On Thu, 19 Jul 2012 01:09:30 +0000, 69roadrunner
wrote:


'Doug[_14_ Wrote:
;2894179']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
'Anthony Watson of Camas, WA' (http://www.anthonywatson.us)
'Mountain Software Home - Recipe Software, Address Book Software,
Calendar Software' (http://www.mountain-software.com)-


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

Anthony, thanks for your detailed reply. All your assumptions are
correct except for the load which, if I understand the truss specs
correctly, is only 43.3 psf.

Anyhow, I went to the city and they said any lumberyard could do the
calc as well as supply engineered drawings (which appear to be all
computer generated these days anyhow). The local yard I went to had them
within minutes and recommended an engineered beam of 3" x 10". However,
since I want to temporarily enclose this each winter I'm thinking I just
may add some additional posts so there's something to attach the wall
material to. However this is a lot more work (which I was trying to
avoid with only 2 posts) since it involves cutting into an existing
poured slab in order to install the sonotubes and poured piers. Nothin's
ever easy !!!!!!!!!


I hope they also checked for your deflection??? I don't know still
what your PLF is but my hunch is your deflection is okay with a 10"
beam depth. The width does little for the deflection.