Lap cement, also known as roll roofing cement or asphalt roof cement, plays a crucial role in sealing and securing roll roofing materials. This guide explains what lap cement is, when to use it, application steps, and best practices to ensure a durable, weather-resistant roof. It covers common applications on residential and small commercial roofs, including patching, sealant work, and edge detailing.
What Is Lap Cement For Roll Roofing
Lap cement is a thick, asphalt-based adhesive designed specifically for roll roofing materials. It provides a flexible, weatherproof bond that remains pliable in temperature extremes. This cement is formulated to adhere mineral-surfaced roll roofing, asphalt-saturated felts, and similar sheets to roof decks, joints, and flashing. It resists wind uplift, rain infiltration, and thermal cycling when used correctly.
When To Use Lap Cement
Lap cement is ideal for:
- Sealing and securing roll roofing seams and lap joints
- Repairing small gaps, splits, or punctures in roll roofing
- Adhering new edges or patches over existing roofing surfaces
- Sealing around flashings, chimneys, vents, and gutters
- Temporary or semi-permanent weatherproofing on aging roofs
For larger leaks or structural damage, professional inspection and possible replacement may be required. Lap cement should be compatible with the specific roll roofing product in use, so always check the manufacturer’s recommendations.
Preparing The Surface
Proper surface preparation ensures maximum adhesion and longevity. Steps include:
- Clean the area: remove dirt, loose granules, and debris with a stiff brush or broom.
- Dry before application: allow the surface to dry completely to avoid trapping moisture.
- Repair damage: address any obvious tears or holes with appropriate patching material if recommended by the product guidelines.
- Protect nearby areas: shield flashing, shingles, and ventilation openings to prevent accidental cement contact.
Surface temperature should typically be above 40°F (4°C) and not exceed 90°F (32°C) for optimal adhesion. Follow product-specific guidance for temperature tolerances.
Application Steps
Follow these general steps, adjusting for manufacturer instructions:
- Open the container and stir to a uniform consistency if required by the product.
- Apply a generous bead along seams and lap joints using a caulk gun or trowel, ensuring complete coverage.
- Press down and smooth the material to eliminate air pockets; use a roller or flat tool as needed.
- Overlap edges according to roofing practice, typically 3/4 inch to 1 inch for lap joints, ensuring a continuous seal.
- Allow curing time as specified by the product before exposing the roof to foot traffic or precipitation.
When sealing around penetrations like vents or pipes, apply a continuous bead, then reinforce with a patch or flashing as required by the roof design.
Best Practices And Tips
- Work in moderate weather to avoid rapid drying or softening of the cement.
- Use a properly sized nozzle or trowel for even application and full joint coverage.
- Apply in thin, even coats if multiple layers are recommended; avoid thick, globbed applications that can crack.
- Store unused cement in a cool, shaded area sealed tightly to prevent skinning or drying out.
- Test a small area first to confirm adhesion and compatibility with the roll roofing product.
Common Problems And Solutions
Be aware of these potential issues and remedies:
- The cement dries too quickly: re-wet or reapply according to product guidance; ensure the surface remains clean and dry.
- Poor adhesion on rough surfaces: lightly abrade the area to create a better bond, then reapply as directed.
- Cracking after curing: check for excessive application thickness or temperature fluctuations; consider a second, thinner coat if approved.
- Peeling at edges: ensure proper surface preparation and complete edge coverage with an adequate overlap.
Safety And Storage Considerations
- Ventilate work areas to avoid inhaling fumes; wear gloves and eye protection.
- Avoid open flames or sparks around solvent-based products, and follow all label warnings.
- Dispose of containers and leftover material per local regulations; do not pour into drains.
- Store in a cool, dry place away from direct sunlight and extreme temperatures.