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.

Example of rise and run

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.
  • 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.
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.



Determine the Rise and Run
To Calculate the Area of Your Porch Roof

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.
  • 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 overhang.

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
Based Upon the Slope

If the slope (rise and run) is 4/12, then you multiply your square footage by 1.06.
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.

Another example of rise and run

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.

Rafter Length

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.



The Amazon ads are our affiliate links. Thank you.


Our site is a labor of love. We appreciate your comments very much.

~~~ Signup for free porch tips in your in box. ~~~

Helpful Links

Shop Amazon for Porch /Garden
Aff link

Popular Pages

Please Join Us Here Also

Join us on Facebook for porch talk Join us on Pinterest Subscribe to get our updates. Thank you!

Our Sponsors

The Porch Store
Porch Store
Custom Porch Products

Shop for Porch Parts at Vintage Woodworks.
Vintage Woodworks
For Your Porch Parts

Screened porch windows at diyEzeBreeze.com
DIY Eze Breeze
Screen Porch Windows

Aluminum screen doors from PCA Products
PCA Products
Complement Any Home

End sponsor section

Copyright© 2008-2018
All rights reserved.

Content and photos may not be reproduced
in any way without our written permission.

small rocking chair

Get up to 4 free local contractor estimates for work you need done
Search Our Site


Our Sponsors
Screened porch windows from DIY Eze Breeze
DIY Eze Breeze - Screen Porch Windows

Shop for Porch Parts at Vintage Woodworks.
Vintage Woodworks for Your Porch Parts

The Porch Store
Porch Store - Custom Porch Products

Aluminum screen doors from PCA Products
PCA Products Complement any Home

House Plans
See wonderful house plans that include porches.
Home Plans with Porches

Get Our Online Newsletter,
Front Porch Appeal

Learn more.

1-Enter Your Name

2-Enter Your Email

3-Click Button Below

You'll also receive
our free eBook.
Enjoy our back issues.

What's New
What is new or updated on our site
See Our Latest Updates

Porch Market
Enjoy browsing our Porch Market
Browse Our Porch Market

Get Our Porch Ideas Generator eBook
Front Porch Ideas Generator eBook
Ten Before & After Case Studies
Showing the Difference a Porch Makes!
Only $9.97

See Our Porch Illustrator eBook
Porch Illustrator Vol 1 & Vol 2 eBook
Over 300 Pages of Porch Illustrations
From Various Angles
Just $6.97

Porch Ideas Network
Porch Ideas Network
Listen to Our Podcasts
Plus Featured Builders and Landscapers

small rocking chair
We may earn a commission by referring you to some products on merchant sites. See our disclosure policy. Thank you.

Front Porch Designs Home | Return to Top
Contact Us | About Us | Advertise with Us | Search | Site Map
Media | Privacy Policy | Disclosure | Legal Notice | What's New

XML RSS   Add to My Yahoo!   Add to My MSN  
By Mary and Dave Morris, Copyright © 2008-2018 Front-Porch-Ideas-and-More.com. All rights reserved.
Content and photos may not be reproduced or copied in any way without our express written permission.

Page copy protected against web site content infringement by Copyscape