Rise and Run for Your Porch Roof
Your roof's slope, also called pitch, is determined by the rise and run.
The slope is the steepness of your roof and is the ratio between any two points on your roof.
A 7/12 roof (normally expressed as a ratio) means that for every 12 inches horizontally (the run), your roof rises 7 inches.
Mary and I know that our roof pitch table and rafter information below will make it easier for your calculations.
The slope is critical to building a porch roof, cutting rafters to the correct length, and even determining the amount of shingles you will need.
When building your own porch roof the
slope should match the roof of your home.
This is not feasible in all cases but it should be as close to it as possible.
Change the slope of your roof and the effect can be dramatic.
Steeper roofs tend to last longer and are more pleasing to the eye.
There are only two kinds of roofs, flat and sloped.
Slope also affects costs.
A 12/12 roof (pretty steep) will require a taller chimney stack and definitely more lumber.
However, the roofing material could last 50% longer and it will require less maintenance.
- A roof with a slope of 2/12 or less is considered flat.
Flat roofs have restrictions as to the type of material that can used to cover them.
In addition, the flatter the roof, the more problems you will have with water, snow, and ice.
- A low-sloping roof ranges between 2/12 to 4/12.
Definitely a roof you would want to avoid especially in snow country because of the weight of the snow.
- Conventional roofs range from 4/12 to 9/12
- Anything over 9/12 is considered a steep roof.
Determine the Rise and Run
To calculate the area of your roof, first determine its rise and run (slope or pitch).
We found some fairly easy ways to do this (you'll need a tape measure and a level).
Before you begin, mark the bottom of your level 12" from one end.
To Calculate the Area of Your Porch Roof
- Option 1: The no ladder option. Put the level against a rafter in your attic
with the 12-inch mark at the bottom of one of the rafters. Measure from the end of the level up to the bottom edge of
the rafter. If it measures 4 you have a 4/12 roof.
- Option 2: Find a safe place to position a ladder next to your roof. (Use caution
on the ladder; many emergency room visits are made due to falling off one; been there, done that!). Pick a spot along the
roof edge and hold the level until it reads "level". Then measure down from that 12 inch mark you made straight down to the
top of the roof. If it measures 6 you have a 6/12 roof, if it measures 8 inches you have an 8/12 roof.
- Option 3: If you have a gable end on your house, place the level against
the rafter on the roof's overhang. Again, use the 12 inch mark on your level and measure up to the bottom of edge of that rafter.
If your roof has a pitch of 3/12 or less, multiply length times width. That was easy.
For roofs over 3/12 measure multiply the length of your home by the width of your home after you add in the
If you have a 1 foot overhang add 2 feet to the width and 2 feet to length. Multiply them together.
Example: Your porch is 42 feet long and 10 feet wide and has a 1 foot overhang.
42 + 2 feet for the overhang
(1 foot on each side) = 44 feet
10 + 2 feet for the overhang (1 foot on each side) = 12 feet
44 x 12 = 504 square foot porch
Not quite finished yet!
Calculate Square Footage for Roofing Materials
Next, you will need to multiply your calculated porch square footage (as done above) by a factor correlated to the slope of your roof.
We've provided those factors for you to make it easy.
Determine the Square Footage Factor
If the slope (rise and run) is 4/12, then you multiply your square footage by 1.06.
Based Upon the Slope
If the slope (rise and run) is 5/12, then you multiply your square footage by 1.08.
If the slope (rise and run) is 6/12, then you multiply your square footage by 1.12.
If the slope (rise and run) is 7/12, then you multiply your square footage by 1.16.
If the slope (rise and run) is 8/12, then you multiply your square footage by 1.20.
If the slope (rise and run) is 9/12, then you multiply your square footage by 1.25.
If the slope (rise and run) is 10/12, then you multiply your square footage by 1.30.
If the slope (rise and run) is 11/12, then you multiply your square footage by 1.36.
If the slope (rise and run) is 12/12, then you multiply your square footage by 1.42.
So going back to our example:
If your rise and run (or slope) is 4/12, multiply the square footage in our example above, 504, by 1.06.
504 x 1.06 = 534.24 square feet
In this example, the area of your porch roof would be 534.24 square feet.
So How Many Shingles Will You Need
How many? A square of shingles cover 100 square feet.
In the example above, you will need 6 squares to cover a porch roof that is 534 square feet.
Porch Roof Rafter Length & Rafter Plate Location
OK, now you want to install rafters. How long should they be? Being too short or too long are not very good options.
Here's how you can calculate not only the length of your rafters but also where you need to place the rafter plate on your exterior wall.
You will need either a rafter square or rafter tables to make the proper calculations.
PLEASE READ: The following are "general" calculations and are not to be used for actual construction.
I'm providing them here as information only so you can see what's involved in constructing an actual roof.
Each roof construction job is different and therefore we recommend you work with a licensed contractor who has the experience to do it right.
Rafter Plate Location
The rafter plate on your wall, that is, where your rafters will be attached to your home), is critical to maintaining
the correct rise and run for your porch roof.
Multiply the rise by the total run in inches. Then divide by "12".
This gives you the height for your rafter plate from the top of the ledger board.
(If not using a ledger board it would be the distance from the top of the outside beam (as measured against the wall).
For example, if you have a rise of 4/12 and a run of 96 inches: Multiply 4 x 96 = 384 and divide by 12 = 32".
You would place your rafter plate 32" above your ledger board or joist as shown in the diagram above.
For a rise of 4 the length factor is 12.65 (factors are marked on a rafter square or in a rafter table).
Multiply this by the width of your rafter (2 x 6 which would be 5.75 inches) and then divide that by 12 and then add 2 inches
for roofing materials.
Mark this measurement from the bottom of the rafter bottom.
If it is a minimum of 3 inches from the window sill you will be ok; if not, decrease the rise and run or lower your porch ceiling.
To find the length of your rafters, multiply the run in inches by the rafter length factor and divide by 12.
For example: The run is 96" x 12.65 (length factor for a 4/12 roof) = 1214.4 divided by 12 =101.2 inches. Not quite done yet though.
Add the overhang space the same way.
If your overhang is 14" then multiply 14" by 12.65 = 177.1 divided by 12 = 14.75 inches.
Add that to the 102.2 inches (101.2 plus 14.75 = 115.95 or 116 inches).
Your rafters will need to be at least 116 inches long.
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