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I’ve got a 60s era carport with a rotted post. It’s a low slope cantilevered roof about 20 feet long. It has two posts on each corner at the front, and the back is supported by the house.

The rotted post is a 2x4 sandwiched by 2x6s like an I beam.

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enter image description here I could replace it as built, Doug fir 2x. The roof is big enough that I’m a little concerned about a 2x4 holding up all that load.

The pier this rests on is perfectly sized for a 4x6. What holds me back from putting in a 4x6 is I don’t know about cutting a birds mouth into an existing beam, and the pier is not very level.

What do you all think? As built or how to sub in a bigger column?

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  • The problem with a 2x4 is not the compression strength (Douglas Fir is good for 7000 psi - which means a 2x4 can withstand 56,000 lbs / 28 tons). The problem is that it may buckle - and the 2x6's will stop that.
    – Martin Bonner supports Monica
    Oct 1, 2019 at 16:42
  • True, but good luck finding a modern stud cut from doug fir. :) I'd bet that much of the fast-grow pine nowdays isn't much better than half that. Still, should be adequate.
    – isherwood
    Oct 3, 2019 at 1:33
  • Had to call around a bit, but found some decent Doug Fir studs. I’m in the PNW, so Doug Fir land I guess
    – monknomo
    Oct 4, 2019 at 3:51

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The post looks reasonably adequate as it was originally built even by modern standards. If it wasn't you'd have seen disaster long ago, when it first started to decay.

To maintain the style detail I'd rebuild to match, using pressure-treated lumber. By doing so you eliminate the need to cut that notch and you end up with a more robust post. A single post member may have weaknesses that are addressed by using multiple boards. To be clear, each board should be cut accurately to fit its bearing points on each end. It's not just the center board carrying the roof load.

I'd fit everything and screw it all together at 18" intervals with pairs of 3" corrosion-resistant deck screws from each side. After it dries out for a few weeks, use a high-quality wood filler to flatten out the screw heads, then prime and paint.

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    I agree with everything but the deck screws. I think stainless steel would be better. I have used those deck screws on pt and had really bad results.
    – JACK
    Sep 30, 2019 at 21:41

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