Dormer Roof Calculator

Calculate dormer roof area, shingle materials, and construction cost for gable, shed, and hip dormers

Calculate roof area and materials for a single dormer

Quick presets

ft
ft

Estimated Materials

0 bundles

0.0 squares • 0 sq ft

PRO

Professional Calculator

Extended parameters for precise calculations

sq ft

Estimated Materials

60 bundles

Roof Area

1,792 sq ft

Squares

17.9

Detailed Breakdown

Roof Area1,792 sq ft
With Waste1,971 sq ft
Roofing Squares17.9
Bundles60
How to Use This Calculator
The Dormer Roof Calculator helps you plan dormer additions with accurate area, material, and cost estimates:

Single Dormer tab: Select your dormer type (gable, shed, or hip), then enter the width, face height, projection depth, and roof pitch. The calculator computes the dormer roof surface area based on the geometry of the selected type. Gable dormers calculate two sloping triangular panels plus a ridge. Shed dormers calculate a single rectangular slope. Hip dormers calculate three sloping panels. The result is the additional roof area that needs shingles, underlayment, and flashing.

Multiple Dormers tab: If you are adding two or more identical dormers, enter the count and dimensions for one dormer. The calculator multiplies the per-dormer area by the count, adds the waste factor, and provides a combined materials list. This is common for Cape Cod and Colonial-style homes where two or three matching gable dormers are evenly spaced across the roof face.

Cost tab: For a complete project budget, enter your dormer specifications along with the finish level and state. The calculator estimates total construction cost including structural framing (headers, trimmers, cripple rafters), roofing (shingles, underlayment, flashing), exterior finish (siding, trim, window), and interior finish (drywall, insulation, paint). Basic finish covers the shell only; standard includes a window and interior finish; premium adds upgraded materials throughout.

The Formula
The dormer roof area calculations use these formulas:

Gable Dormer: Slope Length = Dormer Depth / cos(arctan(Pitch/12)) Each Side Area = Slope Length x (Dormer Width / 2) / cos(arctan(Pitch/12)) Simplified: Each Side = Dormer Depth x (Dormer Width / 2) x Pitch Multiplier / (Dormer Width / 2) Total Roof Area = 2 x Slope Length x (Dormer Width / 2)

Shed Dormer: Slope Length = Dormer Depth x Pitch Multiplier Roof Area = Slope Length x Dormer Width

Hip Dormer: Front Face Area = 0.5 x Dormer Width x Dormer Depth x Pitch Multiplier Two Side Triangles = 2 x (0.5 x Dormer Depth x Side Slope Height) Total Roof Area = Front + Sides (approximately 1.15x shed dormer area)

Cost Estimate: Framing Cost = $40-80/sq ft of dormer footprint Roofing Cost = Roof Area x $5-9/sq ft (installed) Window + Exterior = $1,200-4,000 per dormer Interior Finish = $25-60/sq ft of floor area added Total = Framing + Roofing + Exterior + Interior + Permit
Example Calculation
Example: Twin Gable Dormers on a Cape Cod in Massachusetts

Sarah wants to add two 5-ft-wide gable dormers to her Cape Cod home. Each dormer will be 5 ft tall at the face with 5 ft of projection and a 6/12 pitch.

Step 1: Calculate roof area per dormer
• 6/12 pitch multiplier = 1.118
• Slope length = 5 ft depth x 1.118 = 5.59 ft
• Each side width = 5 ft / 2 = 2.5 ft
• Each dormer roof area = 2 x 5.59 x 2.5 = 27.9 sq ft

Step 2: Total roof area for two dormers
• 27.9 x 2 dormers = 55.9 sq ft
• With 15% waste: 55.9 x 1.15 = 64.3 sq ft
• Less than 1 roofing square — 3 bundles of shingles

Step 3: Estimate construction cost (standard finish)
• Framing per dormer: 5 x 5 ft footprint = 25 sq ft x $60/sq ft = $1,500
• Roofing per dormer: 28 sq ft x $7/sq ft = $196
• Window + exterior: $2,200 per dormer
• Interior finish: 25 sq ft x $40/sq ft = $1,000
• Per dormer: $4,896

Step 4: Total project cost
• 2 dormers x $4,896 = $9,792
• Permit + engineering: ~$1,800
• Massachusetts labor premium (+20%): $9,792 x 0.20 = $1,958
• Total estimated cost: $13,550

Frequently Asked Questions

What are the main types of dormers?
The three most common dormer types are gable, shed, and hip. Gable dormers have a peaked roof with two sloping sides and are the most traditional style, typically 4-8 ft wide. Shed dormers have a single flat-sloped roof and are best for maximizing interior space, spanning 8-30 ft or more. Hip dormers have three sloping sides converging at a point and offer a softer, more integrated look. Less common types include eyebrow dormers (curved), barrel dormers, and wall dormers that extend the exterior wall upward.
Do I need a permit to add a dormer?
Yes, in virtually all jurisdictions in the United States, adding a dormer requires a building permit because it involves structural modification of the roof. You will typically need architectural drawings, a structural engineering review (especially for larger shed dormers), and inspections at framing, electrical, insulation, and final stages. Permit costs range from $500-2,500 depending on your municipality. Some HOAs also require approval. Expect the permitting process to take 2-6 weeks.
How much does it cost to add a dormer in 2026?
A standard gable dormer costs $5,000-12,000 including framing, roofing, window, siding, and interior finish in 2026. Shed dormers cost $6,000-18,000 depending on width. A full-width shed dormer across the entire roof face can cost $15,000-35,000. Hip dormers run $7,000-16,000 due to more complex framing. These prices include materials and labor but not permits or engineering fees. Costs are 15-30% higher in high-cost markets like California, New York, and Boston.
What structural requirements are needed for a dormer?
Adding a dormer requires cutting through existing rafters, which means the load must be redistributed. A structural header is installed at the top and bottom of the opening, supported by trimmer rafters (typically doubled). The dormer sidewalls are framed with studs tied into the main roof rafters. For shed dormers wider than 8 ft, an engineered beam or LVL header is usually required. The existing ceiling joists must also support the added load. A structural engineer should review any dormer wider than 6 ft or on roofs with long rafter spans.
How much living space does a dormer add?
A single 6-ft-wide gable dormer adds roughly 30-40 sq ft of usable floor area (where ceiling height exceeds 5 ft) to a second-story room. A 16-ft shed dormer adds 100-140 sq ft of usable space. A full-width shed dormer (24 ft) can add 200-300 sq ft by raising the ceiling along the entire room. The exact gain depends on the main roof pitch, dormer depth, and the headroom threshold you consider usable. Dormers also dramatically improve natural light and ventilation.

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