Cedar Shake Calculator

Follow these steps to estimate cedar shake bundles for your roof.

1. Enter the roof area in square feet. Measure the sloped surface, including pitch, rather than the footprint of the house.
2. Select the shake length, 18 in or 24 in, as listed on your shake bundles.
3. Choose the weather exposure, 7.5 in or 10 in. Eighteen inch shakes allow a maximum of 7.5 in on roofs of 4:12 pitch and steeper.
4. Pick the shake grade, medium or heavy. This is shown for your records and does not change the math.
5. Enter hip and ridge length in feet, or 0 if the roof has no hips or ridges needing ridge units.
6. Select a waste factor and read the results for field bundles, squares and hip and ridge bundles.

A cedar shake calculator is a tool that turns a roof measurement into a shopping list of shake bundles. Cedar shakes are sold in bundles, and a roofing square is 100 square feet of roof, so the calculator works in squares and then applies the number of bundles needed to cover each square at your chosen exposure.

The Cedar Shake and Shingle Bureau, or CSSB, publishes the coverage and exposure tables that roofers use for this work. Shakes come in 18 inch and 24 inch lengths, and the exposure is the part of each shake left visible to the weather. A smaller exposure covers less area per bundle, so it needs more bundles per square. Hips and ridges are covered with separate ridge units sold in their own bundles, so they are counted on their own.

The calculator first converts roof area to squares. It then multiplies by the bundles needed per square, which depends on shake length and exposure. A waste factor is applied to the field bundles, and the result is rounded up to whole bundles because shakes are not sold in part bundles. Hip and ridge bundles are counted separately from the length of those runs.

Constants used:

• 100 sq ft per roofing square
• 5 bundles per square for 18 in shakes at 7.5 in exposure, per the CSSB coverage table for an 18 in, 12/12 pack
• 5 bundles per square for 24 in shakes at the 10 in maximum exposure
• 7 bundles per square for 24 in shakes at 7.5 in exposure, a simplified rounding of the CSSB ratio of 10 divided by 7.5 applied to 5 bundles
• 20 ridge units per bundle, from the CSSB Certi-Ridge product, so one bundle covers 20 times the exposure divided by 12 feet
• Waste factor of 10%, 15% or 20% applied to field bundles

A 2,000 sq ft roof covered with 24 in heavy shakes at 10 in exposure, 10% waste and 60 ft of hip and ridge:

1. Squares: 2,000 / 100 = 20 squares
2. Bundles before waste: 20 x 5 = 100 bundles
3. With 10% waste: 100 x 1.10 = 110 field bundles
4. Coverage per ridge bundle: 20 x 10 / 12 = 16.67 ft
5. Hip and ridge: 60 / 16.67 = 3.6, rounded up to 4 bundles

Shake Length

Longer shakes can be laid at a wider exposure. The CSSB lists a maximum exposure of 7.5 in for 18 in shakes and 10 in for 24 in shakes on roofs with a 4:12 pitch or steeper.

Weather Exposure

Exposure is the biggest driver of the bundle count. Setting 24 in shakes at 7.5 in instead of 10 in raises the bundles per square from 5 to roughly 7 in this calculator.

Roof Pitch

Lower pitches call for reduced exposure and added underlayment, so check the CSSB tables for your slope before choosing an exposure. Use the true sloped area, not the plan area.

Shake Grade and Butt Thickness

Medium shakes have a butt of about 1/2 in and heavy shakes about 3/4 in. Heavy shakes give a deeper shadow line and last longer, but the bundle count here is the same.

Roof Complexity

Valleys, dormers and hips create cut waste. A cut-up roof needs a higher waste factor than a plain gable.

Costs depend on the grade, the bundle price in your area and the labor to install. Multiply the bundle count by your supplier price, then add ridge units, underlayment, nails and labor. Pricing is not provided here because it varies by region and supplier and was not verified from a primary source.

Planning a New Roof

Use it when you need a bundle count before requesting quotes, so each supplier prices the same quantity.

Re-Roofing a Shake Roof

When replacing worn shakes, the calculator gives a starting order and shows how a change in exposure moves the count.

Repairing a Section

For a partial repair, enter only the affected area to see how many bundles to keep on hand.

Checking a Contractor Quote

Compare the quoted bundle count to your own estimate to spot padding or gaps, and ask whether the quote includes hip and ridge units.

Using Footprint Instead of Roof Area

A sloped roof has more surface than the house footprint. Multiply by the pitch factor or measure the sloped surface.

Exceeding Maximum Exposure

Laying 18 in shakes at 10 in exposure goes beyond the CSSB maximum for that length. This calculator blocks that combination.

Forgetting Hips and Ridges

Field bundles do not cover hips and ridges. Those use separate ridge units, so enter their length.

Skipping Waste

Cuts at valleys, hips and edges use extra shakes. A waste factor avoids a second order that may not match in color.

Ignoring the Starter Course

The first course is doubled, and the CSSB manual calls for a starter layer of 15 in shakes or shingles under it. Add a few extra bundles or starter material if your waste allowance is thin.

This calculator gives a planning estimate. Check it against the coverage chart printed on the bundles you buy.

It depends on length and exposure. The CSSB coverage table lists 5 bundles per square for 18 in shakes at 7.5 in exposure. This calculator uses 5 for 24 in shakes at 10 in and about 7 for 24 in shakes at 7.5 in.

On roofs of 4:12 pitch and steeper, the CSSB lists 7.5 in for 18 in shakes and 10 in for 24 in shakes. Lower pitches need a reduced exposure, so check the CSSB tables for your slope.

Common lengths are 18 in and 24 in, which are the two options in this calculator. Both are available in medium and heavy grades, with butt thicknesses of about 1/2 in and 3/4 in.

Not in this calculator. Grade affects thickness, appearance and durability, but the bundle count here depends on length, exposure, area and waste. The grade selector is for your records.

The CSSB roof manual calls for 2 nails per shake. A 6d box nail at 2 in is listed for 18 in and 24 in handsplit-and-resawn shakes. This calculator does not count nails because shakes per bundle vary.

A bundle of CSSB Certi-Ridge holds 20 units. Each unit covers the exposure, so a bundle covers 20 times the exposure divided by 12 feet, about 16.67 ft at 10 in exposure.

Many roofers allow 10% on a plain roof and more on a cut-up one. The percentages here are a selectable assumption and were not verified from a primary source, so confirm with your supplier.

The CSSB manual calls for a 3/8 in to 5/8 in space between adjacent shakes so they can swell when wet. Joints in successive courses should be offset.

Yes. The first course is doubled, and the CSSB manual calls for a starter layer of 15 in shakes or shingles beneath it. Allow extra material for it.

Measure the sloped roof surface. A pitched roof has more area than the footprint of the building, and using the footprint will undercount the bundles you need.

• CSSB Roof Manual 2020 (Cedar Shake and Shingle Bureau installation manual).
• CSSB Coverage and Exposure Tables (exposure rates and bundles per square).
• CSSB Product Specification Guide (shake sizes, grades and coverage formula).
• CSSB Certi-Ridge Products (hip and ridge units per bundle).