Replacing a Deck Joist

This is Part 4 of 9 in our Series on Restoring Wood Decks and Building New Wood Decks. Navigate to the first article: Restoring a Wood Deck

In the last article, we looked at replacing just a few boards. But you may find that the joist under the bad board is also bad. A bad joist must be repaired. Putting a new board over a rotting joist is like painting a car with rusted metal.


If only a small section of the joist is bad--say, no more than a quarter of it--the repair is fairly simple using a method called "sistering."deck joists

To sister, cut out the bad section of the joist--make sure you cut back to good wood, leaving none of the rot present. Then bolt and glue a new section of joist to the remaining old joist. Things to keep in mind:

  1. The overlap of the new joist onto the old joist should be at least 3 feet--longer is better. The longer the span of the repair, the more it should overlap the old joist.
  2. If possible, your sister piece should rest on one or more supporting beams or a joist hanger.
  3. Use only pressure-treated wood, even if the old joists were not pressure-treated. The wood should be the same dimension as the old joist.
  4. Soak the cut ends of the old joist with wood preservative. Also, soak new cuts you have made in the new, treated wood.
  5. Use at least four ½-inch to 5/8-inch, hot-dipped, galvanized bolts with large washers to bolt the new joist to the old. They should be staggered every nine inches. While bolts are best, you can nail, using several 10d galvanized common nails (three inches long), staggering the nail pattern.
  6. To drill holes for the bolts, you may need a 90-degree chuck on your drill to get in between the joists. You can clamp the joist and sister together to hold them in place while you drill.

You shouldn't need to support the cut joist while you repair it, unless there is extra weight atop the deck. The remaining boards of the deck should hold it in place. But if you notice any sagging, prop up the joist with a couple of two-by-fours or a four-by-four to position it during repair.

You may be sistering in the middle of the joist, requiring an overlap and bolts at both ends, or you may be able to anchor one end of the sister atop a beam or in a joist hanger.

New Joist

If too much of the joist is rotted, you must replace the whole board. If you can't get under the deck, your only choice is to pull all of the boards nailed to the joist and tear it out. If you can get under the deck, you should be able to cut the nails with a sawzall and drop the errant joist to the ground.

You can then punch the nail stubs out of the deck boards, install a new joist and re-nail the boards into the new joist (preferably using a nail one size larger).

This takes care of individual joist removal and repair. Next we will look at total deck deconstruction.

Next article: Wood Deck Demolition. How to clear out that old deck to make room for the new.>


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