Gaco SF42 Silicone Seam Sealant For Roof Waterproofing

• 100% silicone, solvent-free, moisture-curing sealant – not a spray foam and not a roof coating
• Fibre-reinforced – contains a reinforcing agent, so separate mesh or fabric is not required on typical seams
• High adhesion to metal, cement-fibre, asbestos-cement, single-ply and asphalt roof surfaces
• Coverage: 107 linear metres per 19 litre pail at a 102 mm wide bead, 1.5 mm dry film thickness
• Fastener encapsulation: approximately 2,000–2,260 fasteners per 19 litre pail
• Solids content 94.4% by volume – minimal shrinkage
• VOC below 50 g/L
• Brush, roller or trowel applied; the detail component of the GacoFlex S42 silicone roof system

Read the coverage figure carefully: it is in linear metres of seam, not square metres of roof. The 107 m figure assumes a 102 mm (4 in) wide bead at 1.5 mm (60 mil) dry film thickness. Change the bead width or the film build and the figure changes with it, so always quote the geometry alongside the number. Source: GW-5-1 GacoFlex™ Silicone Roof Coatings & Accessories Coverage Rates, revised 08/2024.

SF4200 SeamSeal is the detail component of the GacoFlex S42 silicone roof system. Gaco’s 20-year manufacturer-backed material warranty applies to a complete installed S42 system, not to an individual component; material warranties are issued by Duratite.

GacoFlex SF4200 SeamSeal is used to seal and reinforce roof details before and beneath a GacoFlex S42 silicone coating:

• Laps and seams on corrugated, profiled and standing-seam metal roofs
• Fastener heads and washers
• Penetrations – vents, pipes, curbs and rooflights
• Joints and cracks in cement-fibre and asbestos-cement roof sheets
• Seams and laps on aged built-up asphalt (BUR) and modified bitumen
• Detail work on EPDM, TPO and PVC single-ply membranes
• Filling and levelling “alligatored” built-up roofing, and blister repairs

Substrate note. Gaco requires E5320 2-Part Epoxy Primer/Filler on metal panels with factory-applied finishes. GRP / fibreglass is not among the substrates Gaco names for SF4200 — email us at info@duratite.ie before specifying it there.

• 1. Surface preparation
  The seam or detail must be clean, dry and structurally sound. Remove debris, loose coatings, algae and contaminants using GacoWash and a high-pressure rinse. On asbestos-cement, encapsulate rather than disturb the substrate (consult HSA regulations). Carry out an adhesion test.
• 2. Priming where required
  E5320 2-Part Epoxy Primer/Filler is required on metal panels with factory-applied finishes. Allow the primer to cure fully before sealing.
• 3. Sealing the seam
  Work the sealant into the seam and out to a 102 mm (4 in) wide bead, finished at 1.5 mm (60 mil) dry film thickness. At that geometry a 19 litre pail seals 107 linear metres of seam.
• 4. Fasteners
  Encapsulate fastener heads and washers completely. Allow roughly 2,000–2,260 fasteners per 19 litre pail.
• 5. Filler use (area rate – different figure)
  Where SF4200 is used as a filler over alligatored BUR or for blister repair rather than as a seam bead, the manufacturer’s theoretical rate is 4 US gallons per 100 ft² at 1.5 mm DFT — 1.63 litres per m², about 11.7 m² per 19 litre pail. This area rate applies to filler use only. Never use it to estimate seam work, and never use a roof-coating rate to estimate this sealant.
• 6. Overcoating
  Once cured, overcoat with GacoFlex S42 silicone roof coating at that coating’s own published rate. The coating rate and the sealant rate are separate figures — do not apply one to the other.

• Brush
• Roller
• Trowel or spatula (filler work)
• Airless sprayer (contact Duratite for advice)

• Property: Value

• Product type: 100% silicone, solvent-free, moisture-curing sealant with reinforcing agent
• Solids content: 94.4% by volume
• VOC: Below 50 g/L
• Standard dry film thickness: 1.5 mm (60 mil)
• Coverage – seams: 107 linear m per 19 L pail at 102 mm wide x 1.5 mm DFT
• Coverage – fasteners: Approx. 2,000-2,260 fasteners per 19 L pail
• Coverage – filler use only: 1.63 L/m2 at 1.5 mm DFT (approx. 11.7 m2 per 19 L pail)
• Pack size: 19 litre pail

Figures above are from the Product Data Sheet: GacoFlex™ SF4200 SeamSeal (February 2019) and GW-5-1 GacoFlex™ Silicone Roof Coatings & Accessories Coverage Rates (revised 08/2024).

Gaco publishes no solar reflectance value for SF4200 — it is a sealant, not a reflective roof coating. Any solar reflectance, tensile or elongation figure you have seen attached to this product belongs to the GacoFlex S42 roof coating and does not apply here.

• Flat roofs. Check the existing deck: no trapped water beneath the existing covering, no rot, decay or structural weakness, and the existing covering’s own seams and laps fully bonded. Clean the surface with GacoWash and allow it to dry. Seal all seams and penetrations with SF4200 SeamSeal, working to a 102 mm wide bead at 1.5 mm dry film thickness and a minimum 50–75 mm overlap onto sound material. Where asphalt or EPDM is aged, use GacoFlex A4271 Bleed Trap first to prevent oils staining through. A primer is not normally required — carry out an adhesion test first.

• Pitched and sloped roofs. Check the deck is not at the end of its life. Clean the roof free of moss, dirt, grime and contaminants using GacoWash; pressure washing is recommended (consult HSA regulations for asbestos roofs). Using a brush or roller, seal every sheet seam, joint and penetration with SF4200 SeamSeal at a 102 mm wide bead, 1.5 mm dry film thickness, with a minimum 50–75 mm overlap. A primer is not required on most cement-fibre or asbestos roof sheets, but E5320 2-Part Epoxy Primer/Filler is required on metal panels with factory-applied finishes. Always conduct an adhesion test first.

Allow the sealant to cure before the S42 coating is applied over it; cure time depends on temperature and humidity.

• E5320 2-Part Epoxy Primer/Filler — required on metal panels with factory-applied finishes.
• On other substrates a primer is not normally required. Carry out an adhesion test first, and email info@duratite.ie if the test is marginal.

SF4200 SeamSeal is overcoated with GacoFlex S42 silicone roof coating, applied at the coating’s own published rate — which is a different figure from this sealant’s and must be worked out separately. For ease of installation, consider a different colour for base coat and topcoat. On flat roofs over asphalt or EPDM a white coating helps solar reflectance, protecting the deck and reducing solar gain.

• VOC compliant with EC regulations — SF4200 is below 50 g/L.

Approvals published for the GacoFlex S42 silicone roof coating (for example UL 790 Class A, FM, CRRC) belong to that coating and are not transferable to this sealant. Gaco is an Amrize brand.

Refer to the product-specific Safety Data Sheet for complete guidance on handling, PPE requirements and storage. Where the substrate is asbestos-cement, encapsulate rather than disturb it and follow HSA regulations throughout.

• SF4200 SeamSeal is a silicone sealant. It is not a spray foam, and spray-foam density terminology does not apply to it.
• Coverage is a linear rate. 107 linear metres per 19 litre pail at a 102 mm wide bead, 1.5 mm DFT. Do not substitute a roof-coating rate quoted in gal/100 ft² or litres/m² — those are S42 coating figures and will under-order this product substantially.
• Application temperature must exceed 5 °C and be at least 3 °C above dew point.
• Avoid moisture during cure. High humidity and low temperatures prolong cure times.
• Allow the sealant to cure before overcoating with S42.
• E5320 2-Part Epoxy Primer/Filler is required on factory-finished metal panels.