Roofing
Roofing calculators for pitch, roof area, shingles, rafters and trusses, checked against IRC 2021
Six roofing calculators take a roof from the first measurement to the material order: the pitch and its angle, the roof surface in square feet and squares, the shingles, underlayment and nails, the rafter cut and the trusses. Lengths come out as tape fractions, the formulas are written out on this page, and every slope a tool warns about is tied to its section of the 2021 IRC. Free, no signup, and no price on anything.
Every roofing calculator on this site starts from one number, the slope factor: √(rise² + 12²) ÷ 12, the length of roof per foot of level run. At a 6/12 pitch it is 1.1180, so 1,000 sq ft of plan area is 1,118 sq ft of roof, a 12 ft level run is 13 ft 5 in measured up the slope, and an asphalt shingle roof needs the 2:12 the IRC sets as its floor (R905.2.2). What changes from tool to tool is what the factor is multiplied by: an area, a run, an overhang or a truss.
Pick the calculator by the job in front of you, not by its name. If you are not sure, start with the which roofing calculator do I need table; for the numbers behind the warnings, the IRC 2021 roofing limits are listed with their sections.
The six roofing calculators, and what each one runs, returns and checks against the code
Each tool covers one job and hands the rest to its neighbors. The roof pitch calculator and the pitch to degrees chart are the slope tools: one is a roof slope calculator that solves a pitch from a measured rise and run, the other is the reference chart and converter for angles. The roof area calculator is the roof square footage calculator and roofing square calculator, the one that measures the roof. The shingle calculator is the roof shingle calculator that turns squares into bundles and everything that goes with them. The rafter length calculator and the roof truss calculator are the framing tools for a stick-framed roof and a trussed one. Together they make a free online roofing calculator set for a gable, hip, shed or flat roof: the all-in-one roof calculator split into its six jobs, so each answer can be checked. The figure at the foot of each card is what that tool shows when it loads, so the page can be checked against the tool.
Roof pitch calculator: rise and run to x/12, degrees, percent and the slope factor
Runs: pitch = rise ÷ run × 12; angle = arctan(rise ÷ 12); percent = rise ÷ 12 × 100; slope factor = √(rise² + 12²) ÷ 12; hip and valley factor = √(rise² + 288) ÷ 12. Enter rise and run as measured with a level and tape, a pitch read off a roof pitch gauge, an angle or a percent, and it solves for the rest.
Returns: pitch as x/12, angle, percent slope, exact rise in 12, ratio, the old span-fraction pitch, slope factor, hip and valley factor, rafter inches per foot of run, the nearest standard pitch, and a rafter line length if you give the run.
Code: the 2:12 floor for asphalt shingles and the 2:12 to 4:12 band that needs two layers of underlayment (R905.2.2), with the minimum pitch for every other covering in a table on the page.
Default 6 in of rise on 12 in of run: 6 / 12, 26.57°, 50 %, slope factor 1.1180, hip factor 1.5000, 13.416 in (13 7/16) of rafter per foot of run
Roof pitch to degrees chart: every pitch as an angle, and degrees back to pitch
Runs: the same trigonometry run the other way as well: a pitch in quarter-inch steps to its angle, an angle or a percent back to x/12 with rise = 12 × tan(angle), and the nearest standard pitch when the answer falls between steps.
Returns: the full pitch chart above the converter, degrees for every pitch, the pitch for every whole degree, percent, ratio and metric slopes, radians, the slope and hip factors, and the plumb, seat and hip cut angles a saw is set to.
Code: the code minimums written in degrees, because the IRC states every slope as units in 12 and never in degrees: 2:12 is 9.46° and 4:12 is 18.43°.
Default 6/12: 26.57°, 1.1180, 1.5000, 13.416 in (13 7/16); 4/12 is 18.43° and 12/12 is 45.00°
Roof area calculator: roof square footage, roofing squares and edge lengths
Runs: plan area = (length + overhangs) × (width + overhangs); roof area = plan area × slope factor; squares = roof area ÷ 100; squares to order = squares × (1 + waste); bundles = squares to order × 3, rounded up once. Gable, hip, shed and flat shapes, with eave and rake overhangs entered apart.
Returns: the plan area, roof surface area, measured squares, squares to order, bundles at three per square, and the eave, rake, ridge, hip and drip edge lengths measured along the roof.
Code: the same 2:12 shingle floor, roll roofing from 1:12 (R905.5.2) and membranes from ¼:12 (R905.9.1, R905.11.1 to R905.13.1) for a flat or near-flat roof.
Default 40 × 30 ft gable at 6/12 with 12 in overhangs: 1,502.6 sq ft of roof, 15 squares measured, 16.5 to order, 50 bundles, drip edge 155.6 ft
Shingle calculator: bundles, shingles, nails, underlayment, cap and starter
Runs: squares from the footprint and pitch, or from a roof area you enter; then bundles, pieces and nails from the shingle type, rolls of underlayment from the roll size with two layers below 4:12, ridge cap and starter from their lineal feet per bundle, and drip edge in 10 ft pieces. Waste goes on the quantities you buy, never on the roof.
Returns: squares to buy, bundles, shingle pieces, nails, underlayment rolls, ridge and hip cap bundles, starter bundles and drip edge pieces, with bundles per square for 3-tab, laminated, heavyweight and premium shingles.
Code: R905.2.2 for the 2:12 floor and double underlayment, four fasteners per strip shingle (R905.2.6), drip edge at eaves and rakes (R905.2.8.5) and the ice barrier rule in R905.1.2.
Default 40 × 30 ft footprint at 6/12, laminated, 10 % waste: 1,341.6 sq ft, 14.8 squares to buy, 45 bundles, 945 shingles, 3,780 nails, 2 rolls
Rafter length calculator: cut length with overhang, ridge deduction and birdsmouth
Runs: run = half the width less half the ridge; line length = run × slope factor; tail = overhang × slope factor; cut length = line + tail. Then the plumb and seat angles, the birdsmouth seat and heel, the notch against one fourth of the depth, the ridge depth, hip and jack lengths, the stock length to buy and the rafter count.
Returns: rafter length to cut to the sixteenth, line length, sloped tail, total rise, plumb and seat cut angles, plumb cut depth, smallest 2x ridge, birdsmouth seat depth, height above plate, notch depth, hip or valley line length, jack steps, stock length and rafters per side.
Code: R802.4.1 for a run measured on the horizontal, R802.3 for a ridge at least as deep as the cut end of the rafter, and R502.8.1 for a notch at the end of a member no deeper than one fourth of its depth.
Default 24 ft wide at 6/12, 1½ in ridge, 12 in overhang, 2x8: cut 14 ft 5 9/16 in, line 13 ft 4 3/16 in, tail 1 ft 1 7/16 in, ridge 2x10 (9 1/4 in), buy 16 ft
Roof truss calculator: how many trusses, overall height and top chord length
Runs: trusses = building length ÷ spacing, rounded up, plus one, with two of them gable end trusses; overall height = run × pitch ÷ 12 + heel; top chord = run × slope factor; tail = overhang × slope factor. Common, scissor and mono profiles.
Returns: trusses required, commons and gable ends, overall height with the heel, rise above heel, top chord, sloped tail, bottom chord, width with tails, bays and the actual spacing, roof area and the run of permanent bracing.
Code: R802.10, which leaves member sizes, bearing and bracing to the truss design drawing and bars cutting or altering a truss without an approved design; the section is cited by number and paraphrased, as the reprint text the site reads stops before it.
Default 24 ft span at 6/12, 3½ in heel, 40 ft long at 24 in: 21 trusses (19 commons + 2 gable ends), overall height 6 ft 3 1/2 in, top chord 13 ft 5 in, roof area 1,220.9 sq ft
Which roofing calculator do I need? Pick by the roof job in front of you
The table routes by the job. Read the second column as a check on the tool: if the arithmetic there is not what your roof needs, the row is wrong for you, whatever the page is called. The hip roof area, shed roof rafter, gambrel truss and metal roofing panel calculators are being added to this hub; their rows say what each one will run.
| What you are doing | Arithmetic it runs | Returns | Code figure | Calculator |
|---|---|---|---|---|
| Read the pitch of a roof you can measure | rise ÷ run × 12; arctan for the angle | x/12, degrees, percent, slope and hip factors | R905.2.2 | Roof pitch calculator |
| Turn degrees from a level, an app or a plan into x/12, or the reverse | rise = 12 × tan(angle); arctan(rise ÷ 12) | pitch, nearest standard pitch, percent, cut angles | code minimums in degrees | Pitch to degrees chart |
| Measure a roof in square feet and roofing squares | plan area with overhangs × slope factor ÷ 100 | roof area, squares, squares to order, edge lengths | R905.2.2; R905.5.2; R905.9.1 | Roof area calculator |
| Order shingles, underlayment, nails, cap and starter | squares × bundles per square; rolls by roll size; lineal feet per bundle | bundles, pieces, nails, rolls, cap, starter, drip edge | R905.2.2; R905.2.6; R905.2.8.5; R905.1.2 | Shingle calculator |
| Cut common rafters for a gable | run less half the ridge × slope factor, plus the tail | cut length, birdsmouth, plumb and seat angles, ridge depth | R802.4.1; R802.3; R502.8.1 | Rafter length calculator |
| Check that a rafter size will carry the span | IRC span table lookup by species, grade, spacing and load | the largest span allowed for a size | R802.4.1 rafter span tables | Rafter span calculator |
| Count and size up roof trusses | length ÷ spacing + 1; run × pitch ÷ 12 + heel | truss count, overall height, top chord, roof area | R802.10 | Roof truss calculator |
| Ceiling joists under a rafter roof | IRC ceiling joist span table lookup | span by size and spacing | R802.5 | Ceiling joist span calculator |
| A hip roof: area, hip and jack rafters | four planes; hip = half the short side × hip factor | area, hip length, jack rafters | R905.2.2 for shingles | Hip roof area calculator |
| A shed or lean-to roof with one slope | one plane; rafter from the high wall to the low wall | rafter length, rise, area | R802.4.1 | Shed roof rafter calculator |
| A gambrel barn or shed roof | two pitches per side, each with its own factor | truss or rafter lengths for both slopes | R802.10 | Gambrel truss calculator |
| Metal roofing panels | roof width ÷ panel coverage; panel length along the slope | panels, lengths, trim | R905.10.2 | Metal roofing panel calculator |
| Roof sheathing under the shingles | roof area ÷ 32 sq ft per sheet | plywood or OSB sheets | none checked | Plywood sheet calculator |
A reroof and a new roof start in different places. A reroof starts from the roof that is there: the pitch, then the area, then the shingles, and the framing tools never come into it. A new roof starts from the walls: the pitch is a choice, the rafters or trusses are figured from the span, and the area and shingles follow once the overhangs are fixed. Both orders meet at the same slope factor, which is why one wrong pitch moves every number downstream. For the frame under the roof, the structural span calculators have their own router, and the beam load calculator works out the load a ridge beam or header carries.
Rafters and trusses are two answers to the same roof. A rafter roof is cut on site from a line length the rafter tool gives to the sixteenth, and its sizes come from the IRC span tables. A truss roof is ordered from a fabricator, and the truss tool gives the count, height and chord lengths you need to order and to check that the trusses fit, not the member sizes, which belong to the truss design drawing. The comparison in full is in the rafters vs trusses guide.
One 40 × 24 ft roof read by the roofing calculators: where the numbers agree and where they differ
The rafter and truss tools both load with a 24 ft wide building at 6/12, and the truss tool with a 40 ft length, so a 40 × 24 ft gable with 12 in overhangs is the easiest roof to follow across the site. The rows below are that roof entered in each tool. The area and shingle figures were worked by the same arithmetic the tools run, checked against each tool's own default result first, so you can enter the same inputs and expect the same answer.
| Calculator | What you enter | What it returns | How it ties to the others |
|---|---|---|---|
| Roof pitch calculator | rise 6, run 12 | 6 / 12, 26.57°, factor 1.1180, hip 1.5000 | the factor every other row multiplies by |
| Roof area calculator | 40 × 24 ft gable, 12 in eave and rake, 6/12 | 1,220.9 sq ft, 12.2 squares, 13.4 to order, 41 bundles | plan 42 × 26 ft = 1,092 sq ft × 1.1180 |
| Shingle calculator | footprint 42 × 26 ft (overhangs in), 6/12, laminated, 10 % | 1,220.9 sq ft, 13.4 squares to buy, 41 bundles, 860 shingles, 3,440 nails, 2 rolls | same area and bundles as the area tool |
| Rafter length calculator | 24 ft wide, 6/12, 1½ in ridge, 12 in overhang | line 13 ft 4 3/16 in, tail 1 ft 1 7/16 in, cut 14 ft 5 9/16 in, 31 per side at 16 in on 40 ft | tail is the truss tail to the sixteenth |
| Roof truss calculator | 24 ft span, 6/12, 3½ in heel, 12 in overhang, 40 ft at 24 in | 21 trusses, height 6 ft 3 1/2 in, top chord 13 ft 5 in, area 1,220.9 sq ft | area matches the area tool; top chord is 13/16 in longer |
Three tools land on the same 1,220.9 sq ft of roof surface from three directions: the area tool from the plan with overhangs, the shingle tool from a footprint entered with the overhangs in it, and the truss tool from its span, tails and length. That is the slope factor doing one job three times. Where they part company is on what they are for. The area tool rounds the order once, 13.4 squares and 41 bundles; the shingle tool gets the same 41 bundles and goes on to 860 laminated shingles and 3,440 nails at four per shingle.
The framing tools differ by one deliberate deduction. The truss top chord is 13 ft 5 in, the full run of 12 ft times 1.1180. The rafter line is 13 ft 4 3/16 in, because a stick-framed rafter stops at the face of a 1½ in ridge board: half the ridge comes off the level run, 3/4 in, which is 13/16 in along a 6/12 slope. A truss has no ridge board, so nothing comes off. Both tools give the same sloped tail, 1 ft 1 7/16 in, for a 12 in level overhang, and both count framing on the same 40 ft: 21 trusses at 24 in, or 31 rafters per side at 16 in.
Enter the footprint without the overhangs and the same shingle tool gives 1,073.3 sq ft and 36 bundles, five short. The roof is the same; the input is not. That one difference, footprint against plan area with overhangs, is the most common reason two roofing calculators disagree, and it is why the area tool asks for the eave and rake overhangs as separate inputs.
IRC 2021 roofing limits behind the calculator warnings, with the section for each
Every slope warning a roofing calculator on this site prints traces to a line below. The values are Chapter 9 of the 2021 International Residential Code, Section R905 for roof coverings, as carried in the Seattle SDCI 2021 Chapter 9 reprint the site reads from. The IRC writes every slope as units of rise in 12 of run; the degrees column is the arithmetic, arctan(rise ÷ 12), not the code.
| Roof covering | Minimum slope | In degrees | IRC 2021 section |
|---|---|---|---|
| Asphalt shingles | 2:12 | 9.46° | R905.2.2 |
| Two layers of underlayment under asphalt shingles | 2:12 up to 4:12 | 9.46° to 18.43° | R905.2.2; Table R905.1.1(2) |
| Clay and concrete tile | 2½:12 | 11.77° | R905.3.2 |
| Metal roof shingles | 3:12 | 14.04° | R905.4.2 |
| Mineral-surfaced roll roofing | 1:12 | 4.76° | R905.5.2 |
| Slate shingles | 4:12 | 18.43° | R905.6.2 |
| Wood shingles | 3:12 | 14.04° | R905.7.2 |
| Wood shakes | 3:12 | 14.04° | R905.8.2 |
| Built-up roofs (coal-tar ⅛:12) | ¼:12 | 1.19° | R905.9.1 |
| Metal panels, lapped, no lap sealant | 3:12 | 14.04° | R905.10.2 |
| Metal panels, lapped, with lap sealant | ½:12 | 2.39° | R905.10.2 |
| Standing-seam metal | ¼:12 | 1.19° | R905.10.2 |
| Modified bitumen, thermoset and thermoplastic membranes | ¼:12 | 1.19° | R905.11.1; R905.12.1; R905.13.1 |
The shingle rule is short enough to read in full. This is the first sentence of Section R905.2.2 as printed on page 9-5 of that reprint:
R905.2.2 Slope. Asphalt shingles shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater.
The other figures the tools check or state: two layers of underlayment on asphalt shingles from 2:12 up to 4:12 and one layer at 4:12 or steeper (R905.2.2 and Table R905.1.1(2)); an ice barrier, where the local climate table calls for one, from the eave to a point at least 24 in inside the exterior wall line, and at least 36 in along the slope on a roof of 8:12 or steeper (R905.1.2); a drip edge at the eaves and rakes of a shingle roof (R905.2.8.5); and not less than four fasteners per strip shingle (R905.2.6). On the framing side, rafter spans are measured on the horizontal projection (R802.4.1), the ridge board is at least 1 in nominal and no shallower than the cut end of the rafter (R802.3), a notch at the end of a rafter is no deeper than one fourth of its depth (R502.8.1), and the roof and ceiling must tie across the building to resist thrust (R802.2). Trusses are designed and drawn by the fabricator under R802.10, and are not cut or altered without an approved design.
What the IRC does not set matters as much on a roof. It gives no waste allowance, no bundles per square, no shingle count per bundle and no nails per square: those are the shingle maker's figures and the tools label them that way. It gives no truss spacing or truss span, which belong to the design drawing. It gives no common or standard pitch: 4/12 to 9/12 on most houses is a rule of thumb, as is a roof being walkable up to about 6/12 or 7/12. And it does not define a steep roof for safety; that is the federal construction safety rule, OSHA 29 CFR 1926.500, which calls a roof low-slope at 4 in 12 or less and steep above it, an employer rule rather than a building code. Your jurisdiction can amend any line, and how the site reads its sources is on the methodology page.
Where a roof takeoff goes wrong, in numbers
These are the roofing mistakes that come up again and again, each with the arithmetic that catches it. The overhangs left out. A 40 × 30 ft house at 6/12 is 1,341.6 sq ft of roof on its walls and 1,502.6 sq ft with 12 in overhangs all around: 161 sq ft, 1.6 squares and five bundles apart, 45 against 50. The house square footage used for the roof. Living area counts floors, not roof: a two-story house has about half the roof of the same floor area on one level, and a roof over a garage or porch is not in the living area at all. Start from the footprint at the walls, never from the listing. The pitch left out. The same 40 × 30 ft house with no factor is 12 squares; at 6/12 it is 13.4 before any overhang. A hip factor on the area. A hip roof and a gable over the same footprint at the same pitch have exactly the same surface area, 1,502.6 sq ft on the default house; the hip changes the waste and the cap, 106 ft of ridge and hip, not the area.
Squares rounded before the bundles. Rounding 16.53 squares up to 17 and then multiplying by three orders 51 bundles; the tools multiply first and round once, 50. Waste added to the roof. The roof is a measurement; waste goes on what you buy. Add it to the area and then to the order and it is counted twice. The overhang taken as the tail length. A 12 in overhang is measured level; along a 6/12 rafter it is 1 ft 1 7/16 in, and a tail cut to 12 in leaves the fascia line short. The truss top chord taken for a rafter. A truss chord runs to the peak, 13 ft 5 in on a 24 ft span at 6/12; a rafter stops at the ridge board, 13 ft 4 3/16 in. A 4/12 minimum for shingles. The code floor is 2:12, with two layers of underlayment up to 4:12; 4:12 is where one layer is enough.
What these roofing calculators do not do
They do not put a dollar figure on anything, and they are not a roof replacement estimate or a quote. They do not size a rafter, ridge beam, truss member or connection for load, or check a roof for snow or wind: rafter sizes come from the IRC span tables in the rafter span calculator, truss members from the truss design drawing, and snow and wind loads from the local design values. They do not lay out a truss web pattern, a valley, a dormer or a roof with two pitches in one pass; a cut-up roof is measured plane by plane and added. They do not measure a roof from a satellite image or an address; the area comes from the dimensions you enter.
They do not cover tile, slate or membrane takeoffs in pieces or rolls, only their minimum slopes; they do not count gutters, vents, flashing pieces or fasteners for anything but asphalt shingles; and they do not rate a shingle for wind, impact or fire. Bundles per square and pieces per bundle are the shingle maker's figures and vary by product, so check the wrapper. They do not cover commercial or low-slope design work, where the IBC and the membrane maker set the numbers, and they do not know your local amendments or which code edition your jurisdiction has adopted.
Roofing guides and question posts under this hub
The posts below take the roofing questions the calculators answer in two lines and give them the long form. Each one links back to the tool it supports.
- How to calculate roof pitch, and convert it to degrees: the hand method with a level and a tape, from the ground and from the attic.
- Roof square footage and roofing squares explained: squares, the multiplier and the area of every common roof shape.
- How to measure a roof for shingles and how many you need: measuring, waste by roof shape, and the order list.
- Shingle bundles, squares and coverage: bundles per square by shingle type and why three bundles do not always make 100 sq ft.
- How to figure rafter length, spacing and how many you need: the rafter layout by hand, with the spacing and the count.
- Roof truss spacing, span and how to calculate: spacing, spans and ordering trusses.
- Rafters vs trusses: span and when to use each: the choice between a cut roof and a trussed one.
Roofing calculators FAQ
Which roofing calculator do I start with?
Start with the roof pitch calculator unless you already know the pitch, because every other roofing tool multiplies by the slope factor it gives. For a reroof the order is pitch, then the roof area calculator for square feet and squares, then the shingle calculator for bundles, underlayment, nails, cap and starter. For a new roof the pitch is a choice, so start from the walls: the rafter length calculator for a stick-framed roof or the roof truss calculator for a trussed one, then the area and shingles once the overhangs are fixed. If the pitch reads in degrees from an app or a plan, the pitch to degrees chart turns it into x/12 first.
Why do the shingle and roof area calculators give different squares for the same house?
Almost always because one was given the overhangs and the other was not. The roof area calculator asks for the walls and then adds the eave and rake overhangs; the shingle calculator takes the footprint you type, so it needs the overhangs already in it. A 40 × 30 ft house at 6/12 is 1,341.6 sq ft on its walls and 1,502.6 sq ft with 12 in overhangs, 45 bundles against 50. Enter the same plan in both, 42 × 32 ft, and they agree. The other causes are a different waste allowance, a different shingle type, which changes bundles per square, and rounding squares before multiplying, which the tools do not do.
Is the top chord of a roof truss the same length as a rafter?
No, the truss chord is longer by the ridge deduction. A truss top chord runs to the peak, so on a 24 ft span at 6/12 it is the full 12 ft run times 1.1180, 13 ft 5 in. A stick-framed rafter stops at the face of the ridge board, so half the ridge thickness comes off the level run first; with a 1½ in ridge that is 3/4 in level, 13/16 in along the slope, and the rafter line is 13 ft 4 3/16 in. The tails match: a 12 in level overhang is 1 ft 1 7/16 in along the slope on both. Use the rafter length calculator to cut rafters and the truss calculator to order trusses.
What does the IRC say about roofing, and what does it leave to the maker?
The 2021 IRC sets the minimum slope for every roof covering in Section R905, from ¼:12 for membranes and standing-seam metal to 4:12 for slate, with 2:12 for asphalt shingles. It sets the underlayment, two layers from 2:12 up to 4:12, the ice barrier where the climate calls for one, drip edge at eaves and rakes, and the fasteners per shingle. It leaves out waste, bundles per square, pieces per bundle and nails per square, which are the shingle maker's figures, and it leaves truss spacing and spans to the truss design drawing. The calculators label which figures are code and which are the maker's or a rule of thumb.
Can I use these roofing calculators for a metal roof?
Yes for the geometry, not yet for the panel count. The roof pitch, area and framing calculators work the same under any covering, so the area in square feet and squares, the rafter lengths and the truss count are all good for a metal roof. What is different is the minimum slope: lapped metal panels without lap sealant need 3:12, with lap sealant ½:12, and standing-seam systems ¼:12 (R905.10.2), so a metal roof can go flatter than shingles. Panels are counted across the roof width by the panel's coverage and cut along the slope, which is the metal roofing panel calculator's job when it is added to this hub.
Can I use them for a hip, gambrel or shed roof?
Yes for the area of a hip roof and a shed roof, and with care for a gambrel. The roof area calculator has gable, hip, shed and flat shapes; a hip roof has the same surface area as a gable over the same footprint at the same pitch, 1,502.6 sq ft on the default house, and the tool adds the hip lengths and cap. A shed roof is one plane with a low eave and a high edge. A gambrel has two pitches per side, so measure each slope as its own plane and add them. Hip and jack rafters, shed rafters and gambrel trusses each get their own calculator under this hub.
In what order do I run the calculators for a new roof?
Pitch first, framing second, area and materials last. Fix the pitch with the roof pitch calculator, or the degrees chart if it is set by an angle. Then run the rafter length calculator for a cut roof, or the roof truss calculator to count and check the trusses, with the overhang you will build. Only then measure the roof in the area calculator, with that overhang in the plan, and take the squares to the shingle calculator for bundles, underlayment, nails, cap and starter. Run the rafter span calculator alongside the rafter tool to check the size will carry the span; a truss is sized by its maker. Changing the overhang late changes every number after it.
Do the roofing calculators include waste?
Yes, on the quantities you buy and never on the roof. The roof area, squares measured, rafter lengths and truss count are measurements and carry no waste. The waste allowance you enter, 10 % by default, is added once to the squares to order and to the bundles, shingles and nails, and it is not added to underlayment rolls, cap, starter or drip edge, which are counted from their own lengths. As rules of thumb, a simple gable takes about 10 %, a hip or cut-up roof 15 %, and a complex roof with valleys and dormers up to 20 %. The IRC sets no waste figure.
How were the figures on these roofing pages checked?
By script against the calculators. Each roofing page is built by a script that mirrors the tool's arithmetic, recomputes every area, square, bundle, length and angle printed in the copy and tables, and refuses to write the page if a number disagrees with what the tool displays at the same inputs. Code slopes were read against the Chapter 9 reprint named on the methodology page; truss sections that could not be re-read are cited by number and paraphrased, and say so. There are no ratings or user counts on the site. If a number on a page disagrees with the tool, that is a fault to report through the contact page.
Do these calculators size rafters or trusses for snow and wind?
No, they work out geometry and quantities, not loads. The rafter length calculator gives a cut length, angles and a birdsmouth, but whether a 2x8 will carry the span under your snow load is a table lookup by species, grade, spacing and load, which is what the rafter span calculator does from the IRC tables. A truss is designed for its loads by the fabricator and the design drawing governs it. Snow and wind enter through the local design values, and a steep pitch does not by itself make a roof stronger. Nothing on these pages replaces the span tables, the truss drawing or the building department.
What is the lowest pitch each roofing material can go on?
From ¼:12 for membranes and standing-seam metal up to 4:12 for slate, with asphalt shingles at 2:12 (R905). The full table with the code text is on the roof pitch calculator page, and the same minimums in degrees are in the pitch to degrees chart.
How do I turn roof square footage into squares and bundles?
Divide the roof area by 100 for squares, add waste, then multiply by three bundles per square for most shingles and round up once. The roof area calculator does it from the footprint, and the roof square footage post has the long form.
Should I build my roof with rafters or trusses?
Trusses for most new houses and long clear spans ordered from a fabricator; rafters for attics you want to use, short runs and one-off roofs cut on site. The rafters vs trusses guide sets out the choice.