Replacing Support Posts

Replacing A Rotten Support Post

Exterior decks on older homes are often supported by 4×4 posts that are “untreated” lumber and become rotten over time, requiring replacement. A recent client of mine needed this kind of work done and contacted us to help with the project.

This type of project is pretty straightforward for an able-bodied homeowner, especially if he/she is able to obtain help from a friend or neighbor.

The first step is to determine specifically what materials are needed for the repair. This includes the length of the new pressure-treated 4×4 post, and any other lumber required, such as trim pieces, shims, etc. In many instances the existing support post will be sitting on a concrete pier or pad, but not actually tied into the concrete. In such cases, you may find that the top end of the post has simply been toe-nailed into the adjacent joists (usually 2″ x 10″ or similar). When replacing a support post of this type, I recommend using a metal “box” bracket to secure the post to the concrete pier on the bottom, and “L” brackets to secure the post to joists at the top. Naturally, you’ll need masonry anchors to go into the concrete and fasten to the metal bracket at the bottom, and deck screws to secure the L brackets at the top. So, you’ll need 1/2-inch anchor bolts sufficiently long to drive 2.5-3 inches into the concrete and still have at least an inch of threads above the bracket. Be sure to purchase matching washers, and nuts. You’ll also need 2-4 lag bolts the right size to go through side holes in the box bracket, as well as L-brackets and and some deck screws.

Beyond the above hardware and lumber components, you’ll need primer and topcoat paint for the new post. It’s much easier to prime and paint support posts (especially very tall ones) while they are horizontal, rather than having to stand on a ladder to paint them once they’ve been installed. It’s also wise to purchase concrete sealer to seal the pad after you have drilled it to receive the anchor bolts.

In addition to the above materials, you’ll need to assemble all the necessary tools:

  1. Pole jack — critical to providing support for the deck while removing the existing rotted support post and installing the new one. (This is a tool you can rent at many tool rental shops; there’s no need to purchase one unless you expect to be doing this sort of repair frequently.)
  2. Hammer drill and 1/2-inch masonry bit — to drill holes for the masonry anchor bolts. A regular electric drill will work, but a hammer drill makes the job a lot easier…it makes the masonry bit cut through concrete like butter.
  3. Plastic mallet — to help drive metal “box” bracket onto post bottom, and to set the anchor bolts into the concrete pad.
  4. Hammer — to help remove the old post and toe-nailed fasteners
  5. Drill / screw gun — to pre-drill holes and drive new deck screws at top; drive lag bolts
  6. Saw (hand saw or power circular saw) — to cut new post to final length before installation
  7. Ratchet wrench and socket set (3/8- oir 1/2inch drive)
  8. Crow or pry bar — to help remove trim pieces, and to break the old post free of joists
  9. Paint brush, roller, roller pan, etc. — to apply primer and paint to new post, and to apply sealant to the concrete pad
  10. Plumb bob — for ensuring the new post is positioned vertically
  11. Level — to check level of top of deck when setting height with new post
  12. Ladder.

Doing the Job:

First, position the pole jack adjacent to the existing post at both top and bottom, ensuring you have a good, stable point on which the jack sits — both on the concrete and on the bottom surface of the deck structure.

Before you jack up the deck, check existing level to see how close everything is to being level. A longer level is best, but even a short one will give you an idea. If you’re using a 2-foot level and the deck side is 10 feet long, you’re multiplier factor is 5. If you’re off by what appears to be 1/16-inch of level (need to raise one end of the level this much to center the bubble when measuring at the end of the deck), then multiply by the factor (5 in the example given) and that’s how much the deck should be raised (or lowered) to hit dead level when you install the new post. If you measure the length of the existing post and add/subtract this amount, you’ll be very close to the length dimension you need to cut the new post. I say “close” because you’ll natgurally need to take the thickness of the box bracket into account in the final calculation.

Next, begin cranking the jack up until it begins to take the weight of the deck off of the old support post. Check this periodically as you apply increasing pressure with the pole jack. I also recommend using deck screws to secure the pole jack in place so it can’t scoot or slip sideways under pressure.

Once the pole jack is supporting essentially all of the weight of the deck, push the old post out of position (the bottom may be simply sitting on the concrete pad), and then wrest the top end free of the joists above, using a hammer, small sledge, and/or pry bar.

Next, use the plumb bob to ensure you locate the proper center point for the new post on the concrete pad, and mark it, using the metal box bracket. I use a small cup hook to hang the bob line from the center point of the deck mounting point, and adjust the length until the bob tip just brushes the concrete pad. When the bob becomes stationary (you may need to help it, especially if it’s a windy day), mark the center point, and then extend that mark so you have center marks for all mounting-hole positions (from “ears” of box bracket) on the concrete pad. Now you’re ready to drill the anchor holes.

Drill the anchor holes with a masonry bit, taking care to ensure the bit doesn’t travel. It only takes a small amount of bit travel to shift a hole enough that it will make it difficult to get all the anchor bolts to align with holes in the bracket. Most anchor bolts are designed to be sunk at least two inches in the concrete, and this depth will magnify any error caused by drill bit drift on the surface. One way to minimize drift is to clamp (or have someone secure by standing on) the bracket against the concrete pad and drill the anchor holes with the bit threaded through the mounting bracket holes.

Drive the anchor bolts into the holes taking care not to bugger the top ends while driving. I use a plastic or hard rubber mallet for this purpose, but you can also use a regular hammer or small sledge together with a scrap piece of wood, such as a 9-inch piece of 1″ x 2″ as a buffer. Hold one end of the wood scrap and position the wood on top of the anchor bolt. Then strike the wood directly over the bolt. Move the wood around as you continue striking, so that the top of the bolt doesn’t fully penetrate the wood. When you’re finished, the wood scrap will be destroyed, but you’ll have protected the bolt ends. Prep the concrete with sealant, to ensure against scaling or spalling. This is especially important if you live in climate with serious freeze/thaw cycles.

Prep the post by priming and painting. You may want to consider a special waterproof coating for the bottom 4-5 inches, to ensure against rot, especially if you live in a climate where snow may accumulate around the bottom of the post.

Slide the bracket into the bottom end of the new post, ensuring the post bottoms out in the bracket. Secure the bracket to the post with lag bolts through side holes. Most brackets are pre-drilled with 8 or more mounting holes for this purpose; however, you should only need 2-4 lag bolts. As you pre-drill the wood post for the bolts, be sure to select holes that are offset from each other so the bolts won’t interfere with each other when you drive them in.

Set the bottom end of the post in place, so that the anchor bolts fit through the holes in the box bracket, while leaning the top end of the post off at an angle (usually inside, under the deck, to clear outside overhanging trim pieces), and then swing the top of the post up and into position.

Check to ensure the post is plumb (both directions) and then slowly lower the deck weight onto the new post by lowering the pole jack. As weight transfers to the new post, you should be able to confirm if the length is correct by checking level on the deck railing above. If you’re a bit high, jack up again, remove the post and trim it slightly, as appropriate. If you’re low, you’ll need to add a shim of appropriate thickness. Once you have the correct length and are level on top when the weight is fully on the new post, secure the top end with L brackets and deck screws, then secure the bottom box bracket with washers and nuts.

All that remains to be done is installation of any exterior trim pieces that may have been removed. All done!