Lap Sealants

Picking the right sealant for the job

The single biggest mistake is grabbing whatever tube is handy. Sealant is job-specific: what self-levels beautifully across a flat roof seam runs right off a vertical wall, and a silicone meant for a window won't always bond to a rubber roof membrane. Match the product to the surface and the orientation first, then worry about color.

• Self-leveling vs. non-sag. Self-leveling lap sealant, like the Lippert Alpha Systems tube here, is made for horizontal roof surfaces — it flows out to a smooth, watertight bead over seams and screw heads. For vertical seams, sidewalls, and around windows, you want a non-sag silicone like the AP Products Sikasil-N Plus that stays put where you tool it.
• Membrane compatibility. Check that the sealant is rated for your roof type — EPDM, TPO, or fiberglass. The wrong chemistry may not bond and can even attack a rubber membrane over time. The Sikasil GP and Sikasil-N Plus cartridges are versatile construction silicones; self-leveling lap sealant is purpose-built for RV roofs.
• Color match. Lap sealant comes in white and almond to match common roof and trim colors. It doesn't have to be pretty on top of the roof where no one sees it, but on visible sidewall seams the right shade matters.
• RTV and gasket makers. The Permatex Ultra Blue, Form-A-Gasket, and Super High Tack products are engine and mechanical sealants — gasket makers for your tow vehicle or generator, not roof seams. Sensor-safe formulas like the Ultra Blue matter on anything with oxygen sensors nearby.

Cure time and tooling window vary, so read the tube before you start. Clean the old sealant and surface well first — new sealant bonds to a clean, dry surface, not to a cracked bead or a dusty seam. When you're topping off a seal, pair the right sealant with fresh repair tape for tears, and if the whole roof is chalking it may be time for a coating or membrane instead of chasing every seam.