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Roof slope definitions & measurement methods:
This article defines different types or degrees of roof slope and explains how roof slope, ptich, angle, or grade slope are measured & expressed. We give the required roof slopes for different types of roofing materials and we illustrate several methods by which the slope or pitch of a roof is easily measured.
We also show how simple measurements can give the roof area without having to walk on the roof surface. We include simple calculations and also examples of using the Tangent function to tell us the roof slope or angle, the rise and run of a roof, the distance under the ridge to the attic floor, and how wide we can build an attic room and still have decent head-room.
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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Roof Slope or Roof Pitch Definitions
Summary of Definitions of Roof Slope Types: Flat, Low Slope, Steep Slope Roofs
Our photo illustrates a roof whose slope has become irrelevant after the building collapsed. I suspect the roof in the photo was a bit steeper before the building fell in.
[Click to enlarge any image]
The illustration at page top and used again below, courtesy of Carson Dunlop & Associates, summarizes the ranges of roof pitch or slope for flat, low-slope or conventional or "steep slope" roofing.
What is the Difference in Slope Between Low Slope & Flat Slope Roofs?
Actually most flat roofs are not dead flat and in good design also include slope towards their drains.
Flat roofs (0" to 2" in slope) are flatter than low sloped roofs and pitch just enough to drain water.
In our photo below the roof slopes less than 1" per foot - notice the two areas of ponding and staining on this roof.
What is the definition of roof slope and how are rise, run, slope, angle and percent slope related?
Roof pitch or slope is the angle of the roof surface above the "flat" or horizontal plane.
Roofers express roof slope as "rise" or "pitch", measured in inches of vertical rise per foot of horizontal distance or "run". So a 3-inch rise roof, also described as a 3 in 12 roof, means that for every 12" (or foot) of horizontal distance, the height of the roof increases by 3".
Our sketch above shows the relationship between horizontal distance or "run" and roof slope or "rise".
[Click to enlarge any image]
While roof slope is typically expressed in "rise", it can also be expressed in degrees or in percent of slope. A 3-in-12 roof rises 3" for every 12" of run. That's the same as a 14 degree slope, or a 25 % slope. Why is the roof slope 25%? 3" of rise per 12" of run is the same as 1" of rise per 4" of run or 1/4 = 25%.
Definition of Run: the horizontal change in distance or the horizontal distance covered by an un-supported rafter or truss.
Rafter run: the horizontal or level distance covered by the rafter - the bottom chord or base of a right triangle.
In our sketch above the rafter run is shown as RUN under the left half of the roof.
Definition of Rafter Span: the un-supported length of the rafter along its slope - the diagonal or hypotenuse of a right triangle. In our sketch the "rafter span" is referred to as the RAFTER LINE LENGTH.
Definition of Roof Span: normally a roof span is the same as the building width between the outer edges of the wall top plates.
Definition of Rise: the vertical change in height per unit of horizontal distance or run.
Unit rise is a is a snonym for slope or roof slope. For example a 3/12 roof has a unit rise of 3" of height increase or rise per 12" of horizontal run or distance.
Many framers use "rise" and "slope" interchangeably, calling a 3/12 rise roof a 3/12 slope roof.
Definition of Roof Slope: roof slope is the amount of (vertical) change in height as a ratio of horizontal distance traveled, also expressed as inches of rise per foot of horizontal run, or cm of rise per m of horizontal run.
Roof slope can also be described as the angle of change in elevation, expressed in one of several forms
typically in construction we express roof slopes as units of rise per single unit of run, such as 3 inches of rise per 12 inches of run or 3" per ft.
Slope as rise / run, e.g. 3 in 12 or 3/12
Slope as an angle: e.g. 14 degrees
Slope expressed as a percent or "grade": Slope is calculated as rise / run or 3 / 12 = .25 x 100 to convert to a percent or 25% grade
Also see ROOF SLOPE TABLE, TYPES, WALKABILITY for a guide to roof slopes versus walk-ability versus roof type and for an explanation of the roof slope ranges that define the catetories of "low slope" and "steep slope" roofs.
Definitions of Low Slope & Steep Slope Roofs
The National Roofing Contractors' Association (NRCA) and other authorities and texts use these general definitions of roof pitch or slope:
Steep slope roofing is defined as any roof pitched greater than 3 in 12 or 14 degrees or 25%.
Roof slope affects roof life (steeper roofs drain better so may be more leak resistant), as well as roof installation cost (steeper roofs are harder or even impossible to walk-on without using roof jacks or scaffolding, increasing labor costs to install roofing).
Our photo (above-left) shows a steep slope slate roof. You can see by eye that this is certainly more than 3" of rise for every foot of horizontal distance or run of the roof slope, and you'll also see that the slate roofers were using roof jacks to work on the roof surface.
Low slope roofing is defined as any roof pitched at 3" in 12" or 14 degrees or 25% slope or less. Low slope roofing in other texts refers to roofing systems for pitches below 4" in 12" of slope
Flat roofing is roughly horizontal or "flat" but in fact very few "flat" roofs are really flat, either because it is difficult to build a dead flat surface over a building, or more importantly because even "flat" roofs need to drain water to avoid ponding and leaks.
So most "flat" roofs have at least a little slope either towards one or more roof edges or towards roof drains. So "flat" roofs are really "low-slope" roofs most of the time. To avoid ponding and leaks, flat roofs typically have a nominal drainage slope of 2% to 4%.
What are the Minimum Slope Requirement for Metal Roofs?
Most metal roofing systems can be
installed on slopes of 3:12 and greater and standing-seam
systems from 2:12 and greater.
Special standing-seam systems
designed for slopes as shallow as
1/2 :12 require field
crimping machinery and have sealant in all seams. The
height of the ribs at seams and whether they are protected
with a sealant affect how weathertight a roof will be under
Roof Slope or Pitch Requirements for Clay Tiles
Clay roofing tiles are installed on slopes as low as 4/12, restricted to 6/12 in some jurisdictions. Our photo (above left) shows a low slope clay tile roof in San Miguel de Allende, Mexico.
This roof was built without sufficient pitch (about 2/12) and it leaks badly during heavy rains, as you can see by our photograph of the roof's under-side (above right). Raising the high end of this shed roof a few inches will improve the roof drainage and stop the leak problem.
Where clay tiles are installed on low slope roofs (less than 4/12) for aesthetic reasons, install a waterproof membrane on the roof surface below the tiles.
Continue reading at STAIR RISE & RUN CALCULATIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see ROOF SLOPE CALCULATIONS for an explanation of the mathematics of roof slope and various slope calculation methods
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"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:
copy on file as /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
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).
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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.
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