I'm using Stairtek retreads to resurface previously carpeted stairs. Some current treads have some sag in the middle, with the worst of which being 3/16". Is there a viable way to level that so that the 5/8" thick retreads don't have the additional flex to deal with? enter image description here enter image description here I'm considering something like Bondo putty. I'm not very familiar with most such products and assume some like Durham's Rock Hard would tend to break up too easily. Is something like Bondo or Plastic Wood a reasonable application here?

Please note, my preference would have been to replace the actual treads, but let's say retreads are part of the parameters of the project.

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    Pictures? My hunch is that if you've got that much sag that the problem treads actually move a little when you step on them, and even just a little movement is likely to crack and filler material. Any chance of getting underneath the treads to strengthen them? May 8, 2022 at 19:35
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    I'll get some pictures in later, but you definitely make good points. It would be possible to get underneath and add a center stringer or other shoring. I agree that would be better than these stopgaps. May 8, 2022 at 20:45
  • sag? are the treads bent or worn?
    – Jasen
    May 9, 2022 at 1:23
  • @Jasen They're bent. May 9, 2022 at 2:03
  • If you put some hardening filler on the surface and the steps flex again...it'll probably crack and flake off. Based on your comment, someone screwed up and didn't install a middle stringer. May 9, 2022 at 4:43

2 Answers 2


If you can reasonably get access to the bottom side of the stairs, I would strengthen from below.

  • Cut lengths of 2x4" to fit snugly under each stair tread, running from stringer to stringer (i.e. across the width of the tread).
  • Install the 2x4", crown up, with the 4" dimension running vertically below each tread.
    • "Crown" is any amount of bow along the length of the board. By installing it crown up, any amount of bow in it will point up, then as the board sags over time, it will first sag "flat", before there's any visible dip in the surface. This gives the most life out of the board before any additional remediation work needs to be done.
    • Putting it with the 4" dimension vertically will essentially make this a short joist under each stair tread.
  • Drill a series of clearance holes (not pilot holes) through each tread to run 3-4 construction screws through the tread and into the 2x4 below.
    • A clearance hole is just slightly (1/64" - 1/32") larger than the outside diameter of the threads - the screw should move freely through the hole without the threads catching.
    • A pilot hole is the size of the inner shaft of the screw and leaves enough wood for the screw threads to bite into and hold.
  • Run a bead of construction adhesive along the top of the 2x4", then push it up into place. While holding it firmly in place from below, have a helper drive the screws through from the top, starting in the center and working out.
    • This will ensure that the 2x4 is tight in the center of the dip, then driving the screws on the outside should pull the tread flat as the 2x4 is pulled up against it. The clearance holes will allow the screw to move through the tread and pull the 2x4 up snug.
  • Tie the 2x4s into the stringer:
    • If you can get access to the outside edges of the stringer, drive one or two screws through the stringer and into each end of the 2x4.
    • If you cannot get access to the outside of the stringers, then drive a screw up from the bottom of the 2x4 at an angle so that it will bite into the stringer.

This may be a bit of overkill, but it should make for a rock-solid staircase that won't squeak or sag in the foreseeable future, and will, likely be far easier than trying to put in a 3rd stringer in the middle of the stairs that probably should have been there in the first place.

  • Great explanation and makes good sense. May 10, 2022 at 17:24

I agree with the comment about strengthening from underneath if possible- You can use construction glue (caulk tube type) and run lines perpendicular to the new treads. Then set the new boards and try not to walk on them (at least in the middle of the tread) until the glue has set. This would take a day or 2 depending on temp/ humidity/ etc. The glue would bridge any gaps from the existing treads and lock the old and new together nicely.

Just wondering how you will handle the different riser height where the stairs tie into the top and bottom floors ? Having different riser heights (and 5/8" is quite a lot) can be very dangerous.

  • Shouldn't be a full 5/8" - the old carpet/pad must have had some thickness. May 8, 2022 at 20:17
  • that is very true.
    – Kyle
    May 8, 2022 at 20:24
  • Thanks, Kyle. On the top, @manassehkatz-Moving2Codidact already pretty well covered. It will be a minimal transition between materials but the height itself remains within an 1/8. The bottom step to the floor will be 5/8 higher, but was only about 6" before so will actually make it closer to uniform. May 8, 2022 at 20:52

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