Skylight Shaft Calculator

Calculate light shaft framing, insulation, drywall, and finishing costs for skylight installations through attic space including straight and splayed shaft configurations by skylight size and attic depth

Enter skylight size, attic depth, and shaft configuration

Quick presets

in
in
in

Roof Area

624 sq ft

24×24 ft • 6.2 squares

Estimated Cost Range

$2,495 – $4,990

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 Skylight Shaft Calculator estimates the framing, insulation, drywall, and finishing materials needed to build a light shaft from a roof-mounted skylight through the attic space to the ceiling plane below. Unlike the Skylight Calculator (which covers the skylight unit, flashing kit, and roof installation), this tool focuses on the shaft construction — the tunnel of light that connects the roof opening to the room below. Most skylight installations in homes with attic space require a shaft, and its cost is often underestimated during project planning.

Shaft Dimensions tab: Enter the skylight rough opening dimensions (width and length in inches) and measure the attic depth — the vertical distance from the roof sheathing to the top of the ceiling joists. Choose your shaft configuration: straight (simplest, least light), splayed (angled walls, more light, most popular), or fully flared (all four walls angled, maximum light). The splay angle determines how much wider the ceiling opening is than the roof opening — 45 degrees is the standard recommendation. Deeper attics with splayed shafts produce dramatically larger ceiling openings that flood the room with natural light.

Framing & Materials tab: Select the framing lumber size — 2x6 is standard because it accommodates R-19 insulation. Choose the header size based on the opening span: double 2x8 for most residential skylights, larger for wide openings. Select insulation type and drywall thickness. The calculator computes all framing lumber (studs, plates, headers, jack studs), insulation square footage, drywall sheets, and finishing materials (tape, compound, primer, paint).

Cost Estimate tab: Compare DIY, handyman, and contractor approaches. The shaft construction is separate from the skylight installation and is often quoted as a line item by roofing contractors. The calculator provides a materials-only total for DIY and adds labor estimates for professional installation. White semi-gloss paint is recommended for the shaft interior to maximize light reflection into the room below.

The Formula
The skylight shaft calculator uses these formulas:

Ceiling Opening Size Straight shaft: Ceiling opening = Roof opening (same dimensions) Splayed shaft: Ceiling Width = Roof Width + 2 x (Attic Depth x tan(Splay Angle)) Flared shaft: Both width and length are increased by 2 x (Attic Depth x tan(Splay Angle)) Example: 24" roof opening + 2 x (36" x tan(45°)) = 24 + 72 = 96" ceiling opening

Shaft Wall Area Each wall: Area = (Top Edge + Bottom Edge) / 2 x Shaft Depth along wall slope Shaft depth (sloped) = Attic Depth / cos(Splay Angle) for splayed walls Shaft depth = Attic Depth for straight walls Total interior surface area = Sum of 4 wall areas

Framing Lumber Studs: Perimeter of ceiling opening / 16" OC spacing (both top and bottom plates) Headers: 2 double headers per opening x 2 openings (roof + ceiling) = 4 double headers Jack studs/trimmers: 2 per side per opening = 8 total Plates (top + bottom): 2 x perimeter of each opening Blocking: 1 piece per stud bay for drywall attachment

Insulation Insulation area = Total shaft wall area x (1 + Waste%) Batts: Area / batt coverage (fiberglass R-19 = 48.96 sq ft per bag) Spray foam: Area x depth in inches x $1.00-$1.50/board foot

Drywall Drywall area = Total shaft interior surface area x (1 + Waste%) Sheets (4x8): ceil(Drywall area / 32 sq ft per sheet) Tape: Perimeter of each sheet + seams ≈ Total shaft perimeter x 2 Joint compound: 1 box per 4 sheets

Paint Primer: 1 gallon per 300 sq ft of shaft interior Paint (2 coats): 2 gallons per 300 sq ft of shaft interior

Total Cost Materials = Lumber + Headers + Insulation + Drywall + Tape/Compound + Paint Labor (contractor) = $20-$40/sq ft of shaft wall area State multiplier applied to labor
Example Calculation
Example: Standard Splayed Shaft — 24x48 Skylight, 36" Attic, Contractor in US Average

Jennifer is installing a 24x48 inch skylight in her ranch home with a 36-inch deep attic. She wants a splayed shaft with 45-degree splay on the two side walls to maximize natural light in her kitchen. A licensed contractor will build the shaft.

Step 1: Ceiling Opening
• Roof opening: 24" wide x 48" long
• Side splay: 2 x (36" x tan(45°)) = 2 x 36 = 72" added to width
• Ceiling opening: (24 + 72) = 96" wide x 48" long (uphill/downhill walls straight)
• Ceiling opening area: 96 x 48 = 4,608 sq in = 32 sq ft

Step 2: Shaft Wall Areas
• Two splayed side walls: Each (24 + 96) / 2 x (36 / cos(45°)) = 60 x 50.9 = 3,054 sq in = 21.2 sq ft each
• Two straight end walls (approx): Each 48 x 36 = 1,728 sq in = 12 sq ft each
• Total shaft interior: 2 x 21.2 + 2 x 12 = 66.4 sq ft
• With 10% waste: 66.4 x 1.10 = 73 sq ft

Step 3: Materials
• Framing: 12 studs (2x6x8), 4 double headers (2x8x4'), 8 jack studs, plates = ~$180
• Insulation: 73 sq ft fiberglass R-19 = 2 bags at $45 = $90
• Drywall: 73 sq ft / 32 = 3 sheets 1/2" at $14 = $42
• Tape, compound, primer, paint = ~$60
• Materials subtotal: $372

Step 4: Labor
• Contractor labor: 66.4 sq ft x $25/sq ft = $1,660
• US average multiplier: 1.00
• Labor subtotal: $1,660

Step 5: Total
• Materials: $372
• Labor: $1,660
• Total shaft cost: $2,032

Summary: A splayed skylight shaft for a 24x48 inch skylight through a 36-inch attic, professionally framed and finished with R-19 insulation, drywall, and white semi-gloss paint: approximately $2,030. This is in addition to the skylight unit (~$300-$800) and roof installation (~$500-$1,000). The splayed design creates a 96-inch (8-foot) wide ceiling opening that distributes light across the entire kitchen.

Frequently Asked Questions

What is the difference between a straight shaft and a splayed shaft?
A straight shaft has vertical walls that create a ceiling opening identical in size to the roof opening — essentially a rectangular box from roof to ceiling. Light enters only within the footprint of the skylight and illuminates a relatively small area directly below. A splayed shaft angles one or more walls outward so the ceiling opening is wider and/or longer than the roof opening, spreading natural light across a much larger floor area. The most common configuration splays the two side walls at 45 degrees while leaving the uphill and downhill walls straight or at a shallower angle. A fully flared shaft splays all four walls. Studies show that a splayed shaft distributes 30-50% more usable light into the room than a straight shaft of the same skylight size. The tradeoff is more framing lumber, more drywall, and a larger ceiling opening that requires cutting more ceiling joists.
How much does it cost to build a skylight shaft?
A skylight shaft typically costs $500-$2,000 for materials and labor, in addition to the skylight unit and roof installation costs. A simple straight shaft through a shallow attic (24 inches) with basic fiberglass insulation and drywall runs $500-$800 total. A standard splayed shaft through a 36-inch attic with R-19 insulation, drywall, and paint costs $800-$1,400. A fully flared shaft through a deep attic (48+ inches) with spray foam insulation and fire-rated drywall can reach $1,500-$2,000 or more. These costs include headers, jack studs, insulation, drywall, tape, compound, primer, and paint. Labor typically accounts for 50-60% of the total shaft cost because the work involves cutting structural members, working in a confined attic space, and finishing drywall in an awkward vertical configuration.
Do I need to insulate the skylight shaft walls?
Yes, insulating skylight shaft walls is required by building codes in virtually all climate zones and is critical for energy efficiency and moisture control. The shaft walls separate conditioned living space from unconditioned attic air, creating a temperature differential that can cause condensation, frost, and energy loss if left uninsulated. Most energy codes require a minimum of R-19 insulation in skylight shaft walls, which fits in a 2x6 framed wall with fiberglass or rockwool batts. In cold climates (zones 5-8), R-23 or higher may be required. A continuous vapor barrier on the warm (interior) side of the insulation prevents moisture-laden room air from reaching the cold sheathing. Closed-cell spray foam is the premium option because it provides both insulation and an air/vapor barrier in one application, eliminating condensation risk entirely.
What size headers do I need for a skylight shaft?
Skylight shafts require double headers at both the roof opening (between rafters) and the ceiling opening (between joists) to transfer structural loads around the openings. The header size depends on the span. For openings up to 36 inches wide, double 2x6 headers are adequate. For openings 36-48 inches, use double 2x8 headers. For 48-60 inch openings, double 2x10 headers are needed. For openings over 60 inches, use double 2x12 headers or engineered lumber (LVL). Each opening also requires trimmer rafters or jack studs on each side to support the headers. If you are cutting more than two rafters or joists to create the opening, a structural engineer should verify the header sizing because the load increases with the number of cut members. For splayed shafts, the ceiling opening is larger than the roof opening and may require larger headers at the ceiling level.
Can I build a skylight shaft myself as a DIY project?
Building a skylight shaft is a moderate to advanced DIY project that requires carpentry skills, comfort working in an attic, and an understanding of structural framing. The key challenge is cutting ceiling joists and/or rafters, which are structural members that carry the roof and ceiling loads. You must install proper headers and trimmers before cutting any framing member, and the header size must be adequate for the span. If you are comfortable with framing, insulation, and drywall work, a straight shaft through a shallow attic is a feasible weekend project. Splayed and flared shafts are more complex because the angled walls require compound cuts and careful layout. The total material cost for a DIY shaft is typically $200-$600 depending on size and insulation choice. Where DIY falls short is usually the drywall finishing — taping and mudding inside a shaft is awkward and novice finishers often leave visible seams that are illuminated by the skylight above.

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