Free Rafter Calculator

How to Use This Rafter Calculator

Our rafter calculator helps you determine the exact measurements and materials needed for roof framing. Whether you're building a new structure, adding an addition, or replacing an existing roof, this tool calculates rafter lengths, cut angles, and provides a complete material list.

Understanding Roof Framing Basics

Before using the calculator, it's helpful to understand the key terms:

• Span - The total width of the building from outside wall to outside wall
• Run - Half the span (for standard gable roofs) - the horizontal distance from wall to ridge
• Rise - The vertical height from the top plate to the ridge
• Pitch - The roof slope expressed as rise per 12 inches of run (e.g., 6:12)
• Rafter Length - The actual length of lumber needed, measured along the rafter
• Overhang - How far the rafter extends past the wall (the eave)

Step 1: Select Your Roof Type

Different roof styles require different framing approaches:

• Gable (Two-Slope) - The most common residential roof. Two sloped surfaces meeting at a central ridge. Simple to frame with straight common rafters.
• Hip (Four-Slope) - All four sides slope toward the walls. More complex framing with hip rafters, jack rafters, and common rafters. Better wind resistance.
• Shed (Single-Slope) - One sloped surface, often used for additions, porches, and outbuildings. Simplest framing with rafters running in one direction.
• Gambrel (Barn Style) - Two different slopes on each side, creating more headroom in the attic. Common on barns and Dutch colonial homes.

Step 2: Enter Building Dimensions

Measure carefully - accurate dimensions are critical for proper fit:

• Building Span - Measure from the outside of one wall to the outside of the opposite wall
• Building Length - The length of the ridge (parallel to the rafters)
• Overhang - Typical residential overhangs are 12-24 inches. Longer overhangs provide better weather protection.

Step 3: Determine Roof Pitch

Roof pitch is expressed as a ratio of rise to run. For example, a 6:12 pitch rises 6 inches for every 12 inches of horizontal run.

Understanding Rafter Cuts

Every rafter requires three basic cuts to fit properly:

Plumb Cut (Ridge Cut)

The plumb cut is made at the top of the rafter where it meets the ridge board. This cut is vertical (plumb) when the rafter is in position. The angle is determined by the roof pitch.

How to mark: Set your framing square with the pitch (e.g., 6 on the tongue, 12 on the blade) and mark along the tongue for the plumb cut.

Birdsmouth Cut

The birdsmouth is a notch cut where the rafter sits on the wall's top plate. It consists of:

• Seat Cut (Level Cut) - Horizontal cut that rests on the top plate
• Heel Cut (Plumb Cut) - Vertical cut against the outside of the wall

Rule of thumb: The seat cut depth should not exceed 1/3 of the rafter depth to maintain structural integrity. For a 2x6 rafter (5.5" actual), maximum seat cut is about 1.8".

Tail Cut (Fascia Cut)

The tail cut is made at the end of the overhang where the fascia board attaches. This is typically a plumb cut, but can be modified for different fascia styles.

Rafter Sizing Guide

Rafter size depends on span, spacing, species, grade, and loads. Here are general guidelines for Douglas Fir #2 with 16" spacing and normal residential loads:

Important: Always verify with local building codes and span tables. Snow loads, wind loads, and lumber species significantly affect allowable spans.

What's the minimum roof pitch for shingles?

Most asphalt shingles require a minimum 4:12 pitch. Lower pitches (2:12 to 4:12) may require ice and water shield underlayment. Below 2:12, you'll need low-slope roofing like modified bitumen, EPDM, or TPO.