant, placed correctly and in the correct manner to avoid corrosion.

The design of a penetration flashing or roof lap must not rely on sealants as the primary method of excluding the ingress of moisture. Joints must be designed to allow the discharge of water in the absence of any sealant and all sealants must be compatible with the metals being joined.

Only neutral curing silicone rubber sealants that are acetic acid-free are suitable for and compatible with all metals and organic finishes, including coated steel wall and roof cladding and flashing materials. Acetic cured silicone sealants can be used with stainless steel but are not suitable for zinc or zinc-aluminium coatings as the acid catalyst will attack metallic coatings.

 

Some silicone sealants have been modified to enable them to be painted, but most of these do not have the longevity of unmodified silicones. They are unnecessary for most roofing work as the silicone is placed within the joint and is not exposed. When used in conjunction with mechanical fasteners, the life of unmodified silicone sealants is expected to equal the life of the cladding because they are not exposed to UV radiation. Other cheaper types of sealants do not have the properties required to ensure such longevity.

 

7.9.1 Properties 

 

The properties required for ideal sealants are;
1. Adhesion.
  • Ease of application under construction conditions.
  • Resistance to slump and creep.
  • Low compression resistance.
  • Compatible with both surfaces.
  • Minimal shrinkage.

2. Resistance to weathering.

  • Water repellent.
  • Resistant to Ultra-violet radiation.
  • Heat resistant up to 80°C.
  • Cold resistant down to -20°C.
  • Mould resistant .

3. General properties.

  • Non-corrosive .
  • Flexible .
  • Gap-filling .
  • No primer needed.
  • Ability to accommodate shear movement in thin bands of sealant.
  • Resistant to ageing.
  • No maintenance required.
  • Long life expectancy ( more than 20 years).

Sealants are also known as cold solder because they perform the same function on metals that cannot be soldered with a lead/tin solder.

All soldered or sealant sealed laps and seams must be mechanically fastened for strength.

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

7.9.2 Methods Of Sealing 

There are three main methods of sealing metal sheets:

  • Gun applied sealants.
  • Self-adhesive tapes.
  • Preformed sections.

At low pitches, the weathertightness of the overlapping sheets depends on the effectiveness of the lap seals to provide a continuous effective water barrier for the design life of the roof or wall cladding. Single lap vertical or angled ribs with a capillary groove provide this barrier, but if it cannot be assured, such as with curved roof cladding below the recommended pitch, self-adhesive or foam lap tapes should be used.

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. Compression should be able to be achieved with large variations (up to 4 mm) in the gap thickness for side laps, with long lengths of thin seals.

Any sealant used in end laps should be flexible and be able to accommodate large movements relative to the thickness.

 

Sealants must perform throughout the design life of the building at working temperatures without affecting the seal.
All tapes can be compressed by 20%, and when this happens adhesion to both surfaces of the sheet lap is achieved. Foam tapes have a compression of about 30% but can be subject to shrinkage.

As surface absorption and radiation result in metal sheets achieving much higher and lower temperatures than many other building materials, any sealant should have good resistance to large temperature variations.

The expected life of neutral cure silicone sealant is unknown. However, this material is the best available at the time of writing, and cheaper types are not expected to be as durable.

Butyl lap tapes are also an approved sealant for flashings when used with mechanical fasteners.

Lap tapes should be 15 – 25 mm wide and a minimum thickness of 1.6 mm to provide a seal, but as thin as possible to give a neat joint.

Mechanical fasteners should always be used in conjunction with gun-applied sealants or lap tapes.

Lap tapes are suitable for longitudinal laps but they may not have sufficient flexibility to seal the pans of trapezoidal profiles, because the tape seals against the side of the rib preventing sufficient seal at the pan. Silicone sealants are the recommended sealant for transverse laps.

7.9.3 Surface Preparation 

 

For adequate bonding, all surfaces must be clean, dry, and free from contaminants such as rolling oils, dirt or dust. A detergent can be used to clean the surface, and it in turn must be wiped off with a clean damp cloth or sponge. Sealant must be applied within one hour of first preparing the surface and all surfaces to be bonded must be free from strippable protective film.

7.9.4 Sealant Extrusion And Placement 

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.
 

 

7.9.5 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.

 

 

 

 

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.

Sealants may cause eye irritation and if eyes become contaminated, flush immediately with water and obtain medical attention if the irritation persists.
Sealant must not be applied after the joint has been made and the joint must be left clean. The life of any sealant is compromised if it is submerged and the design of any joint must ensure that this does not occur.
Transverse roof and horizontal wall sealed laps must encapsulate the cut edges of coated steel roof and wall cladding when using both silicone and butyl strip sealants.

7.9.6 Strip Sealants 

Solid sealants in tape form are generally semi-cured butyl rubber possessing positive initial adhesion to both surfaces; as with all other sealants used with flashings, they should be used in combination with mechanical fasteners. The cladding thickness determines the space between fasteners to obtain a seal.

For the tape to make positive contact with both surfaces and form a seal, it is important that the tape's initial thickness is greater than the gap between the components of the finished joint.

Soft grades are needed if joints are to be pulled together without bulging of the sheet between fasteners, and some types may be difficult to compress within a joint during cold weather.

All preformed strip sealants require compression by fastening to ensure contact and adhesion to the joint faces.

Sealed side laps for roof and wall cladding must be fastened at maximum centres of 600 mm.

Preformed sealant strip has a release backing paper attached while being applied to avoid placing the sealant under tension that could reduce the tape's cross-section and the elastic memory of the sealant, which can cause shrinkage and discontinuity. The strip should be applied without removal of the backing, with the paper side uppermost and the paper removed immediately before the other component is positioned.
Cellular foam strip produced from EPDM, PVC or other plastics—in a range of sections and densities—require about 25% to 50% compression to achieve a seal.
Installation requirements for solid tapes apply equally to foam strip sealants.

7.9.7 Soldered Joints 

Lapped joints used with galvanised steel and non-ferrous metals (excluding aluminium) can be soldered with lead/tin solder. The joint should be clean, free of grease, and should be fluxed immediately after cleaning with a suitable proprietary flux. In the absense of a propietary flux, these would be suitable:
  • GALVANISED STEEL – Diluted hydrochloric acid (HCl Spirits of Salts), Muriatic Acid (a 1:3 dilution of HCl)
  • ZINC/COPPER/BRASS – Zinc Chloride H C l killed with zinc (Killed Spirits)
  • STAINLESS STEEL – Phosphoric Acid

The joint should be made with mechanical fasteners at 50 mm centres using rivets, screws and lapped 25 mm in the direction of water flow. A locked seam does not have to be mechanically fastened.

The joint should be completely sweated to avoid leaving any flux residue in the joint—preferably using a eutectic solder of 60% Tin and 40% Lead. (See 12.3 Handling.) After completion the joint should be thoroughly washed with water to remove all traces of flux.