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I have a covered patio that extends 10' from the house towards the yard/pool and has a width of 30-33'. The covered patio seems to be part of the roof, I even have access from the attic as we ran some speaker wire a few years ago to the patio.

It is currently supported by (4) 4x4 wood posts which are rotting away. The posts are holding what appears to be (2) side by side 2x6 pieces of wood acting as the beam and span the 30-33' patio.

I would like to replace the (4) existing wood posts and beams with just (2) posts (I'm assuming steel) as far to the edges of the beam as possible. My concerns are due to the span and weight. I want to avoid any sagging of the beam in the middle.

How can this be achieved?

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    At some point you're going to have to get a structural engineer put his stamp on the plans. Might as well go to one now and discuss your options and budget. You could possibly get away with no vertical supports if you were willing to spend enough money by rebuilding a large portion of your home. It's not possible to give you an exact solution without a lot more info. – OrganicLawnDIY May 9 '14 at 3:30
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The vertical posts are probably almost strong enough for only two supports as is. Replacing those with steel is not necessarily needed.

The challenge is the strength of the horizontal span with reduced supports. If left as wood, it will have to be fortified by a factor of more than two. Of course, the added weight of that much more beam means the factor must be increased a little more.

Assuming #1 northern Doug-fir for a span holding roofing only which is 20 lbs/sq ft live load plus 10 lbs/sq ft dead load and assuming that the current spans were engineered at 150%, then increasing the current 10-11 ft spans to 30 feet would require replacing the pair of 2x6s with a pair of 2x12s on edge, firmly tied together.

I tried to figure out an equivalent steel beam, but I am out of my experience. The first result says that a 4x4.5 inch beam is more than twice what is needed. Searching for rough pricing for such beams is surprisingly challenging.

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    Where did you come up with a factor of two? Going from 3 spans to one using simplistic logic would be at least 3 times. Double 2x12s sound pretty spindly to me for a 30' span, especially since they'd have to be built up and spliced since you cannot buy 30' 2x12s. I think we're looking at a fairly substantial glulam or parallam beam here despite it only carrying half of a 10' span. Steel is a viable option, but again, your selection sounds awfully spindly, I'd expect something more like double the depth and weight, perhaps more. – bcworkz May 8 '14 at 21:51
  • 1 + fortified factor 2 = 3. I should have worded it more clearly. Working backwards from the description means that it is built to be lightly loaded only. About an eighth of a living space's loading. – wallyk May 8 '14 at 21:53
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    Did anyone consult a span chart? 33 feet is huge. – shirlock homes May 8 '14 at 22:03
  • @shirlockhomes: I looked at several span charts to average out variability. First I used them to determine the engineering load factor of the current structure. Then I recalculated going from a 10-11 foot span to a 30' span. – wallyk May 8 '14 at 22:05
  • Good work wallyk. I don't like sistered beams. The overlap can be iffy. Lots of hardware. I'd do steel myself. – shirlock homes May 8 '14 at 22:36

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