Project & Insights
Why is water leaking into our pool mechanical room?
August 18, 2026
By Valeriy
Valeriy — Application Specialist, Joint Seal Waterproofing — concrete pool refurbishments across the GTA and Miami
The most common cause is the expansion joint around the pool deck, not the pool itself. The joint runs the full perimeter where the deck slab meets the pool structure, and when the sealant in it fails, deck water goes straight through and lands in the mechanical room below. The pool can be perfectly sound and still be blamed for it.
This catches property managers out because the water appears a floor down and several feet across from where it entered. It looks like a pool leak. It usually isn’t.
What is the perimeter expansion joint for?
The expansion joint around a pool deck exists to stop the deck slab pressing against the pool structure as both move with temperature. Concrete expands in summer and contracts in winter. Without a joint between them, the deck would push against the pool shell every warm month, and something would eventually give.
Outdoor pools always have one. Indoor pools have one too, and it does the same job on a smaller temperature swing.
The joint is filled with a flexible sealant rather than anything rigid, which is exactly why it is the part that fails first. Flexible materials wear out. The joint is doing work every day of its life.
How does a failed joint put water in the mechanical room?
A failed perimeter joint gives deck water a direct path down beside the pool shell into the space below. On a condominium pool that space is the pool mechanical room, because nearly every condo pool is a suspended pool — there is a crawl area under it holding the plant, even when the pool sits at basement level.
That is what makes it an equipment problem rather than a housekeeping one. Water tracks down and lands on pumps, filters, panels and controls.
At a mid-rise on Church Street in Toronto, the pool mechanical room floor needed more than a thousand square feet of membrane across a second phase of work, on a slab that had cracks in it and old epoxy floor paint that had to come off before anything would bond. At a mid-rise condo on Esther Lorrie Drive in Etobicoke, the penthouse-level pool mechanical room was leaking below one of the windows. Neither of those started as a pool problem in anyone’s mind.
The joint sealant fails for ordinary reasons: the material has aged out, or something damaged it — a scraper, a piece of furniture, deck maintenance. There are many ways to break it. The result is the same, and once it is gone the whole perimeter usually needs redoing rather than a patch, because the rest of it is the same age as the part that failed.
Water showing up somewhere other than where it entered is not unique to pools. It is the same behaviour as a garage leak appearing twenty feet from its source, and it is why chasing the wet spot rarely finds the cause.
How is a pool perimeter joint rebuilt?
The joint is rebuilt by lifting the deck paving, stripping every trace of the old material out, and building the detail back in layers — with an unbonded loop over the top.
- Lift the paving or tile around the pool to expose the joint.
- Clean the joint out completely. Every bit of previously injected material comes out. A new sealant bonded onto old sealant is not a repair.
- Apply adhesive to the joint walls.
- Insert backer rod to set the depth.
- Fill with a flexible rubberised sealant — not up to the top. Sika is what we reach for. Leave room for the detail above.
- Bond a geotextile loop over the joint. The edges are bonded to the concrete with epoxy; the loop itself is left deliberately unbonded and slack.
- Water-test it before closing up.
- Relay the paving.
The loop is the part that gets questioned, so it is worth explaining. It has to work like an accordion. Bond the middle and it becomes a strip of fabric being pulled apart every time the slab moves; leave it slack and unbonded, and the movement is absorbed by geometry instead of by the material’s stretch.
Why not just use a softer caulk instead of the loop?
Because in this climate the loop is what actually survives, and soft caulk alone doesn’t. There is a very soft caulking that in principle removes the need for the loop, and on paper the argument is reasonable.
The field record says otherwise. The concrete separates in a matter of minutes and that material just stays as it is, then expands later. It has been proven, especially in Canada — better not to experiment. Put the soft caulk underneath if you like, but do the loop, and then it’s one hundred percent.
That is a straightforward freeze-thaw argument. Toronto puts a pool deck through a movement cycle that a warm-climate deck never sees, and material behaviour that holds up in Florida does not automatically hold up here.
Why does the joint get water-tested before the paving goes back?
The perimeter joint is water-tested the moment it is built, before anything is laid back over it, because once it’s closed you’ll be crying afterwards. It is the one formal hold point in the whole pool process.
The reason is asymmetry of cost. Testing an open joint costs an hour. Finding the same defect after the paving is back means lifting the paving again — which is most of the labour in the job.
Only when the test passes with no leakage does the deck go back down. A property manager should ask to be present for it, or at minimum ask for it in writing with a date.
What most people get wrong
Diagnosing a mechanical room leak by looking at the pool. The pool gets drained, inspected, sometimes even repaired, and the water keeps coming — because the water was never coming through the shell.
The other one is patching a joint locally. Three feet of new sealant in the middle of forty feet of failed sealant buys a season. The perimeter is one system and it ages as one.
What to do about it
Walk the pool mechanical room with a flashlight and look up, not down. Trace the staining on the underside of the slab and on the walls back toward the perimeter. If the wet path leads to the edge of the pool rather than the middle of it, the joint is the first suspect.
Then go up to the deck and look at the joint line itself. If the sealant is cracked, shrunk back from the concrete, missing in sections, or overlaid with something that has been smeared in as a repair, it has failed.
Book an inspection and we will find where the water is entering before anyone talks about draining the pool.
More on swimming pool repair and waterproofing, expansion joint repair and utility and mechanical room waterproofing, or read whether your pool needs draining at all.
See the work. Related projects: Lasting Mechanical Room Floor Coating · Lasting Floor Coating Solutions for Utility Rooms. More in our projects.
Common questions
Frequently asked questions
Why does the leak appear in a different place than where the water gets in?
Water entering a concrete structure travels along the path of least resistance before it appears — down the inside face of a wall, along the underside of a slab, or through the layers of the assembly — and it emerges at the first place it can. The visible wet spot marks where the water left the concrete, not where it entered. Chasing the stain rarely finds the source.
Is a leaking pool expansion joint urgent?
A perimeter joint leaking into a pool mechanical room should be dealt with promptly, because the water lands on pumps, filters and electrical equipment rather than on a floor that can dry. The joint itself is not a structural risk in the short term. What it is putting water onto is the expensive part of the room.
Can we repair just the section of the joint that is leaking?
Rarely worth it. The whole perimeter joint is the same age and has taken the same movement, so a patched section usually just moves the failure a few feet along within a season or two. When the joint is being opened up anyway and the paving is lifted, the incremental cost of doing the full perimeter is small against the labour already spent.
Does the perimeter joint work happen at the same time as the pool repair?
Yes — on a full pool refurbishment the perimeter expansion joint is a separate concurrent work stream with its own two-person crew, running alongside the shell work rather than after it. The joint sits on the deck outside the pool shell, and the work is done from the deck side by lifting the paving.
How can a property manager tell the joint was rebuilt properly?
Ask for the water test before the paving goes back, and be there for it. Also ask to see the joint open, after cleaning and before filling — old sealant left in the joint is the most common shortcut, and it is invisible once the new material is in. Photographs of the clean open joint and of the test are the record worth keeping.
Should an engineer be involved in a pool deck joint repair?
A perimeter joint rebuild on an existing pool deck is normally contractor work and does not need an engineer. It changes if the deck slab itself is cracked or moving beyond what a joint should absorb, or if the leak has been running long enough to corrode reinforcing steel in the slab below — both of which are findings from the mechanical room, not the deck.
