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How to Install Roll Roofing on a Flat Roof?

Installing roll roofing on a flat roof is a practical and cost-effective solution for many homeowners and businesses. This type of roofing offers durability, weather resistance, and an attractive finish without the complexity and expense of other roofing options. Whether you’re a seasoned DIY enthusiast or a first-time roofer, our comprehensive guide will walk you through each step of the installation process, from preparing your roof to laying the final strip.

By following these instructions, you’ll ensure a secure, long-lasting roof that can withstand the elements for years to come.

Here’s a general breakdown:


Safety First… Ensure you have proper safety gear like fall protection and sturdy footwear.

Clean Up… Remove any debris, dirt, or loose gravel from the roof surface.

Roof Inspection… Check for leaks, damaged areas, or ponding water (flat roofs shouldn’t collect water). If you find any issues, address them before continuing.

Laying the Roofing:

Base Sheet (Optional) … Some applications use a base sheet for additional protection. Apply roofing cement and lay the base sheet according to the manufacturer’s instructions.

Mark the Starting Line… Snap a chalk line across the roof, typically 35 inches up from the edge, to ensure the first roll sits straight.

Roofing Cement… Apply a consistent layer of roofing cement using a trowel, following the manufacturer’s recommendations for thickness.

Unroll and Cut… Unroll the first sheet of roofing material and position it along the chalk line. Use a utility knife to cut the roll to fit the width of the roof, allowing some overhang at the eaves.

Lay and Secure… Carefully lay the roofing material onto the wet cement, pressing it down to ensure good adhesion. Use roofing nails along the edges and at predetermined intervals (usually 8-12 inches apart) to secure the roll.

Overlaps… Subsequent rolls should overlap the previous one by about 6 inches. Apply roofing cement to the seam before laying down the next roll.

Flashing and Sealing… Install flashing around roof penetrations like pipes or vents for additional leak protection. Use caulk to seal any gaps or around the perimeter.

Trimming and Cleaning… After laying all the rolls, use a utility knife to trim any excess roofing material around the edges. Clean up any debris or cement spills.

Additional Tips:

– Wear gloves to protect your hands from the roofing cement and sharp edges.

– Maintain a straight line while laying the rolls to avoid uneven coverage.

– Work in sections to prevent the roofing cement from drying prematurely.

– Ensure proper ventilation for the roof after installation.

Safety Reminder… While this is a general guide, installing flat roofs, especially involving roofing cement and working at heights, can be risky. It’s recommended to consult a professional roofer for complex installations or if you’re unsure about your DIY skills.

What is the Minimum Slope for Roll Roofing?

The minimum slope for roll roofing depends on the application method:

Concealed Nail Method… In this method, nails are hidden under the upper layer of roofing. This allows for a minimum slope of 1 inch rise per 12 inches horizontal run (written as 1:12).

Exposed Nail Method… If nails are driven directly through the roll (exposed nail method), the minimum recommended slope is steeper at 2 inches rise per 12 inches horizontal run (2:12).

Important Note… While some sources mention using roll roofing on flat roofs (0 slope), it’s generally not recommended. Flat roofs are prone to water ponding, which can lead to leaks and roof damage. Roll roofing is best suited for slightly sloped surfaces that allow for proper drainage.

How to Fasten Roll Roofing

There are two main ways to fasten roll roofing, depending on whether you want the nails concealed or exposed:

Concealed Nail Method:

Nail the Starting Edge… Use roofing nails (typically 1 inch galvanized) to secure the top edge of the first roll along the eaves. Space the nails every 3-4 inches.

Overlapping and Nailing… When laying the subsequent rolls, ensure a horizontal overlap of 6 inches with the previous roll. Apply roofing cement to the seam before placing the new roll. Nail only the top edge (bare of mineral coating) of the new roll at a similar spacing (3-4 inches)

Exposed Nail Method:

Nail Through the Roll… In this method, nails are driven directly through the entire thickness of the roll, including the mineral surface layer.

Nail Pattern… Use a zig-zag pattern (sometimes called a “Z” pattern) to distribute the nails evenly. Space the nails 8-12 inches apart along the width of the roll.

General Tips for Fastening Roll Roofing:

– Use roofing nails with a galvanized coating to resist rust.

– Maintain a straight line while hammering nails to avoid damaging the roofing material.

– Don’t over-drive the nails; they should sit flush with the surface of the roll.

– Wear gloves to protect your hands from the sharp edges and roofing cement.

Additional Considerations:

– Some applications may involve a base sheet under the roll roofing. Follow the manufacturer’s instructions for fastening the base sheet, if applicable.

– Valleys on the roof may require additional fasteners for better hold.

– Remember, safety is paramount when working on a roof. If you’re unsure about your DIY skills or have a complex roof layout, consider consulting a professional roofer for installation.

Can You Install Roll Roofing Over Shingles?

Installing roll roofing over existing shingles is technically possible, but it’s not always the recommended approach.

Here’s a breakdown of the pros and cons to help you decide:

Pros of Installing Over Shingles:

Cost-effective… Saves money on removing the old shingles.

Easier Installation… Less time and effort compared to removing a whole roof.

Cons of Installing Over Shingles:

Uneven Surface… The bumps and contours of the shingles will create an uneven base for the roll roofing, potentially affecting drainage and leading to leaks.

Hidden Damage… You won’t be able to inspect the roof deck underneath for potential problems like rot or moisture damage.

Shingle Weight… Adding another layer of roofing material increases the overall weight on the roof structure, which might become an issue in older buildings.

Building Code Violations… Some building codes may not permit installing roll roofing over shingles. It’s best to check with your local building department before proceeding.


In most cases, it’s advisable to remove the existing shingles before installing roll roofing. This allows for a proper inspection of the roof deck, creates a smoother surface for the new roof, and ensures better drainage.

However, if your shingles are in good condition, the roof is simple in design (no valleys or complex cuts), and removing the shingles is not feasible due to budget or other constraints, then installing roll roofing over the shingles might be a viable option, but with the understanding of the potential drawbacks mentioned above.

Important Note…  Always consult a professional roofer for their assessment before deciding to install roll roofing over shingles. They can advise you on the best course of action based on your specific roof condition and local building codes.

Shed Plans

How Much Overlap Does Roll Roofing Need

Roll roofing needs a minimum overlap of 4 inches between courses. Some sources recommend up to 6 inches of overlap for extra security.

Here are some additional tips for overlapping roll roofing:

– Stagger the end laps of adjoining sheets to prevent a buildup at the seams.

– Apply roofing cement over the entire lap area for a watertight seal.

– Make sure no two butt joints (where two ends of the roll meet) are closer than 2 feet apart.

Overall, rolled roofing is a good option for temporary applications, outbuildings, or if you’re on a tight budget. However, if you’re looking for a long-lasting, durable roof with more design options, you may want to consider other roofing materials.

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3-Tab Shingles – The Pros and Cons

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