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
How long is each beam? What loads are a part of the 43.3 psf and
what is the plf on the beam? Is there any depth of beam restrictions?
If so, maybe a glulam product might work better or a dual beam. I
think a couple of uploaded construction pics would help explain more
There are probably online beam load calculators that
could answer the question. But every place I know you
would need a building permit to add a porch on to a
house. And here to get that building permit you need
a drawing signed and stamped by a licensed architect.
So, I'd start with finding out what it takes to get a building
permit. If it's like here, you'd be wasting your time
figuring it out because the architect is going to do it
Have you tried googling for online beam load
calcualtors? I've seen them. I find it interesting that
there are municipalities where they will approve something
that is structural like this without an architect or engineer
signing off on it. I mean I don't know that there is a
specific code that just says for X porch you need Y
beam. It would seem an architect or engineer would
have to do the load analysis and make that decision.
And it just seems odd that a local code official wants
to be in the position of engineer of record for free for
someone's building project.
I would not build anything with a 14 foot span. You'll get eventual sag
in the middle no matter what you use, unless it's a steel girder. Use a
center post. Seven feet is easy to span and much stronger.
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
Your room is 14' wide with 16" overhang on each end for a total width of
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).
I wasn't sure where the OP was located, so I erred on the side of heavier
snow loads. I've always used 30psf for live load calculations here in my
area. In any case, I calculated it with the 30 psf and still came up with
about the same size beam.
When we built our house, I asked the truss company to design for 50 psf
live loads, since we occasionally get heavy snow in the winter. Ironically,
the only change this required in the trusses was slightly larger metal
connecting plates. Kind of surprised me.
On Wed, 18 Jul 2012 21:14:38 +0000 (UTC), HerHusband
<grin>. People often overlook the connections and seem to just worryabout the beams / columns. Usually we try to make the connections
stronger than the beam because if it fails at the connection, it can
be sudden vs. the beam, usually bends before failing allowing people
to get off. I'm a retired structural engineer so I speak from
experience. Most of my experience is in steel / concrete but for a
short part of my career I did wood (homes). Be careful tho when
using your liveload because if you take full snowload with a full
liveload (think what liveload is meant to include here), that's not
likely in a real world sense. Sometimes in engineering we have to
make educated guesses what we think the real loads will be over the
life of a structure. Of course we would like to over design but
depending on the structure, that can be too costly and most clients
don't like that. Last, I commend you for thinking about loads when
building your home... most people take this for granted.
BTW, I was still assuming 14' length above. If you shorten it to say
7 feet, likely anything 2x6 or more will be fine for deflection.
That said, do you really need an extra post for temporary walls? Maybe
you do but I was hoping there was another way. I guess it depends on
what the wall material is.
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