A pool deck pulling away from the coping is doing damage you cannot see.

Building a pool means digging a very large hole, putting a rigid structure in it, and pushing the soil back in around the outside. That returned soil is the backfill the deck is then poured over, and it is the least consolidated ground on the property by a wide margin. It goes on consolidating for years, and unlike backfill against a house it is kept permanently damp: splash out, deck washing, rain sheeting off the surface and irrigation around the surround all arrive in the same strip.

The pool itself makes the settlement visible rather than gradual. A shell is a large rigid object that does not settle at the same rate as loose backfill beside it, so when the soil goes down, the deck goes down and the coping does not. A gap at the coping line is not a cosmetic joint failing, it is the difference between two things moving at different speeds, and it grows because water uses it.

The complaint people call about is the step they feel at the coping or the panel that is now lower than its neighbor. The reason to act is underneath. An open joint at the coping line collects surface water and delivers it down the outside face of the shell, straight into the backfill that was already the weakest soil there. That accelerates the erosion, which drops the deck further, which opens the joint wider. It is a loop with a throttle, and the throttle is how often the deck gets wet.

That is why a pool deck repair that stops at the elevations is only half a repair. Restoring the deck to the coping line is the part you can see. Filling the void that let it drop and sealing the joint so the water stops going down there is the part that decides whether you do this again. The same loop is behind most of the offsets and lips people trip on around a surround, in the one place on a property where everyone is barefoot and wet.

The constraint that shapes a pool deck lift is what the injection is allowed to push against. Foam expands along the path of least resistance, so the plan has to know where the deck ends, where the shell begins, and where the bond beam sits, before a hole is drilled. That is established by measurement during the evaluation rather than assumed, because the boundary is not always where the surface finish suggests.

With that in hand the work is ordinary. Holes about five eighths of an inch across go through the settled sections, polyurethane goes in as small watched shots, the void fills, the loose backfill around it densifies, and the deck comes up in fine increments with readings taken between shots. The material adds almost no weight to soil that already failed under the weight it had, which matters more here than almost anywhere else, because the soil beside a pool is the soil holding the pool.

Once the elevations are right the joint at the coping has to be closed, and how it is closed matters. A pool and its deck are two structures on different supports, and they will keep moving relative to each other for as long as they exist. A rigid fill in that joint cannot accommodate the movement, so it cracks, and a cracked rigid fill is a joint that leaks with extra steps. A flexible sealant keeps water out while allowing the two to move, which is what the joint was designed to do in the first place.

The same thinking applies to the voids that are not under a settled section. A hollow sounding area that has not dropped yet is the cheapest version of this problem and the one worth fixing before it is visible, which is the whole argument for filling a void early. Around a pool, where water is constant, the gap between a void forming and a section dropping is measured in seasons rather than years.

A pool deck usually runs into other flatwork: a patio on the house side, a walkway around to the gate, steps down into a lower yard. Because they share drainage and often share backfill, they usually share the problem too, and the readings almost always show movement carrying across the joints. Where that is the case, planning the patio lift into the same visit is cheaper and produces a surround that finishes flush instead of one corrected area beside three that were not.

Where a section is level but tired, or a single small lip survives after the lift, grinding the high edge is sometimes the proportionate finish rather than raising everything around it. The point of holding both tools is that the measurements get to choose. Everything we recommend on concrete leveling for a pool surround comes out of the elevation map and the water reading, and the written findings are yours whether or not any of it happens with us.