Repair Systems — 04 — Roofing Defects
Sheet lap and seam sealants — metal sheet roof
Butyl mastic tape is the primary sealing medium for sheet-to-sheet end laps and ridge capping joints on metal roofs. Neutral-cure silicone is used at penetrations and detail junctions. Acid-cure silicone must never be applied to Colorbond, Zincalume or natural zinc — the acetic acid released attacks the metallic coating and will cause premature delamination and corrosion.
Sheet lap and seam sealants — the three types
Three sealant types are used on metal sheet roofs: butyl mastic tape (for sheet side laps and end laps), neutral-cure silicone (for penetrations, perimeter and detail sealing), and polyurethane sealant (for paintable perimeter and penetration sealing). Selection of the wrong sealant type — particularly acid-cure silicone on metal — causes accelerated corrosion of the substrate.
Technical Reference
Butyl tape for laps, neutral-cure silicone (never acid-cure), polyurethane for paintable applications, and end-lap requirements by pitch
Product Reference
3 sealant types — butyl mastic tape, neutral-cure silicone and polyurethane sealant for metal sheet roof applications
Butyl mastic tape / strip
Butyl rubber mastic tape or strip; for side laps, end laps and capping base sealing; remains permanently flexible; compatible with Colorbond, Zincalume and natural zinc
Product type
Butyl mastic sealant — pre-formed tape or strip — lap and seam sealing
System description
Butyl mastic tape (also called butyl strip, mastic strip, or lap sealant tape) is the primary sealant for sheet lap joints, end laps and under cappings on metal sheet roofing. It is a pre-formed strip of butyl rubber that is peeled from a release liner and applied to the sheet surface before the overlapping sheet or capping is pressed down and fixed. Butyl tape remains permanently flexible — it does not harden or crack with thermal cycling — making it the preferred sealant for metal roofing where differential thermal movement is significant. Major suppliers include Tremco, 3M, Lysaght-branded butyl tape, and various roofing accessory suppliers. Confirm compatibility with the specific sheet substrate (Colorbond, Zincalume, natural zinc) from the TDS.
Technical properties
- Material: butyl rubber — pre-formed strip on release liner
- Application: peel-and-stick — no primer required on clean Colorbond or Zincalume
- Permanently flexible — does not harden or crack with thermal cycling
- Compatible with BlueScope Colorbond steel, Zincalume and most metal substrates
- Typical widths: 10mm, 15mm, 20mm, 25mm — confirm width to suit lap joint dimensions
- Application: apply to lower sheet before fixing the upper sheet or capping over the top
- Operating temperature: confirm from TDS — most butyl tapes function from -20°C to +100°C
Limitations
- Surface must be clean and dry before application — dust, oil or moisture reduces adhesion
- Do not use acid-cure silicone over or adjacent to butyl tape — incompatible
- Butyl tape alone is not a structural adhesive — fixing screws are still required at all laps
- Confirm compatibility with specific substrate if applying to natural zinc — some butyl products are not recommended for direct contact with natural zinc
- Do not stretch butyl tape during application — stretching reduces seal effectiveness
Procurement
Tremco, 3M, Lysaght and other roofing accessory suppliers. Available through roofing merchants, trade hardware and specialist sealant suppliers. Lysaght-branded butyl tape is available through Lysaght branches.
Confirm suitability with the current manufacturer TDS before specifying or applying.
Neutral-cure silicone sealant
Neutral-cure silicone sealant; for perimeter, penetration and detail sealing on metal sheet roofs; NOT acid-cure; must be metal-compatible grade
Product type
Neutral-cure silicone sealant — metal roofing perimeter and penetration use
System description
Neutral-cure silicone sealant is used for perimeter sealing, penetration sealing and detail work on metal sheet roofs where butyl tape is not practical (e.g., complex shapes, penetrations, flashing terminations). It must be a neutral-cure formulation — acid-cure (acetoxy) silicones release acetic acid during cure and will corrode Colorbond, Zincalume and other metal substrates. Major brands include Selleys Roof & Gutter Silicone (neutral-cure), Bostik and various specialist brands. Confirm the product is specifically labelled as neutral-cure and rated for metal contact. Silicone is not suitable for use at high-movement lap joints where butyl tape is preferred.
Technical properties
- Type: neutral-cure silicone — NOT acetoxy/acid-cure
- Compatible with Colorbond, Zincalume and galvanised steel — confirm from TDS
- Application: gun-applied from cartridge
- Service temperature: typically -50°C to +200°C depending on product
- UV resistant — suitable for exposed outdoor metal roofing applications
- Cure time: skin-over approximately 30 minutes; full cure 24–48 hours depending on temperature and humidity
Limitations
- NEVER use acid-cure (acetoxy) silicone on metal substrates — acetic acid corrodes Colorbond, Zincalume and galvanised coatings
- Not suitable as the primary sealant at continuous sheet laps — use butyl tape instead
- Silicone does not paint — if a paint-over finish is required, use a paintable polyurethane sealant instead
- Adhesion to dusty, oily or wet surfaces is poor — clean and dry all surfaces before applying
- Confirm specific product compatibility with the substrate from the manufacturer TDS
Procurement
Selleys, Bostik, Tremco, Sika and other sealant manufacturers. Available through trade hardware, roofing merchants and building product suppliers.
Confirm suitability with the current manufacturer TDS before specifying or applying.
Polyurethane sealant
Paintable polyurethane sealant; for penetrations, flashings and perimeter sealing on metal sheet roofs; flexible; can be overpainted
Product type
Polyurethane sealant — metal roofing penetrations and perimeter — paintable
System description
Polyurethane (PU) sealants are used for penetration sealing, flashing terminations and perimeter detail work on metal sheet roofs where a paintable finish is required. Unlike silicone, polyurethane sealants can be overpainted with compatible coatings, making them suitable where the sealant joint will be in a visible location on a Colorbond roof and a matching colour is required. PU sealants are flexible and adhesive to metal substrates, but have a lower movement capability than silicone and are not recommended for high-movement lap joints. Brands include Tremco THC-900, Sika Sikaflex 11FC and Selleys Roof & Gutter Sealant (PU grade). Confirm substrate compatibility from the TDS.
Technical properties
- Type: one-part polyurethane (moisture-cure)
- Paintable after full cure — compatible paints confirmed from TDS
- Flexible and adhesive to metal, masonry and some plastics
- Cure time: skin-over approximately 1 hour; full cure 5–7 days
- Service temperature: approximately -40°C to +80°C — confirm from TDS
- Application: gun-applied from cartridge
Limitations
- Not suitable as primary sealant for high-movement sheet lap joints — use butyl tape
- Lower UV resistance than silicone — may discolour or degrade in exposed applications; confirm from TDS
- Must prime certain substrates (e.g., some metals) — confirm from TDS before applying
- Cannot be applied in temperatures below 5°C or in rain
- Paintable, but paint adhesion and colour match require testing — confirm with paint manufacturer
Procurement
Tremco, Sika, Selleys, Bostik and other sealant manufacturers. Available through trade hardware, roofing merchants and building product suppliers.
Confirm suitability with the current manufacturer TDS before specifying or applying.
Sealant Type Equivalence
The three sealant types serve different functions — they are NOT interchangeable. Select the correct type for each application.
| Product A | Product B | Equivalence note |
|---|---|---|
| Butyl mastic tape (any brand) | Butyl mastic tape (any brand) | All pre-formed butyl mastic tapes serve the same function and are interchangeable for lap sealing on metal roofs. Confirm substrate compatibility (Colorbond, Zincalume, natural zinc) from the specific product TDS. |
| Butyl tape | Neutral-cure silicone | NOT equivalent — butyl tape is the correct product for continuous sheet lap joints (permanently flexible, pre-formed, no skill in application). Neutral-cure silicone is for perimeter, penetrations and irregular shapes. Do not use silicone at sheet side laps or end laps in place of butyl tape. |
| Neutral-cure silicone | Polyurethane sealant | Both are gun-applied and suitable for metal roofing perimeter and penetration work. Key difference: polyurethane is paintable; silicone is not. Use silicone where long-term UV resistance and non-paintable finish are acceptable. Use PU sealant where a painted finish over the sealant is required. |
System Comparison
Butyl tape vs neutral-cure silicone vs polyurethane for metal sheet roof applications. Confirm all properties from current manufacturer TDS.
| Attribute | Butyl mastic tape | Neutral-cure silicone | Polyurethane |
|---|---|---|---|
| Product type | Pre-formed strip | Gun-applied liquid | Gun-applied liquid |
| Primary use | Sheet side laps, end laps, cappings | Penetrations, perimeter, details | Penetrations, perimeter, paintable |
| Movement capability | High (permanently flexible) | High (±50%) | Moderate (±25%) |
| Paintable | No | No | Yes (after cure) |
| Colorbond compatible | Yes (confirm TDS) | Yes — neutral-cure only | Yes (confirm TDS) |
| Acid-cure risk | N/A | CRITICAL — must be neutral-cure | N/A |
| Application method | Peel-and-stick | Cartridge gun | Cartridge gun |
| UV resistance | Moderate (confirm TDS) | High | Moderate (confirm TDS) |
Critical sealant mistakes on metal sheet roofs:
- Acid-cure silicone on metal — the most common sealant error on metal roofing. Acetic acid released during cure attacks zinc coatings (Colorbond, Zincalume, galvanised). Results in white staining, corrosion and premature sealant failure. Always confirm neutral-cure before using any silicone on metal.
- Applying sealant to wet or dirty surfaces — reduces adhesion of butyl tape, silicone and polyurethane. Clean and dry all contact surfaces before applying any sealant.
- Using silicone at continuous sheet laps instead of butyl tape — silicone is not designed for the compression-seal function at laps. Use butyl tape at all continuous sheet lap joints.
- Over-relying on sealant instead of achieving correct lap dimensions — sealant supplements correct lap dimensions; it does not substitute for them. If the minimum lap length is not achieved, sealant alone will not provide a durable weathertight joint.
Disclaimer
This page provides general technical information only. Sealant type, application method, end-lap dimensions and primer requirements must be confirmed against the current manufacturer TDS, the sheet manufacturer warranty conditions and the structural engineer specification. Do not rely on this reference as a substitute for professional advice.