Roofing a 20 ft Slope - Area, Sheets and Battens

Four roofing materials, five data points each

Asphalt shingles sell by the bundle. Metal panels sell by the linear foot. Clay tiles sell by the piece. Roll roofing sells by the roll. Each material covers a different number of square feet per unit, weighs a different amount per roofing square, lasts a different number of years and costs a different amount per square foot installed. This calculator converts your roof dimensions into the right unit count for whichever material you choose - plus underlayment rolls, ridge cap footage and a cost estimate.

How to use - step by step

Slope length - measure from the eave (gutter line) up the slope to the ridge, not the horizontal span. A 6/12 pitch on a 15 ft half-span gives about 17 ft of slope.
Slope width - measure along the eave. For most houses, this is the building length plus overhang (1-2 ft per side).
Number of planes - shed = 1, gable = 2, hip = 4. For hip roofs, enter the average slope dimensions.
Roofing material - asphalt shingles, metal panels, clay/concrete tiles, or roll roofing.
Waste allowance - 10% for simple gable, 15% for hip, 20% for complex roofs with dormers and valleys.
Prices (optional) - enter material, underlayment and labor costs per square foot.
Read the results - total area in squares, piece count, underlayment rolls, ridge cap and drip edge footage, cost breakdown.

Roofing materials atlas

Coverage, weight, lifespan and cost for each roofing material per roofing square (100 sq ft).

Material	Units/square	Weight/square	Lifespan	Material $/sq ft	Installed $/sq ft
Asphalt shingles	3 bundles	200-350 lb	20-30 years	$3-5	$5-9
Metal panels	varies (3.3 ft wide)	50-150 lb	40-70 years	$5-12	$8-18
Clay/concrete tiles	~90 tiles	600-1,100 lb	50-100 years	$8-15	$13-25
Roll roofing	1 roll	75-90 lb	10-15 years	$1.50-3	$3-6

Roof type classification

Shed

1 slope. Simplest to measure. Common on additions, porches, lean-to structures.

Gable

2 identical slopes. Most common in the US. Measure one slope and multiply by 2.

Hip

4 slopes (2 trapezoids + 2 triangles). More waste at hip ridges. Use average dimensions and 15% waste.

Practical examples

Example 1: Small gable roof - 15 x 30 ft slopes, 2 planes, asphalt shingles, 10% waste
Area: 990 sq ft | Squares: 9.9 | Bundles: 30

Example 2: Standard ranch gable - 18 x 50 ft slopes, 2 planes, asphalt shingles, 10% waste
Area: 1,980 sq ft | Squares: 19.8 | Bundles: 60

Example 3: Hip roof with metal - 16 x 40 ft slopes, 4 planes, metal panels, 15% waste
Area: 2,944 sq ft | Panels: 56 (16 ft long)

Example 4: Shed roof, roll roofing - 12 x 20 ft, 1 plane, roll roofing, 10% waste
Area: 264 sq ft | Rolls: 3

Example 5: Large gable with clay tiles - 22 x 60 ft slopes, 2 planes, clay tiles, 10% waste
Area: 2,904 sq ft | Tiles: 2,614

FAQ - Frequently asked questions

What is a roofing square?

One roofing square equals 100 sq ft of roof area. It is the standard unit used by roofing contractors and suppliers. A 2,000 sq ft roof is 20 squares. Shingles are sold in bundles (3 per square), underlayment is sold in rolls (4 squares per roll), and contractors quote pricing per square.

How do I measure the slope length without climbing on the roof?

Measure the horizontal distance from the outside wall to the center of the house (half the building width). Then measure the roof pitch with a level and ruler from inside the attic. Use the formula: slope = horizontal / cos(pitch angle). For a 6/12 pitch: slope = horizontal x 1.118. For 8/12: x 1.202. For 4/12: x 1.054.

Why does a hip roof need more waste allowance than a gable?

Hip roofs have diagonal ridges (hips) where two slopes meet at an angle. Shingles and panels must be cut to fit along these hip lines, creating triangular offcuts. A simple gable has only straight cuts at the ridge and eaves. Adding valleys, dormers or skylights further increases waste. Budget 15% for a clean hip, 20% if there are valleys or dormers.

Can my existing roof structure support clay tiles?

Clay and concrete tiles weigh 600-1,100 lb per square compared to 200-350 lb for asphalt shingles. Most standard roof trusses designed for shingles cannot support tiles without reinforcement. A structural engineer must verify the load capacity before switching to tile. Metal roofing (50-150 lb per square) is the lightest option and works on virtually any structure.

How much does a full roof replacement cost?

For a 2,000 sq ft gable roof (20 squares): asphalt shingle tear-off and install runs $10,000-18,000. Metal panels: $16,000-36,000. Clay tiles: $26,000-50,000. These include materials, labor, underlayment, flashing, and disposal. Steep pitch, second story, or complex geometry adds 20-40% to labor.

Do I need ice and water shield everywhere?

Building codes in cold climates (zones 4-7) require ice and water shield along the first 3 ft of the eave (from the edge inward). All valleys, around chimneys and skylights also need it. The rest of the roof gets synthetic underlayment. In warm climates (zones 1-2), ice shield is optional but recommended around penetrations. Budget $0.80-1.50/sq ft for ice shield vs $0.20-0.50 for synthetic felt.

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How much roofing does a 20 ft slope need?

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