Hi, folks. I'm planning on building a porch -- that is, a shed-type
roof that will come off the back of the house and shelter an existing
flagstone patio that is mortared to an eight-inch concrete slab.
What I'm trying to figure is how many posts I need.
Dimensions will be about eight by 24 feet. Construction will be
post-and-beam with 6-by-6 posts and beams. The underside of the roof
will be open, so the posts, beams, joists and decking will all be
visible. Rather than plywood, the decking will be tongue-and-groove
planking to provide a beaded effect.
I'm thinking three posts should be enough. That is, one post at each
corner and one in the middle. So there will be support every 12 feet.
Am I OK here?
I don't want to clutter the area unnecessarily, so the fewer posts the
One more might help. A 6x6 with modulus S = bd^2/6 = 5.5x5.5^2/6
= 27.7 in^3 and max bending moment M = fS = 1000S = 27.7K in-lb
= WL/8 might support WL = 222K in-lb, ie a total weight of W pounds
over an L inch span.
With a 100 psf floor load and L in inches, an 8'xL" deck section
supports 8x100L/12 pounds. If the building supports half of that,
the beam along the outer edge of the deck supports W = 400L/12 lb,
and 400L^2/12 = 222K makes L = 82 inches, so an 8' span might work.
I had understood him to mean that he was building a roof
over an existing surface, not anything that was intended to
support foot-traffic. That being the case, the total
load on the roof will be around, 30# x 192 sqft, = 5760,
of which half will be supported on the outer edge, = 2880.
Since that's presumeably the long edge, he needs a
horizontal member that will support about 120 pounds
per linear foot. IIRC, you can cantilever a beam about
1/4 of the distance it would span if it were supported
at both ends, so shifting each post in 4' gives a 16' span
and a 4' cantilever. Using L/240 for the maximum
deflection, that requires a built up beam of two 2x10s.
(Note: I wouldn't use 6x6s for the horizontal beam.)
If he wants the thing to look good, he's going to want
diagonal braces anyway. How big those are will depend
on how high the roof is, but assuming they start at 2'
below the post, he can move the posts another foot in,
and the diagonals buy him another 2' for a total clear
span of only 10'. *THAT*, he can do with a 6x6..
(Note that, if you're going to use the diagonals as
structure and not decoration, you can't just tack
them in place using 16d nails.. A purist will
use a big-ass shouldered mortice and tennon,
with pins. A modern contractor will use 3/8"
iron plates, and 1/2" bolts..)
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