There are several ways to determine the pitch or slope of a roof surface, including the use of satellite imaging technology. Most important however, is that you do not have to climb on the roof or even on a ladder to easily measure a roof slope.
For those who understand basic terminology, you can skip the first section. However, if you are not entirely sure of the lingo involved, take the time to read about the basics first.
PITCH OR SLOPE OF ROOF
Pitch of a roof is simply the slope of the roof surface with respect to a horizontal surface. Slope can also be visualized as slant, or tilt.
Click on the following link for a simple diagram that is essential for easy understanding.
http://structural101.com/RoofSlopeConversionTables.h tml
The following basic terms are used in discussion;
Rise = Change in vertical elevation between two points on a roof surface
Run = Change in horizontal distance between two points on a roof surface
For practical purposes, roof slope is most often reported as Rise divided by Run, also described as Rise "on" Run. When discussing roof slope, this fraction or ratio (yes, basic arithmetic!) is most often reported as the Rise per 12 inches of Run, such as 6 on 12.
Rise and Run must have the same units (inches or feet), although units are generally not reported since they "cancel out" in the ratio.
For single family houses with gable roof shape (A-frame), roof slopes typically range from 3 on 12 (minimum for shingles) to 12 on 12 (maximum). For row homes in older sections of towns and cities, the roof is often monoslope (one way) with very low slope, such as 1 on 12 or less.
Roof slope (pitch) can also be reported as the angle between horizontal line and sloping roof line (see diagram) measured in degrees. This slope angle can then be converted to a Rise-On-Run format using basic trigonometry or taken off a conversion chart (see end of article).
For a gable roof, the slope angle will generally be in the range of 14 degrees to 45 degrees.
WHERE TO TAKE MEASUREMENTS
Safety is of course essential when taking measurements of a roof. Falls from ladders are unfortunately high on the list of causes for serious injury and death.
Experienced contractors and competent, sure-footed homeowners can take measurements on top of a roof with relatively low (shallow) slope. However, the vast majority of homeowners should only take measurements inside the attic space (if feasible and safe), especially if roof slope is relatively high (steep) or the house is two stories or more.
Roof slope can be measured in an attic space since slope of roof rafters (or top chord of roof truss) is the same as slope of the roof surface.
However, even in an attic space, there can be dangerous conditions to consider, especially if there is no flat floor surface (typically plywood). In that case, homeowners should rely on an experienced contractor unless they are competent to walk and balance on the attic floor joists.
USING ANGLE FINDER
Taking into account safety, ease of effort and accuracy, the best method for measuring roof slope is by employing a simple angle-finder tool available in building supply stores and online. The following link lists a wide range of angle-finders, priced as low as $4.99 at Sears (as of 1-4-2010).
Hold the angle finder against bottom edge of rafter (in attic space) or directly on top of the roof surface. However, on the roof surface, make sure the angle finder is not on the edge of a shingle.
Convert the angle reading to a Rise-On-Run format as shown below.
However, the following angle-finder has a scale that converts angle reading to standard Rise-On-12 format;
http://www.amazon.com/Johnson-Level-Tool-750-Locator /dp/B00002N7UH
USING PROTRACTOR
Another easy and safe method that is usually accurate enough is to use a standard carpenter level (2-foot or 4-foot long) and simple protractor while inside the attic space. Plastic protractor can usually be purchased at drug stores such as CVS or Rite Aid.
As you may remember from grade school, using a protractor is very easy, almost intuitive. However, see the following link (or others) for a basic refresher;
http://www.mathsisfun.com/geometry/protractor-using. html
Note that rounding to the nearest degree is more than adequate for almost all practical purposes.
Perform the following steps to measure the roof slope angle;
(1) Tap nail into side face of rafter to help hold carpenter level (in level position) against side face of a rafter. Be careful to avoid nails that will almost certainly be protruding through roof sheathing (between rafters).
(2) With pencil or pen, draw (along bottom edge of level) a level line on the rafter. This line must extend over to the bottom edge of rafter. For further discussion, the intersection of the line with bottom edge of rafter is designated Point A.
(3) Although not necessary, you can also draw a vertical line (through Point A) to help align the protractor.
(4) With protractor up against side of rafter, position center point of the protractor on point A, with horizontal line of protractor over the line you drew on the rafter. Now read the slope angle for the rafter, which is also the roof slope.
(5) Convert slope angle to Rise-On-Run format as shown below.
USING CARPENTER LEVEL AND TAPE MEASURE
If you do not have an angle-finder or protractor, roof slope can be measured the more "traditional" (and difficult) way, using a carpenter level (2-foot or 4-foot long) and tape measure (or ruler).
Note that longer dimensions (measured) result in greater accuracy.
Having a helper is useful to obtain greater accuracy. However, a single person can take necessary measurements rather easily.
From inside attic space, perform measurements as follows;
(1) Hold one end of carpenter level against bottom edge of a rafter, with length of level in the same direction as (parallel to) the rafter. Of course the level must be held in the level position. Your helper is most useful to hold the level. Without a helper, you could tap a long nail into the rafter to support end of the level.
(2) At right end of the level, measure vertical distance from top of level to bottom edge of rafter. It is important for accuracy that the vertical distance is measured straight up from end of level (not angled). To minimize error, you can take readings at two or more rafters.
When taking measurement on the roof surface, the vertical distance (Rise) is measured downward, from bottom edge of level to roof surface.
It is essential that the horizontal distance (Run) you are using is known. This should be the length of the carpenter level, which will usually be 24 inches or 48 inches.
Roof slope (as a pure ratio number) is then the vertical distance (Rise) divided by length of level (Run). However, you must multiply this result by 12 to obtain the standard Rise-On-12 reading.
For example, say you use a 2-foot level and measure 8-1/4 inches as the Rise. Roof slope is then 8.25 inches divided by 24 inches. Multiply this fraction by 12 to obtain the standard Rise number for a 12-unit Run.
(8.25 / 24) x 12 = 4.125
You can use a calculator (or go wild by performing division with pencil and paper) to obtain this result. However, it is easier to realize that the result is simply the 8-1/4 divided by 2 (dividing both 12 and 24 by a common factor of 12). .
Roof slope is then 4.125 on 12, which is also 19.0 degrees
CONVERTING RISE-ON-RUN TO SLOPE ANGLE
From basic trigonometry, the Rise / Run ratio is the tangent of the slope angle. When using a calculator or spreadsheet, the result may be reported in radians, which you would have to covert to degrees (multiply by 180 / Pi).
To convert from slope angle to Rise / Run requires use of the more "exotic" inverse tangent function (or Arc Tangent).
The following conversion tables provide a baseline;
Rise On Run Slope Angle
2 on 12 9.5 degrees
3 on 12 14.0 degrees
4 on 12 18.4 degrees
5 on 12 22.6 degrees
6 on 12 26.6 degrees
7 on 12 30.3 degrees
8 on 12 33.7 degrees
9 on 12 36.9 degrees
10 on 12 39.8 degrees
11 on 12 42.5 degrees
12 on 12 45.0 degrees
Slope Angle Rise-On-Run
10 degrees 2.1 on 12
15 degrees 3.2 on 12
20 degrees 4.4 on 12
25 degrees 5.6 on 12
30 degrees 6.9 on 12
35 degrees 8.4 on 12
40 degrees 10.1 on 12
45 degrees 12.0 on 12
You can download more extensive conversion tables from the following;
http://www.structural101.com/RoofSlopeConversionTabl es.html
HI-TECH MEASUREMENTS
To avoid taking measurements at all, there is actually a way to have roof slope measurements taken using hi-tech equipment from high above (satellite).
Skytek Imaging is advertising such service online, including a roof slope diagram showing slopes of each roof surface for one house, at a relatively low cost of $56 (as of 1-4-2010).
http://www.skytekimaging.com/ourservices.html
This type of service is most likely worthwhile for roofing contractors and design professionals, especially for large houses with complicated roof geometry.