The job
Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.
A US reroof is quoted in squares before it is quoted in anything else, and a square is 100 sq ft (9.29 m²) of sloped roof rather than of house. The job modelled here is a plain one — a 30 × 40 ft (9.14 × 12.19 m) gable at 6/12, one layer of worn shingles stripped to the deck and replaced with architectural laminates — and its quantities follow the sequence of the work: squares first, then what comes off, then what goes back on from the eaves upward, each figure taken from a step already made rather than measured again with a different tape.
THE FIGURE THAT CONTROLS THE REST IS THE SLOPED AREA WITH NO WASTE ON IT. The tear-off, the deck, the underlayment and the nails are all counted on that bare figure. The shingle cut allowance is added once, at the shingle order; every other material carries its own allowance, set for the way that material goes down and applied to its own measurement, never to the shingle order. The costly mistake comes before any of that, in the plan: the wall line gives the size of the house, and the roof is larger than the house by its overhangs — on this roof a 12 in (305 mm) band on every side, about 11% of the plan area.
THE SECOND IS THAT THE ROOF AND THE ATTIC UNDER IT ARE ONE SYSTEM. How wide the ice barrier runs is set by where the wall stands, not by the roll; a ridge vent earns its slot only if the soffits can feed it; and on this roof the cap shingles go on over the vent rather than instead of it. Those three decisions are why the wall thickness is measured on a roofing job, and why ventilation is settled before anything is cut along the ridge.
WHAT THIS PAGE IS, AND WHAT IT LEAVES OUT. Every quantity comes from one of the site's calculators — the one linked at its step, or, for the few lines named here that sit outside the steps, the one named beside them — in US units with metric alongside. Nothing was torn off and nothing was ordered, so the closing section says where the estimate is likely to be wrong and in which direction, never by how much. No calculator here turns squares and layers into the weight that comes off, and none counts shingle starter strip, or step flashing piece by piece where a roof meets a wall — which this plain gable, with no wall rising above it, does not, though many houses do. Drip edge can be counted with the flashing trim calculator in 10 ft (3.05 m) lengths: taken edge by edge, as that calculator's own notes advise, each 40 ft (12.19 m) eave takes four and each 16 ft 9 in (5.11 m) rake two, sixteen in all before the laps at the joints. The two plumbing stacks take a new boot each, and the vent boot calculator's default spare allowance rounds that count up to three.
What was measured, and how
Roof plan, to the drip edge on all four sides
40 × 30 ft (12.19 × 9.14 m); 1,200 sq ft (111.5 m²) of plan
Built from the wall dimensions plus the overhang, plumbed down from the drip edge on every elevation, rather than taped along the siding. The siding figure — 38 × 28 ft (11.58 × 8.53 m) here — loses the overhang band, which on this roof is 136 sq ft (12.6 m²) of plan and a square and a half of sloped roof.
Pitch, read off the framing
6/12 (26.6°), a slope factor of 1.118
Taken off a rafter in the attic, where the rise over a 12 in (305 mm) run can be read directly with a spirit level and a tape, and checked against a photograph of a gable end. The care is worth it: a pitch misread by 2 in (51 mm) of rise — 6/12 taken as 8/12 — puts the same plan at 14.4 squares, a full square more. A photograph used on its own, taken even slightly off square, tilts the rake line it is read from.
Eave overhang and wall thickness, drip edge to the inside of the wall
12 in (305 mm) overhang + 6 in (152 mm) wall = 18 in (457 mm) on plan; rakes also overhang 12 in (305 mm)
The overhang is plumbed on plan from the drip edge to the siding face; the wall is measured across a door jamb from siding to drywall, because the ice barrier's line sits inside the wall and the overhang alone stops short of it. A tape laid up the underside of a rafter tail reads the overhang as 13.4 in (341 mm) instead of 12 in (305 mm), because it follows the slope.
Layers on the roof now
One layer assumed, to be confirmed at both rakes
Counted at the roof's edges, because that is the only place a second layer shows: a shingle lifted at each rake, and the stack of butt edges above the gutter at each eave. Imagery can size a roof but cannot count what lies under its top layer, and a hidden second one changes the tear-off time and the disposal weight together.
Ridge length, and the part of it over the attic
40 ft (12.19 m) of ridge; 38 ft (11.58 m) between the outside faces of the gable walls, 37 ft (11.28 m) over the attic
The ridge runs the full distance between the rake drip edges, but only the stretch between the inside faces of the two gable walls sits over the attic and can be slotted for air. Taking the vent's working length from the full ridge overstates the exhaust it can deliver, however much vent is laid.
Attic floor, inside the walls
37 × 27 ft (11.28 × 8.23 m); 999 sq ft (92.8 m²)
The wall-line plan less the wall thickness at each side, checked from the attic hatch against the ceiling joists. Using the drip-edge plan instead counts the overhangs as attic and raises the ventilation requirement by about a fifth for air space that is not there.
Penetrations and abutments
Two plumbing vent stacks; no chimney, no skylight, no wall meeting the roof
Counted on both slopes from the ground and again from the attic, where every pipe through the deck is in plain view. A stack on the far slope is easy to miss from one side of the house, and each one missed is a boot and its flashing absent from the order.
The takeoff, in order
Each step needs something from the one before it, which is why the order is part of the answer.
Turn the plan and pitch into squares of sloped roof
- Needs
- The 40 × 30 ft (12.19 × 9.14 m) plan taken to the drip edge and the 6/12 pitch read off the framing — the two measurements every later step inherits.
- Produces
- The sloped area with the waste field set to zero: 1,342 sq ft (124.6 m²). That bare figure is what comes off in the tear-off and what the deck, the underlayment and the nails are counted on; the shingle cut allowance goes on once, at the shingle order, and no other line inherits it.
Result13.4 squares — 1,342 sq ft (124.6 m²) of sloped roof
Roof Squares CalculatorPlan the tear-off and book the dumpster
- Needs
- The 13.4 squares from step one — the tear-off is the sloped area, not the plan and not the order with waste — multiplied by the one layer assumed in the measurements, which the rakes have to confirm.
- Produces
- No figure, deliberately. Nothing on this site turns squares and layers into a loose volume or a weight, and none of this calculator's four waste types — light, mixed, rubble, soil — is asphalt tear-off, so running it would mean choosing the density and with it the answer. The weight is a hand sum: 13.4 squares, times the layer count, times what a square of a comparable laminate weighs by its maker's figures — the product coming off is rarely known by name — plus the old felt and nails. The dumpster is then booked against the hauler's weight limit, which is this calculator's own point about dense waste.
Count the deck sheathing as an upper limit
- Needs
- The sloped area from step one entered as its two sides — the 40 ft (12.19 m) ridge by 33.5 ft (10.22 m) of rafter, both slopes together — because the deck can only be judged once step two's tear-off has bared it.
- Produces
- The sheet count for re-decking the whole roof, allowance included. It is the most this line can be rather than the expected order: after tear-off the deck is walked, and only sheets that are soft, delaminated or rotten are replaced, at the thickness the rafter spacing calls for. Whatever is laid, the eave drip edge and then the membrane in the next step go onto it.
Result47 sheets, 4 × 8 ft (47 of the 2440 × 1220 mm panel as well)
Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof)Size the ice barrier along the eaves
- Needs
- The 80 ft (24.38 m) of eave — both eaves, not the rakes — the 18 in (457 mm) from drip edge to the inside face of the wall, and step one's 6/12 pitch, which stretches that horizontal distance along the slope. It goes onto step three's deck over the eave drip edge, before any underlayment.
- Produces
- Self-adhered membrane carried, as the calculator reads IRC R905.1.2, to 24 in (610 mm) inside the wall line: a band 47 in (1.19 m) up the slope, wider than one 36 in (914 mm) roll, so two courses with a lap along each eave. The job is modelled where the local code requires it. Step five turns this band into rolls, and step six takes its area off the underlayment.
Result313 sq ft (29.1 m²) of ice barrier, 47 in (1.19 m) wide on the slope
Roof Eave Ice Dam Protection Membrane FootprintTurn the eave band into rolls
- Needs
- Step four's 80 ft (24.38 m) of eave and its 47 in (1.19 m) band. The band is wider than one 36 in (914 mm) roll, so the width entered is two full rolls, 72 in (1.83 m): the modelled layout runs the upper course at full width rather than ripping it down to the line.
- Produces
- Rolls to buy, at the calculator's 200 sq ft (18.6 m²) a roll with its allowance for laps. Entered at the bare 47 in (1.19 m) band, the same calculator says 2, which is right by area and wrong by length: two courses along both eaves need 160 ft (48.8 m) of roll before a single end lap, and two rolls of that size hold about 133 ft (40.6 m).
Result3 rolls — 528 sq ft (49.1 m²) of membrane with the lap allowance
Ice & Water Shield CalculatorCount the underlayment for the rest of the deck
- Needs
- Step one's 1,342 sq ft (124.6 m²) of sloped roof less the 313 sq ft (29.1 m²) of ice-barrier band from step four — 1,029 sq ft (95.56 m²) — because the calculator deducts nothing itself and would otherwise order the eaves twice. Only the band comes off, not the extra width step five's upper course runs to, so any error falls on the side of more underlayment.
- Produces
- Synthetic underlayment in 10-square rolls with the standard lap allowance. The deduction does not change the count on this roof — the full 1,342 sq ft (124.6 m²) would also be two rolls — but it is the honest input, and on a roof with valleys lined in membrane it would move the figure.
Result2 rolls — 1,131 sq ft (105.1 m²) to cover with laps, at 1,000 sq ft (93 m²) a roll
Roofing Underlayment CalculatorOrder the field shingles
- Needs
- The same 40 × 30 ft (12.19 × 9.14 m) plan and 6/12 pitch as step one — the calculator re-derives the sloped area itself, and its 1,342 sq ft (124.6 m²) row agreeing with step one is the check that nothing was mis-keyed — plus the plain-gable allowance chosen below.
- Produces
- Squares to order with the allowance on, which the calculator turns into 45 bundles at three to the square. The allowance covers cuts at the rakes and along the ridge course only, because the starter strip and the ridge cap are separate products bought as separate lines (the cap is step eleven).
Result14.76 squares (137.1 m² with the allowance) — 45 bundles
Roofing Shingle CalculatorCount the nails through the shingles
- Needs
- The bare sloped area from step one, 1,342 sq ft (124.6 m²) — nails go into laid shingles, not into step seven's cutoffs — and the width and exposure of the laminate ordered in step seven: 39⅜ in (1,000 mm) wide at a 5⅝ in (143 mm) exposure, off the wrapper.
- Produces
- About 873 shingles cover the roof; at the four-nail standard pattern the calculator's field count is 3,490, and its allowance for dropped nails brings the order to 3,664. On a site where high-wind fastening applies — a matter for the local code and the shingle's application instructions — rerunning at six nails a shingle gives 5,496. Starter, rake and cap nails are not in either figure.
Result3,664 nails — 3,490 at four a shingle, plus the calculator's 5% for dropped nails
Roof Covering Fixing CalculatorSettle the attic ventilation before the ridge is cut
- Needs
- The attic floor inside the walls — 37 × 27 ft (11.28 × 8.23 m) — which is step one's drip-edge plan less the overhangs and the wall thickness measured for step four.
- Produces
- The net free area at the 1:150 ratio, split roughly evenly: about 480 sq in (0.31 m²) of exhaust at the ridge and the same of intake at the soffits. The calculator's 1:300 option assumes a vapour retarder and balanced vents, and whether the adopted code allows the reduced ratio on this house is for the building department; 1:150 is the conservative figure and the one taken. The exhaust half is what step ten's vent has to deliver.
Result6.66 sq ft (0.62 m²) of net free area — 959 sq in
Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count)Run the ridge vent
- Needs
- The 40 ft (12.19 m) ridge from the measurements and the exhaust half from step nine — the vent is worth fitting only if its carton rating times the slotted length reaches that half.
- Produces
- Vent laid rake to rake with the calculator's end allowance, so the cap line runs at one height. Only the slotted part moves air: the deck is opened only over the attic, ending inside each gable wall by a setback the vent's instructions fix, and never over the rake overhangs, so the exhaust check is made on less than 37 ft (11.28 m) against the product's rated net free area per foot.
Result42 ft (12.80 m) of ridge vent
Ridge Vent Linear Footage CalculatorCap the ridge over the vent
- Needs
- The same 40 ft (12.19 m) ridge as step ten: a shingle-over vent is capped along its full length, so on this roof the vent and the cap are one run in two layers rather than alternatives.
- Produces
- Ridge cap bundles at the calculator's 33 ft (10 m) a bundle, with the cut-end allowance for the two ends of one unbroken ridge. Over the vent each cap sits higher and covers slightly less, and it takes the longer nails the vent maker names — neither is in this count.
Result2 bundles — 44 linear ft (13.4 m) with the cut-end allowance
Roof Ridge Cap Calculator
The figures
| Step | Calculator | Figure |
|---|---|---|
| Turn the plan and pitch into squares of sloped roof | Roof Squares Calculator | 13.4 squares — 1,342 sq ft (124.6 m²) of sloped roof |
| Count the deck sheathing as an upper limit | Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof) | 47 sheets, 4 × 8 ft (47 of the 2440 × 1220 mm panel as well) |
| Size the ice barrier along the eaves | Roof Eave Ice Dam Protection Membrane Footprint | 313 sq ft (29.1 m²) of ice barrier, 47 in (1.19 m) wide on the slope |
| Turn the eave band into rolls | Ice & Water Shield Calculator | 3 rolls — 528 sq ft (49.1 m²) of membrane with the lap allowance |
| Count the underlayment for the rest of the deck | Roofing Underlayment Calculator | 2 rolls — 1,131 sq ft (105.1 m²) to cover with laps, at 1,000 sq ft (93 m²) a roll |
| Order the field shingles | Roofing Shingle Calculator | 14.76 squares (137.1 m² with the allowance) — 45 bundles |
| Count the nails through the shingles | Roof Covering Fixing Calculator | 3,664 nails — 3,490 at four a shingle, plus the calculator's 5% for dropped nails |
| Settle the attic ventilation before the ridge is cut | Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count) | 6.66 sq ft (0.62 m²) of net free area — 959 sq in |
| Run the ridge vent | Ridge Vent Linear Footage Calculator | 42 ft (12.80 m) of ridge vent |
| Cap the ridge over the vent | Roof Ridge Cap Calculator | 2 bundles — 44 linear ft (13.4 m) with the cut-end allowance |
The waste factors, and why these ends of the ranges
| Material | Applied | Why |
|---|---|---|
| Field shingles | The lower end of the published range, because the roof is a plain gable | The shingle calculator's own guidance runs from 10 to 15%, and this roof has none of the things that push it up — no hips, no valleys, no dormers, one straight ridge. The allowance only has to cover the cuts at four rake edges and the course that meets the ridge, because the starter and the cap are bought as their own products rather than cut from field bundles. |
| Deck sheathing | The calculator's standard sheet allowance, then whole sheets | The roof divides unusually well: 40 ft (12.19 m) is exactly five 8 ft (2440 mm) sheet lengths, so the waste sits up the slope, where about 16 ft 9 in (5.11 m) of rafter per side takes four full 4 ft (1220 mm) courses and a narrow rip at the ridge. The half sheets that start alternate courses pair up from one sheet across the run and waste nothing; the 10% default covers the ridge rip and any sheet rejected on the day. A tighter figure would be defensible on a full re-deck, but the count here is an upper limit rather than an order. |
| Underlayment | The calculator's standard lap allowance, with the roll rounding doing the rest | On a steep, simple gable 10% covers the side and end laps, and rounding to 1,000 sq ft (93 m²) rolls dominates anyway: the eaves-deducted area and the whole deck both come out at two rolls, so neither the allowance nor the deduction changes the order on this roof. |
| Ice barrier | The roll calculator's lap uplift, taken on two full courses rather than on the band | A band 47 in (1.19 m) up the slope does not fit one 36 in (914 mm) roll, so every eave takes two courses with a lap between them, plus end laps along each 40 ft (12.19 m) run. The calculator's 10% is spent on those laps rather than on cutting. What makes the count come out in length as well as in area is entering the two full courses as the width, which is why an allowance put on the band alone would be the wrong tool. |
| Ridge vent and cap | Small end allowances on one unbroken run | A single straight 40 ft (12.19 m) ridge has two ends and no hip junctions, so the vent's 5% and the cap's 10% cut-end allowances are enough. The cap count is decided by the rounding to whole bundles in any case: 44 ft (13.4 m) of cap is two bundles with a good part of the second to spare. |
| Roofing nails | The calculator's allowance for dropped and bent nails, and no more | The fixing calculator adds 5% for nails that fall off the roof or jam in the nailer. A larger pad would hide the real risk, which is the fastening pattern: moving from four nails to six adds half as many again, and that is a specification question for the maker's instructions, not a waste allowance. |
Where this estimate is most likely to be wrong
Deck sheathing
usually overThis page's sheet count is more likely to exceed the sheets replaced than to fall short of them. It is the whole deck, printed as the upper limit for the line: a single layer coming off a deck that has stayed dry usually exposes damage in predictable places — along the eaves, around the two stacks, under old leaks — and the rest is re-nailed rather than replaced. It becomes the order only if the deck proves to be spaced boards or delaminating throughout.
Narrow it by: Checking the underside of the deck from the attic for staining, delamination and daylight before tear-off day, and agreeing a replacement rate by the sheet with the owner rather than a fixed count.
Tear-off weight and the dumpster
usually underAny disposal estimate built on one layer is exposed in the same direction. A second layer hidden under the first roughly doubles what comes off and the time it takes, and the old felt, the nails and any deck boards removed go in the same dumpster. A dumpster that reaches the hauler's weight limit is refused or surcharged however much room is still in it.
Narrow it by: Counting layers at the rakes and eaves before booking, and giving the hauler a weight worked from the squares, the layer count and a comparable laminate's weight per square, not a volume guessed from the ground.
Ice barrier rolls
usually overThree rolls of the calculator's size hold about 200 ft (61 m) of length, and the two courses along both eaves take 160 ft (48.8 m), so once the end laps are made part of the third roll is spare; the patches round the two stacks can come out of it. The count rises only if a local amendment carries the line past what two courses reach, or if the membrane bought comes in shorter rolls than the calculator assumes.
Narrow it by: Reading the roll length off the product actually being bought, laying the courses out eave by eave with it, and checking the local amendment to the distance inside the wall before settling on two courses.
Roofing nails
usually underThe count assumes the four-nail standard pattern across the whole roof. Where the design wind speed or the shingle maker's instructions call for six, the same roof takes half as many nails again, and the starter course, the rake edges and the ridge cap all take nails this figure leaves out. A nailer set to over-drive destroys shingles as well as nails.
Narrow it by: Confirming the fastening pattern for the local wind speed in the maker's application instructions before ordering, and adding starter and cap nails from their own run lengths.
Shingle bundles
either wayThe calculator fixes three bundles to the square, and some of the thicker laminates need four, or even five, to make one; bought by the calculator's count, the squares would still be right and only the bundle figure would come up short, by a quarter at four to the square. Against that, the plain-gable allowance on a roof with no valleys, hips or dormers can prove generous, most of all where the starter arrives as its own product.
Narrow it by: Dividing the squares to order by the coverage printed on the wrapper of the exact product being bought, rather than by three.
Net free area delivered at the ridge and soffits
usually underThe ventilation figure is a requirement, not a delivery. On an existing house the soffit half is the side most often blocked — insulation pushed into the eaves, perforated soffit put up with no slot cut in the boards behind it — and the ridge half is limited to the slotted length inside the gable walls, which is shorter than the vent laid. Both push the working ventilation below what the ratio asks for.
Narrow it by: Looking along the eaves from inside the attic for daylight in every rafter bay, and multiplying the vent's rated net free area by the slotted length rather than the ridge length.
Tools this job needs
Frequently asked questions
- How many squares is a 30 × 40 roof?
- A 40 × 30 ft (12.19 × 9.14 m) plan at 6/12, taken to the drip edge, is 13.4 squares: 1,342 sq ft (124.6 m²) of slope before waste. If 30 × 40 ft (9.14 × 12.19 m) is the house, 12 in (305 mm) overhangs make the roof plan 32 × 42 ft (9.75 × 12.80 m), or 15.03 squares at 6/12. Taped along this house's siding, 38 × 28 ft (11.58 × 8.53 m), the overhangs vanish and it reads 11.9, a square and a half short.
- How many bundles of shingles do I need for a 30 × 40 roof?
- For this roof at 6/12, 45 bundles of architectural shingles: the calculator puts a plain-gable allowance on the 13.4 squares, giving 14.76 squares (137.1 m²), and rounds to whole bundles at three to the square. Ridge cap is separate — 2 bundles for the 40 ft (12.19 m) ridge — and so is starter strip. If the wrapper states a different coverage, divide 14.76 by that figure instead.
- How many sheets of plywood for a 30 × 40 roof?
- 47 sheets of 4 × 8 ft (the same count in 2440 × 1220 mm panels) re-deck the whole 1,342 sq ft (124.6 m²) with the calculator's standard allowance. On a tear-off that is the most the deck can need rather than the likely order: only sheets found soft or delaminated once the old roof is off are replaced, so the real figure is settled on the day and is often a small part of 47.