Roofing: squares, coverings, underlayment, flashing and drainage

What the roofing calculators cover

A roof quantity starts from the slope and not from the plan. This page lists the calculators in the order the work runs: the roof's area and pitch, the framing that sets its shape, pitched coverings, flat and low-slope systems, green roofs, the layers under the covering, the ridge, eaves and drainage, and last the load and safety checks. Each one says what its answer rests on.

The area group turns a footprint and a pitch into a true roof area with the slope multiplier, which is the square root of one plus the square of rise over run, and then into roofing squares, the unit the roof squares calculator names as the North American trade unit. The conversion pages turn squares back into an area for checking a quote, and a pitch into degrees and back.

Pitched coverings are counted product by product: shingle bundles, tiles and slates from the maker's gauge, wood shingles at an exposure, and metal panels by cover width, with the battens, clips and fixings that go with them. Low-slope roofs are counted in membranes, tapes, cover boards and terminations, and green roofs in growing medium, retention and a drainage layer, with the saturated weight set beside them as a figure for the structural designer. The layers under a covering are the underlayment, the eaves membrane and the flashings at curbs, skylights and vents.

How the answers are worked out

Shingles are counted from the true roof area with a stated number of bundles to the square, and the shingle page prints the footprint area, the true surface area and the bundles needed, with the waste factor as an input. Tiles and slates use the maker's figures: the roof tile calculator takes the length, the cover width, the headlap and the gauge from manufacturers' technical specifications, which is why it sets the maximum gauge at the tile's length less the minimum headlap, and it counts battens with the tiles. Wood shingle coverage scales the rated bundle coverage by the actual exposure, and the slate page counts courses from the weather exposure.

Metal roofing counts panels from the cover width left after the side lap, and runs a panel full length from eave to ridge where it can, as the Metal Construction Association guidance the calculator cites advises. The thermal movement page applies linear expansion: the panel length times the metal's coefficient times the expected temperature swing. Built-up base sheets and modified bitumen cap sheets are counted from roll coverage with an allowance that the page states, a lap allowance for the first and a scrap allowance for granule loss and heat damage at the seams for the second.

The load pages follow named methods and do not supply the code's values. The snow load page applies the ASCE 7 flat-roof relation to a ground snow load you enter, and the uplift page says in plain words that the external pressure coefficient is not supplied and must be read from the code edition your jurisdiction has adopted.

Units and documents by market

The market setting chooses the system the answers appear in: inch-pound units and roofing squares for the United States, metric for the United Kingdom, Australia and Canada, with each answer restated in the other system. The attic ventilation calculator is the one page here that checks four codes, the 2021 International Residential Code, the National Building Code of Canada 2020, BS 5250 and Approved Document C for the United Kingdom, and the NCC. The table shows the documents the other calculators cite for each market.

Allowance, laps and whole units

Counts round up because a bundle, a roll, a panel or a length of gutter is bought whole, and the gutter calculator counts sections in the stock length of the market it is set to. Underlayment is counted by the roll from the area the roll covers, the ice barrier by the area along the eave that a code figure asks for, ridge cap by the length of the ridge and hips, and ridge vent by the length of the ridge. Edges and penetrations are counted apart from area because they scale with length or number: the flashing pages count the run around a curb, a skylight or a vent, not a share of the roof.

What the calculators leave to others

The loads group checks a figure against a published method and does not set it: snow, wind, deflection limits and the anchors for work at height are the code's and the designer's. The fall protection page says that anchor spacing and layout must follow the system maker's engineering and the safety rules that apply, and that its layout is an estimate and not a plan. The uplift page stops at a net pressure for the coefficient you enter, and the snow guard page counts guards per row from a layout practice it names.

Whole jobs, taken off

A job takeoff asks for the dimensions once and returns every line the job needs, each from its own calculator. These are the ones where roofing is a main line.

• Asphalt shingle roof

  Shingles, underlayment, ice-dam membrane, ridge cap and ridge vent from the roof's footprint.

• Standing seam metal roof

  A metal roof's panels, clips, thermal movement and underlayment, for a lean-to or a plain gable.

• Pitched roof re-covering

  A pitched roof stripped to the rafters and covered again, with the scaffold, battens, ventilation and flashings.

• Flat roof replacement (warm roof)

  A flat roof rebuilt warm: the deck, the insulation, the waterproofing, the upstands and the gutter.

• Eaves and rainwater

  Fascia, soffit, gutter, hangers and guard, all taken from one total eave length.

Read an answer backwards

When you start from what you hold and want to know what it will cover, these pages run the same calculators in reverse.

• How much roof will my rolls of underlayment cover?
• How much roof will my bundles of shingles cover?
• How much ridge will my bundles of ridge cap cover?
• How much eave will my rolls of ice and water shield cover?
• How much flat roof will my rolls of EPDM cover?
• How much roof will my cover board sheets cover?
• How much green roof will my growing medium cover?

Worked examples

A job worked through with its inputs and its working shown, so you can see the figures before you enter your own.

• Shingle roof replacement

  A house re-roofed in shingles, from the slope area to the ridge, with each line bought in the order it goes on.

• Flat roof recovered

  A flat roof re-covered, so the membrane, the insulation and the edge details are counted together.

• An L-shaped hip and valley roof, measured

  The roof area and edge lengths of an L-shaped house with a hipped main roof and a gabled wing

• A semi's roof re-covered, in tiles and battens

  Concrete interlocking roof tiles for re-covering a 81 m² two-slope roof

• A low-pitch garage roof, in squares and bundles

  Shingles for a detached garage roof 24 by 24 ft on plan, at a 4/12 pitch

• A gable roof, in squares and bundles

  Shingles for a gable roof 34 by 26 ft on plan, at a 6/12 pitch

Reference tables

• Production rates by operation

  How long each operation takes one person, in person-hours per unit of work.

Guides

The practice behind the numbers: how the work is done, what goes wrong and how to check it.

• Measuring a Roof From the Ground

  Quoting a re-roof off plan dimensions and a pitch read from the driveway, then sizing a ladder that actually reaches the eave.

• Shingling a Roof

  Shingling judged the way water judges it: every lap, nail and flashing traced from the ridge slot down to the gutter line.

• Installing a Standing Seam Metal Roof

  Standing seam panels expand and contract every single day, and every clip, seam and trim detail either allows that movement or fights it.

• Laying a Clay Barrel Tile Roof

  Barrel tile is heavy, brittle and counted piece by piece — and how it reaches the roof is settled by a ladder rule, not by how strong the crew is.

• Setting Out a Slate or Cedar Roof

  Gauge, margin and headlap fixed with a chalk line on bare rafters, and the course arithmetic that has to close before slate or cedar is ordered.

• Replacing a Slipped or Broken Slate

  One slate down is a tingle and an hour. Six down in a winter is a nail count and a scaffold — how to tell which of the two you are holding.

• Setting Out Roof Battens: Gauge and Headlap

  The eaves line, the top line, and a whole number of equal steps between them — how a batten gauge gets settled without ever giving up headlap.

• Dry-Fixing Ridge, Hip and Verge

  A cracked ridge, one quote to re-bed it and one to strip it. What the 2014 revision of BS 5534 removed, and what mortar alone can no longer do.

• Low-Slope and Flat Roofing

  On a flat roof every failure has an address: find where the water stops, then price what it costs in build-up to move it.

• Warm Deck or Cold Deck on a Flat Roof Over a Room

  Insulation above the deck or between the joists: on an extension or a dormer the answer is settled by how far the roof can rise.

• Flashing a Roof Curb

  Eleven boxes on a roof are eleven perimeters, eleven blank widths and forty-four corners. Here is how all three reach the shop in one order.

• Leadwork Around a Chimney

  Four separate pieces wrap a stack and none of them seals. Which detail suits your covering, what code of lead to buy, and how far each piece runs.

• Detailing a Parapet and the Cricket Behind a Curb

  A parapet gets weather on three faces and a cricket is the only slope you build by hand — this is what both cost in metal, board and flashing height.

• Fitting a Roof Window Into a Pitched Roof

  A roof window is ordered twice: once for the unit, once for the flashing kit that has to suit the covering it lands in and the pitch it lands on.

• Stopping Ice Dams at the Eaves

  Icicles are a symptom with an address. Read the melt pattern, find the warm air reaching the deck, then clear the eave path.

• Balancing Roof Ventilation

  Roof intake and exhaust form a single pressure loop — the failures all trace back to one side outrunning the other.

• Sizing Gutters and Downspouts

  A rainwater run carries only what its tightest point allows, so size the trough, the outlet drop and the leader against one storm.

• Replacing Fascia, Soffit and Bargeboard

  Capping over a tired fascia and stripping it back to the rafter feet buy the same frontage and commit you to very different buildings.

• Fastening a Roof for Wind Uplift

  Wind never loads a roof evenly, so the fastening cannot be even either — marking the corner and edge zones, then setting the pattern each one demands.

• Designing a Roof for Snow

  Snow does not load a roof where it falls but where wind and slide pile it, so frame the drifts before the field.

• Building a Green Roof the Structure Can Carry

  The saturated dead load and the stormwater credit are the same water counted twice. Settle both before a bag of substrate is ordered.

Terms

• Roof pitch
• Shingle
• Sarking and roof underlay
• Battens, furring and strapping
• Flashing at abutments and penetrations
• Ridge, ridge board and ridge beam
• Roof valley
• Eaves
• Verge and rake
• Soffit and fascia
• Gutter / eavestrough
• Flat roof covering
• Ice dam

How the numbers are worked out

Every calculator names the method it uses and the standard or guidance behind it. These are the methods the most of the calculators above rest on, and the published documents they cite.

Methods

• Quantity, Waste and Rounding

  Why a material count is a ceiling function rather than a division, and why the waste factor — not the geometry — carries the risk. Applies to 20 of the calculators above.

• Slope, Pitch and True Surface Area

  Converting a plan footprint into the sloped area a roof actually has — the factor that decides whether a shingle order is right or a third short. Applies to 10 of the calculators above.

• Perimeter Runs, Stock Lengths and Mitres

  Why a room's area tells you nothing about how much skirting it needs, why adding a door can increase the trim order, and why a crown moulding mitre is neither forty-five degrees nor half the corner angle. Applies to 6 of the calculators above.

• Rolls, Sheets and the Lap Between Them

  Why covering an area with rolls is a strip count rather than a division, why the lap is bought twice over, and why turning the strips the other way can change the order. Applies to 6 of the calculators above.

• Spacing, Pitch and the Count That Follows

  Why a run divided by a spacing gives the number of gaps and not the number of things, why the division rounds up rather than to nearest, and what the leftover bay at the end does to both. Applies to 6 of the calculators above.

• Fastener Schedules, Supports and Counting Over a Grid

  Why a fixing count is never area divided by spacing, why the edges of a panel supply more fasteners than its middle, and why the tighter schedule at a roof corner is a requirement rather than caution. Applies to 4 of the calculators above.

Standards cited

• ASCE 7

  Cited by 2 of the calculators above.

• International Residential Code

  United States. Cited by 2 of the calculators above.

• Approved Documents to the Building Regulations (England)

  England (Wales, Scotland and Northern Ireland have their own). Cited by 1 of the calculators above.

• International Plumbing Code

  United States. Cited by 1 of the calculators above.