Expert: Daniel Humphrey - 10/29/2008

Question
Hi, I'm just putting a roof over my 12 by 28 deck and need to know how many supports I need in 28'.
I live in edmonton alberta so I need for snow load.
Thank You

Answer
Richard,

There are many ways to approach this project, depending on what kinds of openings you want in your porch. For a double header only 2" x 6", the posts (4" x 4") need to be every four feet, or eight posts, with at least two knee braces.

To build a basic post and a beam structure, you'll need a minimum of three 8" x 8" posts, supporting a 8" x 12" beam. This 28' beam can be built up from 2" x 12" boards of various lengths, glued and nailed together. The posts should be braced to the beam at the corners with 4" x 6" knee braces about 52" long, minimum. Knee braces oppose each other, providing sheer strength to the outer wall.

Rafters need to be at least 2" x 12" on 24'' centers to go that 12'. You may get by with 2" x 10", if some sag is acceptable under heavy snow.

The attachment to the building must be very secure, since the building itself will be supplying the sheer strength to keep the new addition from leaning outward. I recommend tying the porch rafters into the main rafters, if the roofs meet at the same level. If the porch is dropped some, then the ledger must be bolted into the wall studs or otherwise secured that it cannot pull out or slip down under the weight of the snow.

I hope this helps.

Daniel