Sikalastic 641 Lo-VOC Fluid-Applied Roofing

Sikalastic 641 Lo-VOC is a single-component, moisture-triggered, aliphatic polyurethane resin made by Sika Corporation. It is cold applied, elastomeric, low odor and low VOC. It is the resin in the Sikalastic RoofPro Fluid-Applied Roofing section (CSI 07 56 00), a fully bonded membrane for concrete, plywood sheathing, cover board, metal substrates and modified bitumen membranes.

The resin embeds and tops Sikalastic systems reinforced with Sika Reemat and saturates systems reinforced with Sika Fleece. It is used on roofs with complex details and geometry, or where access is limited, and white is used for cool roofs and solar roof assemblies. The listed environmental document is a cradle-to-grave Environmental Product Declaration.

• FM Approval Standard 4470 for Class 1 roof covers.
• UL 790 Class A.
• Miami-Dade.
• ASTM D6083, ASTM E108 and ASTM C836.
• CRRC.
• Florida Building Code.
• ICC-ESR-4509.

External fire performance is Class A (ASTM E108).

• Packaging: 5 gal. (19 L) pail, and a 50 gal drum in white and standard gray base only, made to order.
• Shelf life: 15 months for the pail and 6 months for the drum.
• Density: 11.9 lb./gal. VOC content: 38 g/l (ASTM D2369). Solids by volume: 89%.
• Service temperature: -22 to 176°F (-30 to 80°C), intermittent.
• Artificial aging: 5,000 hours under UV light with no cracking or crazing (ASTM C1442).
• Chemical resistance: paraffin, gasoline, fuel oil, white spirit, acid rain, detergents and moderate solutions of acids and alkalis. Some low molecular weight alcohols can soften the material.

Reinforced assembly values are published for Sikalastic 20 builds.

• With Sika Reemat: static puncture over 55 lbf (ASTM D751), tensile strength 1030 psi, elongation at break 21%, tear strength 300 lbf/in (ASTM D624).
• With Sika Fleece 140: static puncture over 55 lbf, tensile strength 900 psi, elongation at break 82%, tear strength 200 lbf/in.

Colors are white, standard gray, pearl gray, steel gray, mushroom and copper green, with custom colors available. Thermal emittance is 0.91 for white, pearl gray, standard gray, steel gray and mushroom (CRRC, ASTM C1371). Solar reflectance values under CRRC and ASTM C1549 are listed by color.

• White: 84.
• Pearl gray: 57.
• Mushroom: 56.
• Standard gray: 37.
• Steel gray: 12.

Solar Reflectance Index for white is 108 (ASTM E1980).

Three system guides are published. Each assumes a primer chosen from the priming guide and detailing with Sika Flexitape Heavy or Sika Joint Tape SA centered over seams, transitions and treated cracks and joints.

• Metal roof guide: a base layer and a top layer at 20 wet mils each (80 sf per gal), for at least 36 mils dry.
• Reemat systems, named Sikalastic 10, 15, 20 and 25: a 50 wet mil base layer (32 sf per gal), the mat, then a top layer of 20 mils (80 sf per gal) for Sikalastic 10 and 15, 30 mils (53 sf per gal) for Sikalastic 20, and two 23 mil layers (69 sf per gal) for Sikalastic 25. Minimum dry film is 62 mils for 10 and 15, 71 mils for 20 and 85 mils for 25.
• Fleece systems, Sikalastic 15, 20 and 25: Fleece 120, 140 and 170 under base layers of 45 mils (35 sf per gal), 50 mils (32 sf per gal) and 66 mils (24 sf per gal), with top layers of 25, 30 and 34 mils. Minimum dry film is 62, 71 and 89 mils.

These coverage rates are optimal figures. A Sikalastic 20 fleece build over modified bitumen could reach 14 to 16 sf per gal as a full system, and a Reemat base coat 25 to 28 sf per gal. The substrates for the reinforced builds are concrete or cementitious surfaces, metals, wood, single-ply or bituminous roofing, and stone.

No mixing is needed. Ambient air is 41 to 95°F (5 to 35°C) with relative humidity at 85% maximum. Substrate is 41 to 140°F (5 to 60°C), at least 5°F (3°C) above the dew point, and at 4% moisture or less by Tramex meter. Material in an opened container forms a film after about 1 to 2 hours at 75°F and 50% R.H.

• At 40°F and 50% R.H.: rain resistant in 1 hour, touch dry in 12 hours, full cure in 24 hours, overcoat after 18 hours.
• At 50°F: 1 hour, 6 hours, 18 to 24 hours, overcoat after 8 hours.
• At 70°F: 1 hour, 4 hours, 12 to 18 hours, overcoat after 6 hours.

After 7 days the surface is cleaned and primed with Sika Concrete Primer Lo-VOC or Sika Reactivation Primer before more coats.

The priming guide for Sikalastic 641 Lo-VOC assigns primers by substrate.

• Concrete and lightweight structural concrete: Concrete Primer Lo-VOC, DTE Primer, GDC Primer, EP Primer/Sealer or EP Primer Rapid.
• Cement and gypsum roof boards, brick and stone: Concrete Primer Lo-VOC, EP Primer/Sealer or EP Primer Rapid.
• Bituminous felts, coatings and SBS or aged APP cap sheets, PVC membranes, fiberglass, unglazed roof tiles, foam, metal and wood: EP Primer/Sealer or EP Primer Rapid.
• Hypalon: Sika Bonding Primer. TPO and EPDM: Sikalastic EPDM / TPO Primer Lo-VOC.

The specification adds Sikalastic Conductive Primer, a two-component primer for high voltage dry leak detection, and SikaFast 3341, a methyl methacrylate adhesive for PVDF finished edge metal. New cementitious substrates are Portland-based and cured at least 28 days.

New concrete cures at least 28 days to 3000 psi (20.7 MPa) compressive strength and 200 psi (1.4 MPa) tensile bond strength, and is wood floated or steel pan finished. Cementitious surfaces are blasted or scarified to ICRI CSP 2 to 4. Plywood decks that receive resin directly are at least 1/2 in. thick, conform to PS 1, preferably APA Grade A-C EXT, and use non-rusting fasteners recessed 1/8 to 1/4 in. and filled with Sikaflex sealant, with panel ends spaced 1/8 to 3/16 in. Existing Sikalastic and Sarnafil or Sikaplan membranes are pressure washed at about 2000 psi.

Repair materials include SikaQuick 1000, a fast-setting trowel-grade mortar, SikaQuick VOH for vertical and overhead work, and Sikadur-22 Lo-Mod or Sikalastic-720 for birdbaths. Cover boards are at least 1/2 in. thick, such as Dens-Deck Prime or Securock Cement Board. Polyisocyanurate insulation meets ASTM C1289 Type II at 20 or 25 psi.

Non-structural cracks up to 1/16 in. need no detail. Cracks from 1/16 to 1/4 in. are routed, sealed with Sikaflex and covered with a 40 to 45 mil resin layer embedding 3 in. Flexitape Heavy. Moving joints get a bond breaker tape at least 1 in. wide. Sika Drainage Mat 420 sits under soil, sand or stone ballast, and Drainage Mat GRS serves vegetated assemblies.

The resin is not placed on grade concrete decks, under continuous immersion, or directly under cementitious tile mortar. Unvented metal pans, sandwich slabs, cinder fill and lightweight insulating concrete need added deck evaluation. Heavy foot traffic areas receive an aggregate wear coat. The product belongs with reinforced liquid-applied roofing systems.