On Wed, 18 Jul 2012 14:51:54 +0000 (UTC), HerHusband
wrote:

I am planning to build a porch roof attached to the back of my house.
It will have a trussed roof. The room will be 14' x 14'. I plan to
support this with one end attached to the house and the other two
corners supported by 6 X 6 pressure treated posts.
What I need to know is what size each of the beams spanning the sides
need to be to support the roof. There will be a 16" overhang on each
side. Roof pitch is 4.5/12. Load is 43.3 psf. Length of one side of the
truss is pretty close to 9' so the area of the roof (that each beam
will support) is about 126 sq.ft.. Total load per beam of 5456 lbs

Just so I'm understanding correctly, you will have two beams running
perpendicular to the house, with the trusses sitting on top parallel to
the house? I assume you will be installing some kind of support posts on
the house side of the wall as well, and not just relying on brackets or
something?

Your room is 14' wide with 16" overhang on each end for a total width of
16.7 feet.

The length is 14' with a 16" overhang for a total length of 15.3 feet.

16.7 width x 15.3 length = 256.5 sq/ft

Half of that load is carried by each beam, or approximately 128 sq/ft.

I will assume a 20 psf dead load (weight of the building materials) and
40 psf live load (people on the roof, snow loads, etc.) for a total load
of 60 psf.

60 psf x 128 sq/ft = 7680 pounds on each beam

If you have a 6x6 post at the outer end, and the house end is supported
inside the wall, your free span would be approximately 13.5 feet. But
I'll err on the side of simplicity and say 14 feet.

The size of the beam will vary depending on the wood species you use, but
I'll assume Douglas Fir since that's common in my area. According the
charts I have, you would need a 6x12 beam on each side to support 7680
pounds over a 14' span (assuming you want less than a 1/360 deflection).
That's probably a lot larger than you were picturing.

If you live in a warm climate without snow loads, you could probably get
by with a 6x10 beam.

However, if you add another post to reduce the span to 7', you would have
60psf x 64 sq/ft = 3840 pounds on each beam. In that case, a simple 4x8
beam would be more than adequate to carry the load.

Of course, you will need to install some kind of diagonal bracing to
prevent the whole structure from swaying sideways in winds or earth
movements (lateral loads).

Good luck,

Anthony Watson
www.anthonywatson.us
www.mountain-software.com


I can't say for sure without more info but I suspect your LL is too
high.