Roll Roofing Calculator

Follow these steps to estimate the roll roofing materials needed for your low-slope roof project.

1. Enter your roof area in square feet. Measure length times width for each rectangular section and add them together.
2. Select coverage type — single-coverage for exposed-nail installation on slopes of 2:12 or greater, or double-coverage (selvage) for concealed-nail installation on slopes as low as 1:12.
3. Choose a waste factor — 10% for simple rectangular roofs, 15% or 20% for more complex shapes.
4. Read the results showing total rolls, roofing cement needed and the coverage breakdown.

A roll roofing calculator estimates the number of mineral-surface asphalt rolls and roofing cement required to cover a low-slope roof. Roll roofing (also called MSR or mineral-surfaced roll) is a cost-effective, single-ply roofing material sold in 36-inch wide by 36-foot long rolls that cover approximately one roofing square (100 sq ft) in single-coverage mode.

Roll roofing is commonly used on sheds, garages, porches and utility buildings with roof slopes between 1:12 and 4:12. It meets ASTM D6380 standards and IRC R905.5 requirements for mineral-surfaced roll roofing on residential structures. While less durable than asphalt shingles (typical lifespan 5-15 years versus 20-30), it is faster to install and significantly cheaper per square.

The calculator divides the waste-adjusted roof area by the effective coverage of each roll. Single-coverage rolls provide about 100 sq ft of coverage after accounting for standard 6-inch side and end laps. Double-coverage (selvage) rolls provide only about 50 sq ft because half the roll width overlaps the previous course to create two waterproof layers.

Standard roll dimensions (ASTM D6380):

• Width: 36 inches (3 ft)
• Length: 36 feet
• Gross area: 108 sq ft per roll
• Effective coverage: 100 sq ft (single) or 50 sq ft (double/selvage)
• Weight: 55-90 lb per roll (single-coverage)

A 600 sq ft shed roof using single-coverage roll roofing with 10% waste:

1. Roof area with waste: 600 x 1.10 = 660 sq ft
2. Rolls needed: 660 / 100 = 6.6, rounded up to 7 rolls
3. Squares with waste: 660 / 100 = 6.6 squares
4. Roofing cement: 6.6 x 2 = 13.2, rounded up to 14 gallons

Double-coverage example: A 500 sq ft garage roof at 15% waste gives 575 sq ft adjusted area. At 50 sq ft per roll: 575 / 50 = 11.5, rounded up to 12 rolls. Cement: 5.75 x 2 = 11.5, rounded to 12 gallons.

Coverage Type (Single vs Double)

Single-coverage rolls cover twice as much area per roll (100 vs 50 sq ft) but provide only one layer of protection. Double-coverage selvage rolls overlap half their width for two complete layers, providing better waterproofing on very low slopes but requiring twice as many rolls.

Roof Slope

IRC R905.5 allows roll roofing on slopes as low as 1:12 (1 unit rise per 12 units run). Exposed-nail single-coverage is recommended for slopes of 2:12 or greater. Concealed-nail double-coverage is required for slopes between 1:12 and 2:12.

Installation Method

Exposed-nail installation uses 6-inch side laps cemented in place with nails visible along lap edges. Concealed-nail installation hides all fasteners under the overlapping course, requiring full-width cement application in the overlap zone. The concealed method uses more cement per square.

Roof Complexity

Rectangular roofs with minimal penetrations waste about 10% of material in overlaps and trimming. Roofs with hips, valleys and multiple planes waste 15-20% due to additional cutting and fitting around intersections.

Climate and Overlap

In cold climates or areas prone to wind-driven rain, wider overlaps (up to 12 inches) may be specified, reducing the effective coverage per roll and increasing the total quantity needed.

Single-coverage mineral-surface rolls (Class M) are the most common for residential outbuildings. They feature a full coating of ceramic granules for UV protection. Double-coverage (Class WS for wide selvage) has half the roll left bare for overlap adhesion. Modified bitumen is a separate product category using polymer-modified asphalt with higher performance and cost.

• Budget projects — Roll roofing costs roughly one-third to one-half the price of architectural shingles per square, making it ideal for utility structures
• Low-slope roofs — Slopes between 1:12 and 4:12 where standard asphalt shingles may not perform well
• Temporary or short-term roofing — When a building needs weather protection for 5-15 years before a major renovation
• Quick installation — A single roll covers 100 sq ft in one pass, making it much faster than individual shingle installation
• Small structures — Sheds, garages, porches, carports and workshop roofs where aesthetics are secondary to function

Using Single-Coverage Below 2:12 Slope

Exposed-nail single-coverage roll roofing leaks on very low slopes because water backs up under the lap edges. Slopes below 2:12 require double-coverage (selvage) with concealed nails and full cement application per IRC R905.5.

Skipping Roofing Cement on Laps

Every overlap must be cemented with ASTM D4586 plastic roof cement. Without cement, water wicks into the laps by capillary action, causing leaks even on adequately sloped roofs.

Applying in Cold Weather Below 45F

Roll roofing becomes brittle below about 45 degrees Fahrenheit. Unrolling cold material causes cracks that allow water entry. Store rolls in a warm area and install only when temperatures are above 45F.

Omitting the Starter Strip

The first course along the eave needs a 9-inch wide starter strip (cut from a roll) cemented and nailed in place. Without it, the exposed nail line at the eave has no waterproof backing.

Under-Ordering Roofing Cement

At approximately 2 gallons per square for exposed-nail installation (and more for concealed-nail), cement is a significant material item. Running short delays the project and leaves unsealed laps exposed to weather.

This calculator gives exact counts based on standard roll dimensions and coverage rates.

Divide your roof area (with waste factor) by the effective coverage per roll. Single-coverage rolls cover 100 sq ft each and double-coverage selvage rolls cover 50 sq ft each. A 600 sq ft roof with 10% waste needs 7 single-coverage rolls.

Single-coverage rolls have granules across the full width and overlap 6 inches at the sides. Double-coverage (selvage) rolls have half the width bare so each course overlaps the previous one by 50%, creating two waterproof layers for better protection on very low slopes.

IRC R905.5 allows roll roofing on slopes as low as 1:12 when using double-coverage selvage with concealed nails. Single-coverage exposed-nail installation requires a minimum slope of 2:12.

A standard single-coverage mineral-surface roll weighs 55 to 90 pounds depending on the product grade. Double-coverage selvage rolls weigh 110 to 140 pounds per square of coverage due to the thicker material.

Roll roofing typically lasts 5 to 15 years depending on climate, slope and maintenance. It has a shorter lifespan than asphalt shingles (20-30 years) but costs significantly less per square.

Yes. Every side and end lap must be sealed with ASTM D4586 plastic roof cement. Plan for approximately 2 gallons per roofing square for exposed-nail installation and more for concealed-nail methods.

It is not recommended. Roll roofing should be applied directly to a clean, smooth roof deck. Installing over existing shingles creates an uneven surface that prevents proper adhesion and leads to premature failure.

Use large-head corrosion-resistant roofing nails (minimum 3/8 inch head, 11 or 12 gauge) that penetrate at least 3/4 inch into the roof deck. Typical nail length is 1 inch for exposed-nail application on 1/2 inch plywood.

Roll roofing can be used on very low slopes (1:12 minimum) but is not suitable for truly flat (0:12) roofs. For dead-flat roofs use modified bitumen, EPDM or TPO membrane systems designed for ponding water.

Roll roofing costs roughly 30 to 60 dollars per square for materials, compared to 80 to 150 dollars per square for asphalt shingles. The labor cost is also lower due to faster installation. These are approximate 2024-2025 retail ranges.

• GAF Mineral Guard Installation Instructions (cement application rates and overlap requirements for exposed-nail roll roofing).
• IRC R905.5 Mineral-Surfaced Roll Roofing (minimum slope requirements for single and double coverage installations).
• ASTM D6380 Standard Specification (roll dimensions, coverage classes and material requirements for mineral-surfaced rolls).
• InspectApedia Roll Roofing Guide (installation methods, overlap patterns and common failure modes).