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How to Caulk your pool's expansion joint
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Salt Chlorine System Maintenance
by Rob Cox, June 2, 2012

How to Caulk your pool's expansion joint

pool caulking close upNow that the pool is open, and the water if finally clear, it's time to take a look at some maintenance tasks for 2012.

I used to say think that caulking the expansion joint was something that only snowbelt pool owners had to worry about, but with freezing temps occuring more regularly in the south these days...it should be on the mind of concrete pool owners everywhere.

Who Needs to Caulk?

If you have an inground gunite swimming pool with coping stones, whether they be brick, flagstone or pre-cast stones, you will have an expansion joint behind the coping stones, to allow for expansion of the pool deck when the weather gets hot. If you have a cantilever pool deck, where the pool deck extends over the pool wall, monolithically, there is no need to caulk - and no need to read further. Likewise, vinyl liner pools usually do not have an expansion joint, although some do.

Why Should I Caulk the Expansion Joint?

As mentioned above, the expansion joint around your pool allows for the pool deck (and the pool itself) to expand, when the weather gets hot. A concrete slab will grow laterally, as the weather gets warmer. Almost imperceptively, but it does expand 1/4 inch or so. The same happens to your pool shell, it grows a little larger when summer heats up, and contracts or shrinks when the weather gets colder. 

The expansion joint serves to allow some room for the pool deck and the pool wall to beam damage on an inground poolexpand, without knocking into each other. Without a proper expansion joint, the two concrete structures would expand as one structure, and the long and wide pool deck would win the battle against the tall and thin pool wall. Over time, the pressure from the pool deck will break the top of the pool wall, resulting in a fairly major repair to what's called beam damage.

When I say "proper" expansion joint, I refer to a gap between the pool wall and pool deck that goes clear to the earth beneath the pool deck. Without caulking in the gap, it soon fills with sand, pebbles, seeds, dirt, etc. When the joint fills with such non-compressible stuff, this removes the gap, and the two structures will move as one. Bad news for the pool wall.

Water can also cause problems during the winter. When the expansion joint is not proper, but is partially filled with debris, rain and snow melt will stand in the gap, and freeze. We all know what happens when water freezes, it expands. Over time, expanding water will cause damage to the pool wall, as it places pressure against it. Once again, the tall and thin pool wall will lose the battle against the short and wide pool deck. Beam damage is again the eventual result.

So, the reason that pool caulk is necessary (besides it looking nice) is that it prevents water and debris from getting into the joint and causing some very common and very expensive problems for your pool wall.

How to Caulk your pool

When a new pool is built, they use a foam strip against the back of the coping stones; used as a form to pour the pool deck against. This can be pushed down, cut down, or burned down with a torch. It works poorly as a base to set your pool caulking on top of, it's usually not tight enough against both sides to prevent the pool caulk from running down in the joint.

Placing a round foam rod known as backer rod into the expansion joint is the preferred method to prep your pool for caulk. Don't use sand. Sand is not very compressible, and remember that we want to keep debris such as sand out of the expansion joint. backer rod is pushed into the joint before caulking your pool

Backer rod is sold in different widths, up to 1" in diameter. Buy the size that is slightly larger than the widest part of your joint. If your expansion joint width varies by a significant amount, you can buy different lengths of different widths. You can also use a razor knife to carefully trim a larger backer rod to size.

Place the backer rod into the expansion joint to a depth of 1/4" below the deck or coping, whichever side is lower (usually the deck may have settled slightly). Don't push it in too deep or you will use a lot of caulking bringing it up to level. And if it's not level, you will get standing water, which will grow algae, which looks awful. If it gets too deep, use a small screwdriver or hooked pin to pull it up. Keeping the rod tight as you insert it into the joint will help prevent it becoming wavy or undulating up and down in the joint.

pool caulkingWhen the backer rod is in place, you are ready to begin filling the expansion joint with self-leveling pool caulk. It's called self-leveling because it is slightly runny and will level itself - as opposed to gun-grade caulk which has a putty consistency and must be placed in the joint with a small trowel or putty knife.

Buy the large tubes of caulk if possible, this prevents having to change the tubes too often. Just like bathroom caulk, you snip the tip and drop the caulk tube into the caulking gun. Be easy on the trigger, it comes out fast and can get messy real quick.

I like to keep a large piece of cardboard on the deck with me, which I move along with the caulking, as I proceed down the edge of the pool. Whenever you need to stop, place the caulking gun onto the cardboard quickly. Continue in this manner around the pool, moving backwards as you squirt the pool caulk into the expansion joint.

Here's some other tips to a clean caulking job:

  1. Wear old clothes and shoes, this can get messy.
  2. Tie up the dog, cat and the kids for 24 hours.
  3. Keep several moist rags around the pool.
  4. Blow off the deck first to prevent blowing leaves.
  5. Do the job when the weather is cool, the caulk is less runny.
  6. Check the forecast for rain before you begin.


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