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My townhouse has some outdoor stairs built out of 6" x 8" lumber. A couple of the steps are rotten and I need to change the lumber. There is also a perpendicular long piece of lumber that crosses underneath the steps, more or less flush with the ground (highlighted in yellow, in the first image below). While I am replacing the lumber for the rotten step I think it would be a good occasion to replace that other rotten piece as well. I don't want to dismantle the whole staircase for that, but I am hoping to cut off the lumber at the base of the second step, so that the new piece extends below the first step (so it is not apparent that the lumber was cut) (the red line in the second picture).

  1. I am assuming that this piece of lumber is not structurally important (it's pretty rotten right now, and the rest of the staircase is still standing), and there will be no other negative side effect to my plan. Does anyone disagree?

  2. What is the best way to cut it off? Maybe it's so rotten that just hacking at it with a hammer / crow bar might do it... but I am not sure.

  3. What is the best way for preventing this from rotting again? It is pressure treated, but it rotted anyway. Is there some sort rubber sheet/lining that I should use between the lumber and the soil?

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    Lumber in contact with the ground, will eventually rot.
    – Tester101
    May 7, 2014 at 18:19
  • It's important to keep water from pooling against the wood. Even pressure-treaded lumbar will rot if it's constantly wet. Typically wood should not be in contact with the ground at all, but if you must keep this general layout, make sure water isn't collecting.
    – Hank
    May 8, 2014 at 2:57

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I would cut off the timber in the ground using a Sawzall type tool (reciprocating saw) that has a long rough cut blade installed. You can get blades as long as 12 inches that can easily reach into tight spots and cut the wood. Soil will wear the teeth away fairly quickly and so be prepared to replace the blade.

I've used this technique to cut roots free from stumps when removing a tree. Works very well.

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I would use a chain saw myself, but if you dig out adequate clearance, even a hand saw will work. You'd have to also dig clearance for a chain saw, running the chain in dirt is a bad thing. Thoroughly clean the chain after doing this work.

You may find the wood is rotted beyond where you plan to cut. It's important to remove all rotted wood or else it will spread into competent wood, it's sort of like cancer. Thus you will be cutting into competent wood, so simply prying out rotted wood is not enough. Go with your plan, but be prepared for the scope to grow substantially.

There are different levels of pressure treatment, foundation grade is the highest treatment level and really the only level suitable for direct burial. Barrier materials will help to some degree, but ultimately all wood in the ground will rot, as Tester101 points out.

You might consider using concrete or masonry materials anywhere the structure is in contact with earth to avoid this.

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