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Roof measurement methods: here we describe various methods for measuring all roof data: roof slope or pitch, rise, run, area, and other features. We include on-roof measurements, roof measurements or estimates that can be made from ground level, and several neat tricks using a folding ruler to measure roof angle or slope.
This article shows how simple measurements can give the roof area without having to walk on the roof surface. This article series gives clear examples just about every possible way to figure out any or all roof dimensions and measurements expressing the roof area, width, length, slope, rise, run, and unit rise in inches per foot.
There are several neat tricks that allow you to just a carpenter's rule to get a close approximation of the roof slope or pitch. We describe all of them here. All of these examples work best for a simple gable end roof. Additional measures may be needed for hip roofs, mansards, and intersecting gables.
How to Measure Roof Slope from On the Roof
To use the direct roof slope measurement procedure shown just abofe, in addition to safe access to a roof that won't be damaged by touching or walking on it, you need: a tape measure and a carpenter's level. This procedure to measure roof slope can be performed on the roof or from a ladder at roof edge. [Click to enlarge the image].
Hold the carpenter's level in a horizontal position with one end touching the roof surface.
Measure out 12" along the level bottom edge - the green line in our sketch. Mark this point on the leve.
Measure straight down to the roof surface from the mark you just made - the blue line in our sketch.
Read that distance in inches. The result is inches of rise per foot of run.
How to Measure Roof Slope from Inside the Attic or Inside of a Cathedral Ceiling Structure
You need: a tape measure and a carpenter's level. The procedure is like the on-roof example above, but flipped upside down.
As with measuring roof slope on the roof, inside the attic allw e need is a simple carpenter's level and a tape measure. We use the level to measure out from the roof surface horizontally for a distance of 12".
Then from that point on our level we measure the distance straight up to point of contact with the roof surface.
That distance in inches will be the inches of rise per foot of horizontal run for this (horrible leaky) roof.
Watch out: if you do not keep the level at right angles to the roof (parallel to the gable end wall of the building) your vertical measurement will be inaccurate. Hold your level with its side tightly against the side of a rafter to keep the level at right angles to the roof surface.
Or you can make this measurement inside the attic or even inside the building if a cathedral ceiling is installed (photo at left) by holding the level right against the gable end wall as we show at left.
How to Measure Roof Slope Outdoors at the Gable End of a Building
You need: a tape measure and, if there is no horizontal siding installed, you will need a carpenter's level. [Click to enlarge the image]
From a ladder or other accessible area giving safe access to the sloping gable end trim or roof edges,
1. Measure out horizontally 12" (one foot) from the gable end sloping trim to a point along horizontal siding, or use your level to make a mark that is level, horizontally measured out 12" from the sloped gable trim or roof edge.
2. From the point you just marked, measure vertically straight up to the edge of the same sloping gable end trim or roof edge. This distance (green arrow in our photo) is the roof rise in inches per foot of run.
How to Measure Roof Slope & All Other Dimensions From the Ground - counting siding courses
All of the roof data: rise, run, and slope can be calculated from the ground if the building sports horizontal siding such as that shown in our photograph below.
Roof rise (b): first we will obtain the total roof rise by counting siding courses. We measure siding width & then we count courses at the building gable end. If we start counting siding at a horizontal lilne even with the lower roof edge or eaves and count up to the ridge, we've got a close guess at the total roof rise.
The number of siding courses from the roof triangle base to the roof peak x siding course width = total roof rise = (b)
Roof length (c): Measure or step off building gable end width.
Roof width (a): This data allows us to calculate the roof triangle as we know two sides (b) and (c) of the three sides of a right triangle (the red lines in our photo at above left). Let (b) = the vertical rise in the roof and (c) = the roof length (building length + gable overhangs). The third side of the triangle, its hypotenuse or the sloping surface of the roof, or side is (a) which is calculated as follows:
a2 = b2 + c2 - the square of the length of the hypotenuse (a) equals the squares of the lengths of the opposite sides of a right triangle (b) and (c).
Given a2 we use our calculator to take the square root and bingo, we have the length of the sloping side of the roof.
Given that we now know all of the lengths of our triangle we can easily obtain roof slope too if we need it.
Now finally to get the roof area, we just need one more figure, the length of the roof along the building eaves or ridge. From the ground we measure or step off building length (L).
The roof area(RA) is calculated easily: we multiply the Roof Length (c) (which is the sum of building length plus the gable end overhangs of the roof) by the Roof Width of slope (a) that we just figured out above when we computed the hypotenuse of the roof triangle (that's why we needed the roof rise number).
RA = (a) x (c)
Measure Roof Slope From the Ground Using a Folding Carpenter's Rule
As you've see from out other roof measurement examples beginning
at ROOF MEASUREMENTS, knowing the roof slope is really handy as something we can measure up close but also from a distance, obtaining a number that plugs into simple formulas for roof area too.
What if the roof is inaccessible from outdoors, and there is no access to the building interior (i.e. its attic) so we cannot make direct roof slope measurements, but we still need to know its slope?
You need a simple folding carpenter's rule. It helps to have a steady hand and to read the instructions on exactly where to read the inches on the bottom scale in the folding rule triangle that we show at left.
Details of how to use a folding carpenter's rule to obtain roof slope, area, and other measurements along with a table that translates inches along the bottom scale of the folded rule into roof slopes are
at ROOF MEASURE by FOLDING RULE
Obtain Roof Pitch or Slope by Using the Rafter Tables on a Framing Square
A long-standing use of the framing square that is missed by many new carpenters is the rafter table imprinted right on the framing square itself.
This data will tell us the required rafter length for a roof of a given pitch or slope.
The rafter table on a typical framing square gives rafter length for roof slopes with a rise anywhere between 2" to 18" per foot of run.
How to Obtain the Roof or Ceiling Slope, Rise & Run Using an Angle-Finder Level & the Tangent Function
It's trivial to obtain the slope of a roof or ceiling if you happen to have picked up an angle finder such as the one shown here and sold by Pro Products Co., Rockford IL.
We can simply place either of the Level + Angle Finder straight sides against a sloping surface and read the surface slope expressed in degrees right off of the tool's scale.
A weight inside the Angle Finder level moves the pointer to the proper position.
For more accurate angle or slope measurements, for example on a rough shingle surface outdoors, use a straight edge between the angle finder level and the roof surface to give a longer footprint for the sensor. If your framing square is steel (not aluminum), the angle finder level will clamp neatly right onto the framing square.
That's how I used the angle finder shown until someone ran over my steel framing square, forcing me to buy a new (aluminum model). The angle finder level still works but I have to hold it in place (leaving too scratches on my ceiling as I fumbled around for these photos).
You can see that the angle finder immediately gives the answer to the roof slope: 81 degrees. [Click to enlarge any image]
Now what? We can use the TAN or tangent feature of a calculator as a trivial way to convert degrees of slope (or grade if we're building a sidewalk or road) into units of run per unit of rise. The Tangent of any angle expressed in degrees is nothing more than a ratio:
Tangent = Rise / Run
Let's calculate rise per foot of run for this roof using the number from an angle level
I'll show that even if we screw up we can still come out ok finding the angle and then the rise and run of a roof using the angle finding level.
I read 81 deg. on my angle level. Now let's figure run for 12" of rise for an 81 degree slope - HOLD ON! something's crazy here. This is a low slope roof, how can it be sloping 81 degrees? Egad! that's nearly straight up! This is a good lesson in thinking for yourself - or performing a sanity check on calculations.
The answer is I was holding my angle level on the wrong scale. I could have made my photos over again holding the angle level the right way, but there's an easier trick:
81 degrees is just 9 degrees off of dead vertical (90 - 81 = 9). So really I could go just 9 degrees off of flat. As "flat" is 0 degrees of slope, flat+ 9 = 9. My roof actually slopes 9 degrees. Whew!
The Tan value for my 9 degree slope roof = Tan ( 9) = 0.1583
The inches of rise for 12-inches of run on a 9 deg low-slope roof is calculated as follows:
Tangent is defined as a ratio: Rise / Run so all we need is a little algebra (don't faint, it's easy):
0.1583 = Rise / Run
Set run to 12-inches because we're going to calculate the rise per foot of run.
0.1583 = 12 / Run
Use simple algebra:
0.1583 x Run = 12" of rise
Run = 12" / 0.1583
Run = 75.8"
That makes sense: we travel about 75 inches horizontally for every 12 inches of vertical rise on this low slope 9 degree roof.
Calculate Unit Rise for the Roof Rise
To calculate total rise if I knew the total run (say we had made an on-roof measurement) we take the following steps:
Total Rise = (Total Run in Feet) x (Rise per Foot)
The Tan value for my 9 degree slope roof = Tan ( 9) = 0.1583
0.1583 = Rise / Run
Using a little high school algebra we can re-write the equation as
0.1583 x Run = Rise
If I want to know the rise per foot of run I calculate
0.1583 x 12 = 1.89 " of rise per foot of run.
Calculate Total Rise for the Roof
I measured the total horizontal run - my building width is 20 ft. + a total of 2 ft. of overhang at the eaves.
0.1583 x 22 = 3.5 ft.
My roof increases in height 3.5 ft. from the eaves to the high end (this is a shed roof).
I can check this result against the rise per foot we got above.
(22 ft. x 1.89" rise per foot) / 12 = 3.5 ft. (thank goodness)
For the Tangential Enthusiast - Usnig Inverse Tangent Function Tan-1
The inverse Tan-1 function can convert a Tan value back into degrees of roof slope.
Tan-1 (1.43) = 55 deg. and wonderfully, Tan-1 (1.00) = 45 deg.
Since Tan is a simple ratio of unit Rise / unit Run, we note that we can quickly convert a roof slope in degrees into the number of inches of rise per 12" of run as follows, using a 55 deg. slope as example:
Tan (55) = 1.43
Since 1.43 = rise / run we can use simple algebra to write:
1.43 x 12" run = 17.16" of rise per 12" of run
If you have safe access to the roof surface you can quickly make the needed area measurements: just measure from the ridge to the lower edge or eaves, keeping your tape straight. With a decent 3/4" or 1" wide 30 ft. tape measure you can extend the tape out to catch the roof eaves without having to walk dangerously close to the roof edge. Also measure the roof edge or length.
RW = Roof Width - dimension a in my sketch at left. Remember to include in (a) the amount by which the roof extends out over the building walls - its eaves.
RL = Roof Length = dimension of the roof from one gable end of the building to the other. RL is simply the building width plus the amount of overhang at the two gable ends of the building.
Slope is simply the relationship between increase in height and horizontal distance traveled. In my sketch above if we travel up my triangle along (a) from left to right, we will cross a horizontal distance (b) and we will ascend in height an amount (c).
Slope or pitch or grade has several means of expression: Rise/Run, Angle in degrees, Tangent, Pitch, or Percent Grade - all compared in our little table below.
Tangent = Rise / Run
E.g. Tan (45 deg) = 1.00 - that is, we will travel 12" up for 12" of run
Tan -1 is the inverse of a Tangent which will convert any rise/run ratio back to degrees.
E.g. Tan -1 (1.73) = 60 deg.
Put another way, a roof with a slope of about 27 in 12 has a Rise/Run of 27/12 = 1.73 (approximately)
So Tangent in its simple use here is just the ratio of increase in unit height for travel in unit run. "Units" can be inches, feet, meters, furlongs, or hammer handle lengths. A table of tangent values for common roof slopes is given at FOLDING RULER to SLOPE DATA TABLE. Below is a tiny summary that shows the range of these values.
Typical Range of Roof Slope Values
Horizontal Scale 90 deg Reading in Inches1 Shed Roof
Horizontal Scale Left Slope Reading in Inches12 Gable Roof
How to Use Horizontal or "Flat" Roof Projections as Rough Estimates of Roof Area for Inaccessible Roofs
Another simplistic approach used by some estimators is to ignore complex roof structure, just measuring the building's footprint and the roof slope - an approach that gets you into the right "ballpark" but will very seriously under-estimage the roof area for steep slope roofs.
Frankly, as we illustrate beginning at ROOF MEASUREMENTS, there are some easy and accurate alternatives that can give a good estimate of roof area while making measurements only from the ground. But to understand how some people use a flat or horizontal projection of a roof to guess at roof area, here is the procedure.
BF: Measure the building footprint or BF
EO: Measure or estimate the increase in footprint size given by the roof eaves overhang. (Tip: look at the drip line under the roof eaves and measure the distance from the outer edge of the drip line to the building exterior wall. This is EF.
GO: Measure or estimage the increase in footprint size given by the gable end overhangs. This is GO.
RF: If the eaves overhang and gable end overhang are the same on both front and back and left and right building ends we just add these up to obtain Roof Footprint or RF.
How to convert the building footprint or roof "footprint" (building footprint + roof overhangs) to roof area
To convert the rectangular footprint of the building roof to roof area we need to increase the footprint area to account for the greater area covered by the sloping roof. Using any of the roof slope estimating or measuring methods described above, just this simple roof slope multipication chart:
Roof Slope Multipliers: convert a "flat" or "projected horizontal" building footprint
to sloped roof coverage area
[Watch out: calculations have not been verified]
Multiplier to convert a flat horizontal
footprint to roof area
Triangle side (a)
Triangle side (b)
Triangle side (c)
(a2 = b2 + c2)
Convert (c) to Inch Scale
(convert fraction to 16ths of an inch)
(c) x 2
(two roof slopes)
2 in 12
3 in 12
4 in 12
5 in 12
6 in 12
7 in 12
8 in 12
9 in 12
10 in 12
11 in 12
12 in 12
Notes: for fractional slopes, when estimating roof area use the next higher slope multiplier.
Roof Pitch = rise / run = Roof Slope = Tangent Function. Tangent calculations are illustrated at ROOF SLOPE CALCULATIONS
Roof slope or pitch can also be expressed in degrees or angular degrees, as we illustrate at ROOF SLOPE DEFINITIONS
The numbers in the table above can be calculated as follows: (and as illustrated below)
For a 12-inch unit-length roof we calculate the hypotenuse dimension to obtain the roof true width for each roof slope. In the table above the roof slope or rise (e.g. 6 in 12) gives us the vertical dimension of a right triangle. The horizontal dimension is fixed at 12 inches.
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 "How to Measure Angles with a Ruler", South Dakota School of Mines and Technology, Website: http://www.mcs.sdsmt.edu, http://www.mcs.sdsmt.edu/tkowalsk/portfolio/downloads/pub_HowToMeasureAngles.pdf retrieved 10/26/2013, copy on file.
"Choosing Roofing," Jefferson Kolle, January 1995, No. 92, Fine Homebuilding, Taunton Press, 63 S. Main St., PO Box 5506, Newton CT 06470 - 800-888-8286 - see http://www.taunton.com/FineHomebuilding/ for the magazine's website and for subscription information.
Owens Corning Corporation, One Owens Corning Parkway
Toledo, Ohio 43659
Telephone: (419) 248-8000
Fax: (419) 248-5337
http://www.owenscorning.com Owens Corning is credited as the inventor of fiberglass when Owens Illinois [O-I] researcher Dale Kleist and his colleague John Thomas stumbled onto and then realized the significance of producing glass fibers in 1932. O-I formed a joint venture with the Corning Glass Works in 1935, leading to the formation of Owens Corning Corporation in 1938. More on Owens Corning's history is at
Focus, Toledo, Ohio, Owens-Corning Fiberglas Corporation, October 1988.
"A History of Innovation," http://www.owenscorning.com, 1997.
Stewart, Thomas A., "Owens-Corning: Back from the Dead," Fortune, May 26, 1997.
International Directory of Company Histories, Vol. 20. St. James Press, 1998.
"Two-Year Wisconsin Thermal Loads for Roof Assemblies and Wood, Wood–Plastic Composite, and Fiberglass Shingles [on file as Roof_Thermal_Loads.pdf] - ",
Jerrold E. Winandy
Cherilyn A. Hatfield, US Department of Agriculture, US Forest Products Laboratory, Research Note FPL-RN-0301
ARMA - Asphalt Roofing Manufacturer's Association - http://www.asphaltroofing.org/
750 National Press Building, 529 14th Street, NW, Washington, DC 20045, Tel: 202 / 207-0917
ASTM - ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA The ASTM standards listed below can be purchased in fulltext directly from http://www.astm.org/
NRCA - National Roofing Contractors Association - http://www.nrca.net/, 10255 W. Higgins Road, Suite 600,
Rosemont, IL 60018-5607, Tel: (847) 299-9070 Fax: (847) 299-1183
UL - Underwriters Laboratories - http://www.ul.com/
2600 N.W. Lake Rd.
Camas, WA 98607-8542
Tel: 1.877.854.3577 / Fax: 1.360.817.6278
copy on file as /roof/Roofing_Historic_NPS .pdf Roofing for Historic buildings", Sarah M. Sweetser, Preservation Brief 4, Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS .pdf From Asbestos to Zinc, Roofing for Historic buildings, Metals", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_3 .pdf From Asbestos to Zinc, Roofing for Historic buildings, Metals-part II, Coated Ferrous Metals: Iron, Lead, Zinc, Tin, Terne, Galvanized, Enameled Roofs", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_4 .pdf From Asbestos to Zinc, Roofing for Historic buildings, Metals-part III, Slate", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_5 .pdf From Asbestos to Zinc, Roofing for Historic buildings, Metals-part IV, Wood", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_5 .pdf From Asbestos to Zinc, Gutters", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
copy on file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_2 .pdf From Asbestos to Zinc, Roofing for Historic buildings, Metals- Roofing Today", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
/exterior/NPS_Preserv_Brief_16_Subs_Mtls.pdf The Use of Substitute Materials on Historic Building Exteriors ",
Sharon C. Park, AIA, Preservation Brief 16, Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source: http://www.nps.gov/history/hps/tps/briefs/brief16.htm
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).