Roof Curb Calculator

Calculate roof curb dimensions, framing lumber, metal cap flashing, insulation, and total installed cost for HVAC units, skylights, and other rooftop equipment on flat and low-slope commercial roofs

Enter the rooftop unit size and calculate curb dimensions including clearance and insulation thickness

Quick presets

inches
inches

Roof Area

1,500 sq ft

15.0 squares • 77 linear 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 Roof Curb Calculator helps commercial roofers, HVAC contractors, and building owners size and budget roof curbs for rooftop equipment installations. Whether you are installing a new HVAC package unit, adding a skylight, or replacing an existing unit with a different footprint, this calculator provides complete material takeoffs and cost estimates.

Dimensions tab: Start by entering the width and length of the equipment that will sit on the curb. The calculator adds the selected clearance to each side to determine the overall curb opening dimensions. Choose the curb height based on your climate and local snow loads — 14 inches is the standard recommendation, but increase to 18-24 inches in heavy snow areas. The insulation thickness selector determines how much rigid polyiso foam will line the interior of the curb to prevent thermal bridging and condensation. The calculator outputs the overall curb exterior dimensions, the board feet of lumber or linear feet of steel needed for the frame, and the square footage of insulation required.

Materials tab: Select the frame type (site-built lumber, steel channel, or pre-fabricated), the metal cap material, sealant method, and the number of identical curbs needed. The calculator produces a complete bill of materials including lumber or steel, cap flashing with formed corners, rigid insulation, fasteners, sealant, and membrane tie-in materials. Quantities are multiplied by the curb count so you get a single material list for the entire project.

Cost tab: Choose between site-built and pre-fabricated options, indicate whether crane access is needed, and select your roof membrane type and state. The calculator estimates the total installed cost broken down by materials, labor, and equipment. Pre-fab curbs have higher material costs but significantly lower labor costs, often making them the more economical choice on projects with multiple curbs or when labor rates are high.

The Formula
The roof curb calculator uses these formulas:

Curb Opening Dimensions Curb_Width = Unit_Width + (2 x Clearance) Curb_Length = Unit_Length + (2 x Clearance) Example: 48" unit + 2" clearance each side → Curb opening = 52" x 52"

Curb Perimeter (for cap flashing and lumber) Perimeter = 2 x (Curb_Width + Curb_Length) Example: 2 x (52 + 52) = 208 inches = 17.33 linear feet

Lumber Quantity (site-built, 2x frame) Board_Feet = (Perimeter_ft x Curb_Height_ft x 2) + (4 x Corner_Post_Height_ft) Example: 17.33 x (14/12) x 2 + (4 x 1.17) = 40.44 + 4.68 = 45.12 board feet

Metal Cap Flashing Area Cap_Area = Perimeter x (Curb_Height + Top_Width + 4" overlap) + 4 corner pieces Example: 208" x (14 + 3 + 4)" = 208 x 21 = 4,368 sq in = 30.3 sq ft

Rigid Insulation Area Insulation_Area = Perimeter x Curb_Height Example: 208" x 14" = 2,912 sq in = 20.2 sq ft

Installed Cost (site-built) Total = Lumber + Cap_Metal + Insulation + Fasteners + Sealant + Labor_Hours x Rate Example: $85 + $120 + $45 + $25 + $35 + (6 hrs x $75/hr) = $760 per curb
Example Calculation
Example: Two 5x4-foot RTU Curbs on a TPO Commercial Roof in Texas

A property manager is replacing two 5-ton rooftop HVAC package units on a single-story retail building in Houston. The new units measure 60" wide x 48" long. The existing roof is TPO single-ply over polyiso insulation. The building is in a low-snow area so standard 14" curb height is adequate. A crane is already scheduled for the HVAC installation.

Step 1: Curb Dimensions
• Unit size: 60" W x 48" L
• Clearance: 2" per side
• Curb opening: 64" W x 52" L (5.33' x 4.33')
• Curb height: 14"
• Insulation: 2" polyiso (R-13)

Step 2: Materials Per Curb
• Perimeter: 2 x (64 + 52) = 232 inches = 19.33 LF
• Lumber (2x14 PT): 19.33 LF x 2 sides + 4 corner posts = ~46 BF → 6 pieces 2x14x8 = $132
• Plywood nailer strips (3/4" x 3.5" x perimeter): $18
• Metal cap (24 ga galvanized, formed): 19.33 LF x 21" wide + 4 corners = $145
• Rigid polyiso 2" (20.2 sq ft interior): $52
• Fasteners (structural screws, cap nails): $28
• TPO membrane strip + weld: $65
• Sealant (polyurethane, 2 tubes): $24
• Material subtotal per curb: $464

Step 3: Labor
• Site-built curb construction: 5 hours x $65/hr = $325
• TPO membrane flashing: 2 hours x $75/hr = $150
• Labor per curb: $475

Step 4: Total for 2 Curbs (Texas pricing)
• Materials: 2 x $464 = $928
• Labor: 2 x $475 = $950
• Crane: Already on-site for HVAC (no additional cost)
• Waste factor (10%): $188
• Total: $2,066 for two complete insulated curbs with TPO membrane flashing

Frequently Asked Questions

What is a roof curb and why is it required for HVAC units and skylights?
A roof curb is a raised frame installed on a flat or low-slope roof that provides a watertight mounting platform for rooftop equipment such as HVAC package units, exhaust fans, skylights, and hatches. The curb raises the equipment above the roof surface so that standing water, snow, and debris cannot pool around the base and cause leaks. Building codes (IBC and IRC) require curbs to extend a minimum of 8 inches above the finished roof surface, though 12-14 inches is standard practice. The curb also provides a structural attachment point and allows the roofing membrane to be properly flashed up the curb sides, creating a continuous waterproof barrier. Without a curb, any rooftop penetration becomes a direct leak path into the building.
Should I use a pre-fabricated curb or build one on-site?
Pre-fabricated curbs are generally recommended for new construction and HVAC replacements because they are factory-welded from galvanized steel or aluminum with precise dimensions, integrated flashing flanges, and optional factory-installed insulation. They install in 1-2 hours versus 4-8 hours for a site-built curb, and the factory welds eliminate the field-solder joints that are a common leak point on site-built metal caps. However, site-built curbs from pressure-treated lumber make sense for odd-sized skylights, one-off penetrations, or budget-constrained projects where the material cost of $150-400 beats the $300-600 cost of a pre-fab unit. For HVAC adapter situations where you are replacing one unit with a different footprint, pre-fab adapter curbs that fit over the existing curb are almost always the best choice because they avoid tearing off and reflashing the original curb.
How tall should a roof curb be in snow country?
In areas with significant snowfall, curb height should be increased beyond the standard 12-14 inches to prevent snow from burying the curb flashing and blocking HVAC intake or exhaust. A general guideline is to make the curb height at least equal to the average peak snow depth on the roof. For areas with 30-50 psf ground snow loads (climate zones 5-6), an 18-inch curb is recommended. For 50+ psf snow loads (zones 6-7, mountain regions), 24-inch curbs are standard. Some jurisdictions and HVAC manufacturers specify minimum curb heights based on local snow data. Additionally, snow crickets — tapered insulation saddles on the uphill side of the curb — should be installed to divert meltwater around the curb rather than allowing it to pool against the flashing.
Why does a roof curb need insulation and what type should I use?
An uninsulated roof curb creates a thermal bridge — a direct conduction path between the cold outdoor air and the warm interior of the building. In winter, the inside surface of an uninsulated metal or wood curb can drop below the dew point, causing condensation to form and drip onto ceiling tiles, equipment, or inventory below. This condensation drip is one of the most common complaints on commercial buildings with uninsulated curbs. The standard solution is to line the inside of the curb with 2 inches of rigid polyisocyanate (polyiso) foam insulation, which provides approximately R-13. In cold climates (zones 5-8), 3 inches (R-19) is recommended. The insulation is adhered to the inside face of the curb with compatible adhesive and should extend from the roof deck to the top of the curb without gaps. Pre-fab insulated curbs have the foam factory-installed for consistent coverage.
How do I properly flash a roof curb to prevent leaks?
Proper curb flashing depends on the roof membrane type but follows the same general principle: the roofing membrane must wrap continuously up all four sides of the curb, with the metal counter-flashing or cap overlapping the membrane by at least 4 inches. On TPO and PVC single-ply roofs, a strip of membrane is heat-welded to the field sheet and run up each curb side, with the corners either factory-welded boots or field-formed with a hot-air gun. On EPDM roofs, adhesive-bonded EPDM strips are used with seam tape at all laps. On BUR and modified bitumen, base flashing sheets are torched or mopped up the curb sides in two plies. The metal cap then goes over the top of the curb, extending down at least 4 inches over the membrane on all sides, with a continuous bead of polyurethane sealant between the cap and membrane. All four corners of the cap must be soldered, welded, or sealed — corners are the primary failure point for curb leaks.

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