Ridge Board Calculator: Sizing & Height

A ridge board calculator works out the length and height of the ridge board for a gable roof, based on the building width, roof pitch, and wall thickness. You enter the building width, the roof pitch (rise per foot), and the wall thickness, and you get the ridge board length and the height the ridge must sit above the top of the wall plates. This is the critical starting point for framing a gable roof, because the ridge board ties the rafters together and sets the entire roof geometry.

Formula

The ridge board length is simply the building width (outside to outside) because the ridge runs the full length of the building, parallel to the walls. The ridge height is calculated from the horizontal run of the rafter (half the building width minus half the wall thickness) multiplied by the roof pitch (rise per foot). The formula for ridge height in inches is:

Ridge Height = (Building Width / 2 – Wall Thickness / 2) × Pitch

Where:

• Building Width (ft) – the total width of the building measured from outside of one wall to outside of the opposite wall.
• Wall Thickness (in) – the combined thickness of both exterior walls (e.g., 6 in for a typical 2×6 wall).
• Pitch (in/ft) – the vertical rise in inches for every 12 inches of horizontal run. A 6/12 pitch means 6 inches of rise per foot of run.

The ridge height is measured from the top of the wall plates to the top of the ridge board. This is the theoretical height where the top of the rafters meet; the actual ridge board may be slightly lower if you use a different rafter connection method.

Worked Example

Using the default values from the calculator: building width = 24 ft, roof pitch = 6/12, wall thickness = 6 in.

1. Ridge board length = building width = 24 ft.
2. Horizontal run of rafter = (24 ft × 12 in/ft) / 2 – 6 in / 2 = 144 in – 3 in = 141 in = 11.75 ft.
3. Ridge height above plate = 11.75 ft × 6 in/ft = 70.5 in.

The calculator gives 24 ft for the length and 70.50 in for the height, matching the manual calculation.

How to Use the Ridge Board Calculator

Measure the building width from outside of one wall to outside of the opposite wall. This is the same dimension you would use for the floor joists or the exterior wall length. The wall thickness is the actual thickness of the framed wall, not including siding or drywall – typically 4.5 in for a 2×4 wall with sheathing, or 6 in for a 2×6 wall. For the pitch, use the standard roof pitch notation (e.g., 4/12, 6/12, 8/12) – the calculator expects the rise in inches per 12 inches of run.

The result gives you two key numbers:

• Ridge board length – the total length of the ridge board you need to purchase, usually a 2×6 or 2×8 board.
• Ridge height above plate – the vertical distance from the top of the wall plates to the top of the ridge board. This is the height you set your ridge board at during framing.

Remember that the ridge height is measured to the top of the ridge board, not the bottom. If you are using a ridge board that is thicker than the rafter thickness, you may need to adjust the height slightly.

Common Mistakes and Misconceptions

One common mistake is forgetting to subtract half the wall thickness from the run. The rafter does not start at the outside edge of the wall; it sits on the top plate, which is inset from the outside. Using the full half-width will give you a ridge height that is too high by the pitch times half the wall thickness.

Another mistake is confusing the ridge board length with the rafter length. The ridge board runs the full length of the building, while the rafters are the sloped members that span from the ridge to the walls. The calculator only gives the ridge board dimensions, not the rafter length (which you can calculate with a rafter calculator).

A common misconception is that the ridge board must be the same thickness as the rafters. In traditional framing, the ridge board is typically 1× or 2× material, and the rafters are nailed to it. The International Residential Code (IRC) requires a ridge board to be at least 1 inch thick (nominal) when the roof slope is 3/12 or greater. For steeper roofs or where the ridge is unsupported, a structural ridge beam may be required – that is a different calculation involving loads.

When to Use a Ridge Board vs. a Ridge Beam

A ridge board is a non-structural member that provides a nailing surface for the rafters. It is only required to be 1 inch thick (nominal) and is used when the rafters are tied together by ceiling joists or rafter ties, which prevent the walls from spreading. A ridge beam, on the other hand, is a structural member that supports the rafters and transfers the roof load to posts or walls. Ridge beams are used when you have a cathedral ceiling or when the roof is designed without rafter ties.

If you are using a ridge beam, the height calculation is the same, but the beam must be sized by an engineer to carry the roof loads. The calculator here is for a ridge board, which is the more common approach for conventional gable roofs with ceiling joists.

Units and Conversions

The calculator uses feet for the building width and inches for wall thickness and pitch. If you have measurements in meters or centimeters, convert them before entering:

• 1 ft = 12 in = 0.3048 m
• 1 in = 2.54 cm
• 1 m = 3.28084 ft

For example, a building width of 7.32 m is 24 ft. A wall thickness of 15 cm is about 5.9 in. The pitch is a ratio (rise over run), so it is unitless – a 6/12 pitch is the same in metric, meaning 6 cm rise per 12 cm run.

Limitations of the Calculator

This calculator assumes a symmetrical gable roof with equal pitches on both sides. It does not account for:

• Overhangs – the ridge board length is the building width, not including any overhang beyond the walls. If you have an overhang, the ridge board may need to be longer if the overhang extends past the gable ends.
• Ridge board thickness – the height is to the top of the ridge board. If you use a thicker ridge board (e.g., 2×8 instead of 2×6), the top of the rafter will be at the same height, but the bottom of the ridge board will be lower.
• Unequal pitches – if the roof has different pitches on each side, the ridge is not centered, and the calculation is more complex.
• Structural requirements – the calculator only gives geometry, not structural adequacy. Always check local building codes and consult a structural engineer for large spans or unusual conditions.

Frequently Asked Questions

What is the difference between a ridge board and a ridge beam?

A ridge board is a non-structural member, typically 1× or 2×, that provides a nailing surface for rafters. It is used when the roof is stabilized by ceiling joists or rafter ties. A ridge beam is a structural member that supports the rafters and transfers loads to posts or walls, used for cathedral ceilings or when rafter ties are not present. The height calculation is the same, but the beam must be engineered.

How do I measure the roof pitch?

Roof pitch is the rise over run, usually expressed in inches per foot. For example, a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. You can measure it with a speed square or a level and a tape measure: hold a level horizontally for 12 inches, then measure the vertical drop to the roof surface.

What is the minimum ridge board thickness?

Most building codes require a ridge board to be at least 1 inch nominal thickness (e.g., a 1×6 or 2×6). The actual minimum may vary by local code, but 2×6 is common for rafters up to 24 inches on center. For wider spans or heavier loads, a thicker ridge board or a ridge beam may be needed.

Can I use the ridge board length as the rafter length?

No. The ridge board runs horizontally along the peak, while rafters run diagonally from the ridge to the walls. Rafter length is longer than the horizontal run because of the slope. Use a rafter calculator to find the actual rafter length, which includes the pitch and any overhang.

Why does the ridge height calculation subtract half the wall thickness?

Because the rafter sits on the top plate, which is inset from the outside edge of the wall. The horizontal run of the rafter is measured from the center of the ridge to the inside edge of the wall plate, not the outside. Subtracting half the wall thickness gives the correct run, which is essential for an accurate ridge height.

What if my roof has an overhang at the gable ends?

The calculator gives the ridge board length as the building width. If you want the ridge board to extend beyond the gable ends to support an overhang, you need to add the overhang dimension to each end. For example, a 24 ft building with a 12 in overhang on each end would need a 26 ft ridge board.

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in b | Building width (outside to outside) | ft | 24 | 0 | 1000
in p | Roof pitch (rise per 12 in run) | in/ft | 6 | 0.25 | 24
in t | Wall thickness (each side) | in | 6 | 0 | 24
out len | Ridge board length | ft | b | 1
out hgt | Ridge height above wall plate | in | (b * 12 / 2 - t) * p / 12 | 2