Slate Roof Calculator — Slates by Size and Headlap

Slates a roof takes, from the published schedule for size and lap, breakage included.

Showing US units

Field slate to order EXAMPLE3,345 slates18 x 11 in each, 5% extra for breakageThat is 19 squares (175 slates each) plus 20 slates.Field slate only: the starter course along the eaves, hip and ridge slates, and wider slates at valleys, hips and rakes are not in this count.

An answer marked EXAMPLE is worked out from the page’s sample values. Type your own over the values marked SAMPLE to make it yours.

sq ftSAMPLE

The area of every roof plane to be slated, measured out to its eaves, rakes, ridge, hips and valleys, up the slope, not off the plan. A plan area is smaller than the roof it describes, and the difference grows with the pitch.

How many slates make a square depends entirely on this. The counts are one distributor’s published schedule at a 3-inch headlap — check them against your own supplier’s sheet.

At a 3-inch (76.2 mm) lap: 175 to the square, 7.5 in (191 mm) exposure.

in

Measured from the bottom edge of one slate to the top edge of the slate two courses beneath it, up the slope. Three inches (76.2 mm) is what North American quarries punch the holes for; a deeper lap is used on lower slopes and needs more slate. Starts at 3 in, the figure the National Slate Association publishes for slate sold by the square.

SAMPLE

Used to tell you when the roof is below the slope the roofing contractors’ association recommends for slate, and to show how much larger the roof is than its plan; it does not change the count. Enter 0 to skip both — once you enter a measurement of your own roof, this starts at 0, skipped, until you enter a pitch.

ftSAMPLE

Measured from the lower end of each hip, valley or other run where slate is cut on an angle to its upper end, along the roof, all of them added together. The National Slate Association’s estimating guide gives, as a common method, one extra square for every 100 linear feet of cut work, and this calculator adds that slate where you can see it rather than hiding it in a percentage. A straight rake is not cut work: the association lists rakes among the places that may need wider slates, which this count leaves out.

Applies to Roofing slate.

5%

Starts at 5%, the figure the National Slate Association publishes for breakage in shipping and handling. Change it to match how roughly the load will be handled between the yard and the roof. Source

Applies to Roofing slate.

Starts at 0 spares — this calculator’s own starting point, not a published figure. The roof itself does not use these slates; they are kept for repairs years later. The National Slate Association calls leaving some best practice, and its sample specification, written for a roofing contract, asks for one extra square of field slate when the contract calls for owner’s stock. One square of this size as slate is sold, at a 3-inch (76.2 mm) headlap, is 175 slates. Type or slide the number of slates to leave for the owner. Source

Field slate to order EXAMPLE3,345 slates18 x 11 in each, 5% extra for breakage

Exposure — how much of each slate shows, from its bottom edge up to the bottom edge of the next course Source 7.50 in

That is 19 squares (175 slates each) plus 20 slates.

An 1,820 sq ft roof, at the headlap and breakage allowance set above.

The job needs3,185 slates
Plus 5% for breakage3,344 slates
Buying 3,345 slates (18 x 11 in each) covers3,345 slates
Left over1 slate

Counted in whole slates, with the squares they arrive in stated on their own line — a square is 100 square feet of roof, but only at a 3-inch lap.

Not counted in these quantities: the hip and ridge slates laid over the hips and the ridge — the cut work counts only field slate cut along the hip and valley lines; the undereave (starter) slates along the eaves, and the slates that restart the courses at a change of slope; wider slates next to valleys and hips, and at rakes and walls where the slate size needs them; underlayment; nails and hooks; flashing; snow guards.

Slates in one square, at this lap Source175

Squares of roof Source18.2

  • The area above is the roof measured up the slope, not off a plan. At the 8 in 12 pitch entered here, a roof is about 1.20 times the plan area beneath it — a figure read off a plan, multiplied by 1.20, is the area to enter. Nothing here applies that multiplier: it is the geometry of the pitch above, not a figure quoted from anyone.
  • A square of slate weighs anywhere from about 700 to 3,275 pounds depending on its thickness — a factor of nearly five. Whether the structure below can carry it is a question for someone who can see the framing, and this calculator does not answer it. Source

The printed list carries the size, the lap, the exposure and the count.

Slate Roof Calculator — Slates by Size and Headlap

ConcreteTally

EXAMPLE — not a measured job. Every quantity below is worked out from this page’s sample measurements. Enter the real measurements and print it again before anyone orders from it.

Job / customer
Deliver to

Exposure — how much of each slate shows, from its bottom edge up to the bottom edge of the next course: 7.50 in [5]

What to buy

Field slate only: the starter course along the eaves, hip and ridge slates, and wider slates at valleys, hips and rakes are not in this count.

MaterialBuyWhich isLeft over
Roofing slate3,345 slates (18 x 11 in each)that is 19 squares (175 slates each) plus 20 slates3,345 slates1 slate

Not counted in these quantities: the hip and ridge slates laid over the hips and the ridge — the cut work counts only field slate cut along the hip and valley lines; the undereave (starter) slates along the eaves, and the slates that restart the courses at a change of slope; wider slates next to valleys and hips, and at rakes and walls where the slate size needs them; underlayment; nails and hooks; flashing; snow guards.

The roof area this sheet uses is measured UP THE SLOPE, not off the plan — a plan area is smaller than the roof it describes. The count comes from one distributor’s published schedule; another publisher differs by a slate on some sizes, so check it against the order sheet before the load is made up.

Worked out from

Roof area to coverThe area of every roof plane to be slated, measured out to its eaves, rakes, ridge, hips and valleys, up the slope, not off the plan.1820 sq ft (sample)
Slate size18 x 11 in
HeadlapMeasured from the bottom edge of one slate to the top edge of the slate two courses beneath it, up the slope.3 in
Roof pitch — rise for every 12 of level run8 (sample)
Hips, valleys and other runs cut on an angle — total lengthMeasured from the lower end of each hip, valley or other run where slate is cut on an angle to its upper end, along the roof, all of them added together.0 ft (sample)
Material needed before breakage3,185 slates
Breakage in shipping and handling — Roofing slate5%, the figure the National Slate Association publishes [1]
Spare slates left on site for future repairs — Roofing slatenone — this calculator’s own starting point, not a published figure. The roof itself does not use these slates; they are kept for repairs years later. The National Slate Association calls leaving some best practice, and its sample specification, written for a roofing contract, asks for one extra square of field slate when the contract calls for owner’s stock. One square of this size as slate is sold, at a 3-inch (76.2 mm) headlap, is 175 slates. [8]

Estimates only — check quantities before you buy. Confirm with your roofing supplier or contractor before ordering.

Where these figures come from

A number in brackets after a figure on this sheet is its entry in this list.

  1. A roofing square is 100 square feet (Estimating slate quantities for roof areas — units); Slate is sold by the square at a 3-inch headlap (Estimating slate quantities for roof areas — what a square of slate means); A second publisher's figure for an 18 x 11 slate — 175 to the square (Estimating slate quantities for roof areas — worked example converting pieces to squares); Typical shipping and handling breakage allowance — 5% (Estimating slate quantities for roof areas — breakage allowance); Leaving slate on site for the owner's future repairs — best practice, and often required by contract specifications (Estimating slate quantities for roof areas — owner's stock); Count what actually arrived before the scaffold comes down (Estimating slate quantities for roof areas — checking quantities on site) — National Slate Association, Trade association standard. The association's guidance page carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  2. Quarries punch the nail holes for a 3-inch headlap (Low pitched slate roof with a 4" headlap — standard practice); The same rule with the headlap left as a variable (Low pitched slate roof with a 4" headlap — exposure) — North Country Slate, Retailer product specification. The article carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  3. A deeper lap on a lower-slope roof needs more slate per square of roof (Greenstone slate guide to roof installations — quantities and headlap); Exposure — take the headlap off the slate's length and halve what is left (Greenstone slate guide to roof installations — exposure) — Greenstone Slate Company, Manufacturer or grower specification. The producer's guide carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  4. The roofing contractors' association does not recommend slate below a 4 in 12 slope (Greenstone slate guide to roof installations — roof slope) — Greenstone Slate Company, quoting the National Roofing Contractors Association, Manufacturer or grower specification. The producer's guide carries no commencement date; current as of the retrieved date. Retrieved August 28, 2026.
  5. Pieces per square, by slate size, at a 3-inch headlap (Schedule of Standard Sizes — Pieces per square at 3" headlap); Exposure by slate length at a 3-inch headlap (Schedule of Standard Sizes — Exposure at 3" headlap) — North Country Slate, Retailer product specification. The distributor's specification page carries no commencement date; current as of the retrieved date. Retrieved October 11, 2026.
  6. How much a square of slate weighs — 700 to 3,275 lb depending on thickness (Slate thickness and weights — approximate weight, pounds per square at 3" headlap) — North Country Slate, Retailer product specification. The distributor's specification page carries no commencement date; current as of the retrieved date. Retrieved October 11, 2026.
  7. Extra slate for cut work — one square for every 100 linear feet (Estimating slate quantities for roof areas — cut work allowance) — National Slate Association, Trade association standard. The association's guidance page carries no commencement date; current as of the retrieved date. Retrieved October 11, 2026.
  8. Owner's stock in the association's sample specification — one extra square of field slate (Sample Slate Specification, Section 07 31 26 Slate Shingles, Part 1 General, 1.11 EXTRA MATERIAL, item A (page 5 of 15)) — National Slate Association, Trade association publication. The specification prints no date; it is posted under a 2020/11 path on the association's site. Current as of the retrieved date. Retrieved October 11, 2026.

This list, with a link to each document: concretetally.com/slate-roof-calculator/#sources-heading

Every table ConcreteTally reads, with its dates: concretetally.com/sources

Check the count per square against your supplier’s own table.

Where these figures come from

Every table ConcreteTally reads, with its dates: all sources.