How do I calculate lean-to shed rafter length?

Use the horizontal roof run and vertical rise as the two sides of a right triangle. Rafter length before cuts is the square root of run squared plus rise squared, then add the overhang allowance if the overhang is included in the run.

What pitch is good for a lean-to shed roof?

Many lean-to sheds use about 2/12 to 4/12 pitch, but the right pitch depends on roofing material, local rain or snow, and the manufacturer's minimum slope.

Does this calculator include birdsmouth cuts?

No. It gives the planning rafter length and plumb-cut angle before final layout details such as birdsmouth depth, ridge or ledger fastening, blocking, and local code adjustments.

Can I calculate high wall height from pitch?

Yes. Enter a pitch such as 3/12 and the calculator estimates the vertical rise between the low wall and high wall across the shed depth.

Single-slope shed framing

Lean-to Shed Rafter Calculator

Calculate lean-to shed rafter length, roof pitch, angle, high-wall rise, roof surface area, rafter count, sheathing sheets, and a rough material estimate before you buy lumber.

Rafter length 8 ft 8.5 in 3/12 pitch at a 14° roof angle.

Shed and roof size

Calculate rise from Shed depth / horizontal run (feet) Roof pitch rise per 12 in Known vertical rise (inches) Shed length along wall (feet) Low wall height (feet) Front overhang (inches) Back overhang (inches) Side overhang each end (inches)

For a lean-to roof, the run is the horizontal distance from the low wall to the high wall. Overhangs are added to estimate roof surface area and lumber length.

Materials

Rafter spacing (inches) Waste allowance (%) Sheathing coverage (sq ft) Price per sheathing sheet ($) Price per rafter board ($) Extra blocking/fastener budget ($)

Use this for planning quantities. Confirm span tables, snow loads, uplift hardware, fasteners, and minimum roof slope before building.

How to use the lean-to shed rafter calculator

Enter the shed depth from the low wall to the high wall.
Choose whether you know the roof pitch or the vertical rise.
Add shed length, overhangs, rafter spacing, waste, and local material prices.
Use the results to plan rafter board length, sheathing, and the high-wall height difference.

The calculator treats the lean-to roof as a right triangle. It converts pitch to rise when needed, then estimates the sloped rafter length, roof angle, roof surface area, rafter count, and basic material quantities.

Lean-to shed rafter example

For an 8 ft deep shed with a 3/12 pitch, the roof rises 24 inches from the low wall to the high wall. With 6 inch front and back overhangs, each rafter is about 8 ft 8.5 in before final cuts, so 10 ft boards are a practical planning length.

A steeper pitch sheds water faster but raises the high wall and increases roof area. A lower pitch can keep the shed shorter, but your roofing material may require special underlayment or may not be rated for that slope.

FAQ

What is the formula for lean-to rafter length?

Rafter length is the square root of horizontal run squared plus vertical rise squared. If you include front and back overhangs, add those overhangs to the horizontal run before calculating the sloped length.

What pitch should I use for a lean-to shed?

Around 2/12 to 4/12 is common for simple lean-to sheds, but snow, rain, appearance, roofing material, and local rules all matter. Always check the minimum pitch listed for your roofing product.

How many rafters do I need?

The calculator divides the shed length by your chosen spacing and adds an end rafter. Common spacing is 16 inches or 24 inches on center, but the correct spacing depends on lumber size, span, load, and local code.

Should I buy the exact rafter length shown?

No. Buy the next common board length above the calculated length so you have room for plumb cuts, seat cuts, end trimming, and small layout adjustments.

How to use the lean-to shed rafter calculator

Lean-to shed rafter example

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