Standing in the driveway with a tape and a phone
Most re-roofs are priced from the ground, and there is nothing shameful about it. The covering is thirty years old and curling, so every trip across it costs shingles somebody then has to replace. The service drop lands on the fascia at the one corner a ladder would want. The owner wants a figure by Friday and three other firms have already been and gone in twenty minutes each. So the survey happens at ground level: a long tape, a phone, a sketch on the back of a delivery note, and a walk that takes rather longer than twenty minutes if it is done properly.
What the supplier needs at the end of it is a count of squares, and a square is a fixed thing — one hundred square feet of finished roof surface, 9.29 m², the unit both the material and the labour on a steep-slope job are sold in. Getting there needs exactly two measurements and one judgement: the area the roof covers seen from above, the pitch of each plane, and how much of the order will end up in the skip as offcut. Everything after that is a square root and a multiplication, and neither of those is where estimates go wrong.
They go wrong in the first two. A footprint taken at the wall line instead of the drip edge is short by more than a tenth on a house with generous eaves. A pitch guessed one step out is worth about seven and a half percent of the whole order. Both errors are silent, both survive right through to the delivery, and both are avoidable with a plumb bob and half an hour. The rest of this is how to avoid them, and where the ground-level method stops being honest and has to say so on the quote.
The footprint is a lap of the building
Plan area is the area the roof covers looked at from directly overhead: the building's outline pushed outward by the overhang on every side. That last part is the whole trick. A tape run along the wall face measures the house, not the roof, and the difference is not small. Take a plain forty by twenty-eight foot ranch with a sixteen inch eave and a sixteen inch rake: the walls enclose 1,120 sq ft and the roof covers 42 ft 8 in by 30 ft 8 in, which is 1,308 sq ft. That is nearly seventeen percent of the job sitting outside the wall line, and it is the single most common reason a truck arrives light.
Find the overhang by dropping a plumb bob from the drip edge and measuring in to the wall, one at a time, on each elevation. They are rarely equal — an addition built later, a rake detailed tight where the original eave is deep, a porch with its own return. Do not take the overhang along the rafter tail, because a tail is sloped and its plan projection is shorter than its length: twenty-four inches of tail on a 6:12 projects to twenty-one and a half inches on plan, and using the sloped figure inflates the whole perimeter band by the slope factor a second time.
Break the outline into rectangles and measure them as rectangles. An L-shaped house is two; a house with an attached garage and a bay is four. Check each one is actually square with a diagonal or a 3-4-5 before trusting a single length and width, because a plan that is out by three degrees over forty feet is out by two feet at the far corner. Record the rectangles separately on the sketch rather than folding them into one number, since anything with its own pitch — a porch, a garage wing built shallower, a bay roof — has to be carried through the arithmetic on its own.
One more thing gets measured on this lap even though it has nothing to do with area: linear feet. Ridge, hips, valleys, rakes and eaves are all bought by the foot and all visible from the ground, and the plane count you write down here is what tells you later whether ten percent waste or eighteen is the honest allowance. A cut-up hip roof with three valleys and a dormer does not have more area than the gable next door, but it certainly has more of everything sold by the metre and a great deal more offcut.
- Walk the perimeter first with nothing but eyes, and mark every change of direction, every roof that meets the wall, and every plane you can see on the sketch.
- Measure each rectangle at the wall face, at ground level, taking the longest available run rather than adding up short ones.
- Plumb down from the drip edge on all four elevations and record each overhang separately.
- Diagonal-check anything you intend to treat as a rectangle before you treat it as one.
- Pace out and record the ridge, hip, valley, rake and eave lengths while the sketch is still in front of you.
- Photograph each elevation square-on from as far back as the site allows, for the pitch work and for the argument six weeks later.
Reading the pitch without leaving the ground
A gable end hands you the pitch for free: it presents the slope as a triangle standing vertically, facing you, at a scale you can photograph. Stand as far back as the plot allows, zoom in rather than walking closer, and shoot from a point roughly level with the middle of the gable — perspective is what corrupts this method, and distance plus zoom is what suppresses perspective. Measure the angle of the rake line off the photograph and convert it. Anything that reads within a quarter of a pitch step of a whole number almost certainly is that whole number, because roofs are framed to whole pitches.
There are better methods when the building gives you one. Best is the loft: hold a level horizontal against the underside of a rafter, mark twelve inches along it, and measure straight down to the rafter — that is the rise per twelve, measured on the framing, which is the number the framer actually used. Next best is a pitch gauge or a phone laid on a rake board within reach at a low eave. Counting brick courses up the gable triangle and dividing by the width works too, if the brick is regular. What matters in all of them is that the reading comes off the structure and not off the covering, because tiles, laminates and thick starter courses all sit proud of the deck and bias the answer by a degree or more.
Cross-check with a second method and take the disagreement seriously. Two readings a degree apart is nothing — a degree at 6:12 moves the area by well under one percent. Two readings a whole pitch step apart is a different problem: 6:12 gives a slope factor of 1.118 and 8:12 gives 1.202, so being one step out changes the order by seven and a half percent, which on most re-roofs is the entire margin. If the two methods will not agree, that is the point at which somebody gets on a ladder at the rake and settles it.
Pitch also decides a question that is not about quantity at all. Minimum slope for a given covering is set by the manufacturer's application instructions and by the roof assembly provisions of the adopted code — International Residential Code Chapter 9 in most of North America, as amended locally — and below the stated slope the product either needs a specified low-slope underlayment treatment or is simply not permitted. A shallow porch or a converted carport hanging off a normal house is where this bites, and finding it out from the driveway is a great deal cheaper than finding it out at the inspection.
An angle off a photograph or an inclinometer is in degrees, and every material schedule, minimum-slope table and slope factor downstream wants rise per twelve — convert it here before it goes anywhere near the takeoff.
The roof angle measured from horizontal, in degrees.
Equivalent pitch
6.928 in per 12 in
Rise per 12 = tan(angle) × 12. Round to the nearest common pitch when ordering — material schedules and minimum-slope tables are written against whole ratios.
They open the calculator with your figures already in it
Degrees to Roof Pitch Calculator: 6.93 in per 12 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 6.928 in per 12 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Surveying an existing roof gives an angle, but almost everything you then need to buy is specified against a ratio: shingle manufacturers publish minimum slopes as 2:12 or 4:12, flashing details assume standard pitches, and material take-off tables are indexed the same way. Converting the measured angle back into rise-per-12 is what lets a survey talk to a supplier. Expect an untidy number — a real roof rarely sits on a whole ratio — and round to the nearest standard pitch for ordering while keeping the measured figure for cutting. Where the rounding crosses a manufacturer's minimum slope threshold, round down rather than up: a covering installed below its stated minimum slope is a warranty problem and, in driving rain, a leak.
One plan area, one slope factor, one order
The slope factor is the ratio of true sloped surface to its own shadow, and it is the square root of one plus the pitch ratio squared — identical to one over the cosine of the roof angle, which is the same statement in the other notation. On a 6:12 it is 1.118. Multiply the plan area by it and you have the surface the covering must actually cover; divide by a hundred square feet and you have squares. Carrying the earlier example: 1,308 sq ft of plan area at 6:12 is 1,462 sq ft of roof, which is 14.6 squares before a single offcut.
Waste goes on as a separate, visible line, not folded quietly into the area. Ten percent is a reasonable allowance for a plain gable with two planes; fifteen and upward belongs to anything with valleys, hips, dormers or a plan that changes direction, because every valley is a diagonal cut on both sides and every hip throws away roughly half of what it cuts. That is why the plane count went on the sketch. Applying 10% to 14.6 squares gives 16.1 squares of material, and a quote that shows the 14.6 and the 10% separately is a quote you can defend line by line when the owner asks why they are buying more roof than they have.
Squares then have to become things the supplier stocks. Most three-tab and standard laminate lines run three bundles to the square and the heavier architectural products run four or five, but the only figure that counts is the coverage printed on the wrapper for that exact product. Sixteen point one squares at three bundles is 49 bundles once you round up to whole ones. Starter strip, hip and ridge cap, drip edge, valley metal and underlayment are all counted off the linear measurements from the lap, not off the square count, and forgetting that is how a job that is right on shingles still stops on a Tuesday morning.
| Pitch | Angle | Slope factor | Roof surface (sq ft) | Squares before waste |
|---|---|---|---|---|
| 4:12 | 18.4° | 1.054 | 1,379 | 13.8 |
| 5:12 | 22.6° | 1.083 | 1,417 | 14.2 |
| 6:12 | 26.6° | 1.118 | 1,462 | 14.6 |
| 8:12 | 33.7° | 1.202 | 1,572 | 15.7 |
| 10:12 | 39.8° | 1.302 | 1,703 | 17.0 |
| 12:12 | 45.0° | 1.414 | 1,850 | 18.5 |
Plan area and pitch resolve into the squares the material is sold in and the labour is quoted in; run each rectangle from the sketch separately where the pitches differ, and read the pre-waste sloped area out of the breakdown so nothing downstream adds an allowance twice.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The area the roof covers, measured on plan.
Rise in inches per 12 inches of horizontal run.
Cutting, starter courses, hips and valleys.
Roofing squares
15.86 squares
Geometry is exact for a plane roof. Hips, valleys and dormers add area beyond the plan projection, which the waste factor is there to absorb.
- Slope factor
- 1.12 ×
- True sloped area
- 1,442.26 ft²
- With waste
- 1,586.49 ft²
- Sloped area in ft²
- 1,586.49 ft²
- Pitch angle
- 26.57 °
They open the calculator with your figures already in it
Roof Squares Calculator: 15.86 squares — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- One slope factor is derived from the single pitch figure and applied across the entire footprint, so a gambrel, a mansard or a lower-pitched extension roof has to be run as separate calculations and the square counts added together.
- Pitch is read as rise per 12 of run whatever measurement system the area is entered in, and the field stops at 24 in (610 mm) 12 — an angle in degrees or a percentage grade has to be converted to that form before it is typed in.
- Nothing is subtracted from the footprint you enter, so a chimney stack, a large rooflight or an open stairwell well is still counted as covered surface and should be taken out of the plan area first if it is big enough to matter.
- Waste is applied as a flat percentage of the finished surface, which spreads the cost of starter courses and cap shingles evenly over the field rather than counting the ridge and hip lengths that actually generate them; a small roof with a lot of edge needs a higher figure than the same percentage on a large plain one.
- The answer is left as a fractional square rather than rounded up, and no particular product sits behind it — bundles, rolls and sheets each cover their own share of a square, so turning this figure into an order quantity is a second step.
What the flat picture is allowed to hide
There is a piece of geometry worth holding onto, because it settles an argument that comes up on every hip roof. If every plane on a roof stands at the same pitch, the total surface is the plan area multiplied by the slope factor, full stop — the projections of the four planes of a hip roof tile the rectangle underneath exactly, so a hip roof and a gable roof over the same footprint at the same pitch contain precisely the same number of squares. Cutting a roof up does not add area. It adds waste, and it adds hip and valley footage, and those are separate lines that you already measured.
The identity breaks the moment a plane sits at a different pitch, and that is where careful ground surveys still go wrong. A porch roof run shallower than the main house, the lower slope of a gambrel, a mansard, a gable dormer framed steeper than what it sits on: each one needs its own rectangle, its own pitch and its own slope factor, and a dormer also brings cheek walls and a small area of main roof it displaces. Rounded, conical and swept-eave work is outside what a footprint-times-factor method can honestly produce at all, and the right answer there is to say so rather than to publish a number.
Then there is everything a plan view cannot see by definition. How many layers are already up there — which drives tear-off labour, disposal weight and whether the fasteners will reach — is knowable only by lifting an edge at a rake or looking hard at the gutter line and the cut ends at the eave. Deck condition, board gaps, delaminated sheathing, previous repairs, flashing that was caulked instead of replaced, and ventilation that was never adequate are all strip-day discoveries. Aerial measurement reports bought from imagery vendors solve the geometry very well and none of this, which is exactly the trap: a report with three decimal places on the area still tells you nothing about the second layer of shingles it is measuring.
Choosing a ladder by its working length
The ladder question is not what is on the van. It is what length reaches the eave at the place the ladder will physically stand, and that place is often not the elevation the owner points at. Ground falls away, basements walk out, window wells and planted beds occupy the six feet of standing room the setup angle demands, and the eave height on the low side of a sloping plot can be four or five feet more than the height at the front door. Measure it where the ladder goes.
Three ways to get that height from the ground, in descending order of trust. Run a tape up with a second person and a plumb line, which is exact and needs two people. Count courses of something regular — modular brick at three courses to eight inches, or lap siding at its stated exposure — and multiply. Or use the phone: stand a measured distance out on level ground, sight the eave with the inclinometer, and add your eye height, since height equals distance times the tangent of the angle plus eye height. Twenty-seven feet back, thirty degrees up, five feet six at the eye gives 27 × 0.577 + 5.5, or 21.1 ft. Treat that as a cross-check against the course count rather than as the answer on its own.
Two OSHA requirements then size the ladder. Non-self-supporting ladders are set with the horizontal distance from the foot to the top support at about a quarter of the working length, the 4:1 rule of 29 CFR 1926.1053(b)(5)(i), which is roughly 75.5 degrees from horizontal; and where a ladder is used to get onto a surface, 29 CFR 1926.1053(b)(1) requires the side rails to extend at least three feet above that landing so there is something to hold while stepping across the eave. Together they say the ladder must work to about 1.03 times the eave height plus three feet: 24.7 ft of working length for that 21 ft eave, with its foot standing just under six feet out from the wall. Confirm that six feet exists and is level before going any further, because the fix for a foot in a flower bed is a different ladder position, not a steeper angle.
Now the part that catches people out: a ladder's labelled length is not its working length. Extension ladder sections have to overlap, and the overlap schedule published under ANSI ASC A14.2 runs three feet for ladders up to thirty-six feet, four feet above that to forty-eight, and five feet from there to sixty. A twenty-four foot ladder is two twelve foot sections overlapped three feet, so it works to twenty-one feet — it would not reach our eave with the required extension. A twenty-eight foot ladder is two fourteens and works to twenty-five. The eave measured at twenty-one feet therefore takes a twenty-eight foot ladder, and the maximum working length printed on the ladder's own label is the number to buy against. European ladders under BS EN 131-2 carry the same kind of marking, and the UK guidance in HSE INDG455 states the same one-in-four setup.
Duty rating is the other half of the specification and it is routinely ignored. The ANSI ASC A14 series rates ladders from Type III at 200 lb through Type IAA at 375 lb, and that rating covers the climber together with clothing, tools and anything being carried; BS EN 131-2 tests to a 150 kg load. A roofer with a laden belt has spent a serious share of a light-duty rating before anything else joins him, which is one of several reasons material goes up by hoist or handler rather than by hand. Choose fibreglass rails where the ladder or the person on it could come near the service drop, which on most houses lands at the eave — the clearances for those conductors sit under NFPA 70 Article 230 and moving or shrouding them is the utility's job, not the roofer's. Fit a standoff so the rails bear clear of the gutter rather than crushing it, secure top and bottom before the first climb, and remember that the roof edge itself remains a fall exposure under 29 CFR 1926 Subpart M no matter how well the ladder is set.
Enter the eave height measured at the ladder's actual footing and this returns the minimum working length the 4:1 angle and the three-foot extension demand — then compare it against the maximum working length on the ladder's label, not against the length in its name.
The height of the support point (eave, wall top, or landing) you need to reach.
Minimum ladder length
15.4 ft (minimum ladder length)
Extension ladders have overlap between their sections, so a ladder's labeled length is longer than its maximum safe extended reach — check the manufacturer's duty rating and maximum working length, not just the labeled overall length, before buying.
- Height to reach
- 12 ft
They open the calculator with your figures already in it
Extension Ladder Size Calculator: 15.39 ft (minimum ladder length) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- The setup angle is fixed at OSHA's quarter-of-the-working-length rule, a single multiplier of 1.0328 on the height, so a lean forced shallower or steeper by a fence, a flowerbed or a driveway you cannot plant the feet on changes the rail length you actually need and nothing in the arithmetic responds.
- Three feet of extension above the support point is added on every run, including the case where you will stand on the rungs and work rather than step off at the top, and that allowance is not offered as an input you can switch off.
- The support point is taken to sit against the wall face the 4:1 setback is measured to; a projecting eave, gutter, windowsill or standoff bracket pushes the base further out and adds rail length the formula carries no term for.
- The answer comes back as a raw decimal — about 15.2 ft at the 3.6 m default — rather than a size ladders are sold in, so round up to the next stocked length yourself.
- Ground condition sits outside the calculation entirely: sloping, soft or stepped footing, levelers under the rails and tying off the top all decide whether the climb is safe, and none of them is asked for.
- Reach is capped by the input itself, which refuses anything above 15 m, so the tool covers house-height access rather than the taller sectional and pole ladders used on commercial frontages.
Pricing from a measurement you have declared
Squares are the unit the whole quote is built on: material by the square, installation labour by the square, tear-off by the square for each existing layer, and access and staging priced against the pitch and the storey count because both change what the crew has to stand on. Disposal is the one line that is not area at all — it is weight, and the installed weight per square is on the shingle manufacturer's product data sheet, multiplied by the number of layers coming off and by the squares. A transfer station charges by the ton and a skip has a weight limit as well as a volume, so a two-layer tear-off on a steep roof is the case where a quote priced on area alone quietly loses money.
This site publishes no rates, and the reason is that they are not publishable: labour, tipping fees, bundle prices and permit costs vary too much between two neighbouring counties to put a number on a page and call it useful anywhere. What does travel is the structure — squares, layers, linear items, access, disposal weight — and the discipline of pricing each of them from something you measured rather than from a per-square figure remembered off the last job on a differently shaped roof.
Last, write down on the quote how the measurement was taken. Something as plain as measured from ground level, pitch taken as 6:12 from the gable elevation, one existing layer assumed, quantities to be confirmed on strip is honest, takes one line, and turns the awkward conversation about a second layer from an argument into a clause. Estimators who skip that sentence are not saving anything; they are just deferring it to the day the crew is on the roof and the owner is standing in the driveway.
What goes back to the office
The sketch is the deliverable, not the total. It should carry each rectangle with its own dimensions, the overhang on each elevation, the pitch of each plane and how that pitch was obtained, the plane count, the linear feet of ridge, hip, valley, rake and eave, the eave height at every position a ladder would stand, and a note on the ground at each of those positions. Add the photographs, the evidence you found about layer count, and the state of the gutters and fascia, since a re-roof quote that ignores them gets amended in front of the customer.
The table below is a short list of what actually goes wrong when a roof is measured from the driveway, with the cost of each. None of them is exotic and all of them are cheaper to avoid than to explain.
| What was measured | How it goes wrong | What it costs |
|---|---|---|
| Footprint at the wall line | Overhang left out of the plan area | Around 17% on a small ranch with 16 in eaves |
| Overhang along the rafter tail | Sloped length used as a plan projection | The perimeter band inflated by the slope factor |
| Pitch off the finished covering | Reading biased by the covering standing proud of the deck | A degree or more, and sometimes a whole step |
| Pitch assumed rather than measured | 6:12 taken where the roof is 8:12 | 7.5% of the entire material order |
| Waste inside the area figure | A second allowance added downstream by the product calculator | Squares bought twice and stored |
| Eave height at the front elevation | Ladder set on the low side of a falling plot | A ladder that will not legally reach the eave |
| Layer count assumed at one | Second layer found on strip | Tear-off labour and disposal weight doubled |
Taking the driveway survey into the workspace
Six things come off the sketch and nothing else does. The stack opens on the worked example, the 121.6 m² plan area of that 1,308 sq ft footprint at 6:12 with 10% waste, so overwrite all three with your own before reading anything out of it.
- Plan area, rectangle by rectangle, overhang included — Measured at the wall face and pushed out by the plumbed overhang on each elevation; anything with its own pitch stays its own rectangle.
- Pitch per plane, and how it was obtained — Off the framing where possible, off a square-on photograph otherwise, and cross-checked — a whole step of error is worth more than the job's margin.
- Waste, sized by plane count — Driven by valleys, hips and dormers rather than by area; shown as its own line on the quote so it can be defended rather than discovered.
- Linear items off the same lap — Ridge, hip, valley, rake and eave in feet, for cap, drip edge, valley metal, fascia and gutter — none of them come out of the square count.
- Eave height at each ladder position — Measured where the ladder will actually stand, not at the front elevation, together with a note on whether six feet of level standing ground exists.
- Evidence about the existing covering — Layer count, cut ends visible at the eave and rake, gutter and fascia condition — the inputs to tear-off labour and disposal weight.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.