Movement joints are flexible, sealant-filled joints in a tiled pool that let the tiling, background and structure move without cracking tiles or grout. They are typically found at perimeters, changes of plane, over structural joints and at intervals across larger areas. If they are missing or filled with rigid grout, stress builds up and can crack or loosen tiles.
Why tiled pools need room to move
A swimming pool is never completely still. The shell and its tiling expand and contract with temperature, respond to the load of filling and draining, and continue to shrink slightly as concrete and render age. Ground and building movement can add to that.
Tiles and grout are rigid. Without somewhere for movement to go, stress builds in the tiling until something gives, usually a tile, a grout line or the bond beneath.
What a movement joint is
A movement joint is a gap through the tiles and the adhesive bed, down to the background, filled with a flexible sealant over a suitable backing instead of grout. In a pool the sealant system has to be suitable for permanent immersion and for the pool's water treatment.
Some sit over joints that run right through the structure. Others are in the tiling and bedding only, to break up larger areas and relieve stress at corners and edges.
A good joint is sized for the movement expected and the sealant bonds to the two sides only, not the bottom, which is what the backing is for. Stuck on three sides, sealant cannot stretch and soon tears.
Where they are commonly found
Exact positions depend on the pool's design and the relevant guidance. BS 5385-4 covers ceramic tiling in specific conditions, including swimming pools, and the pool designer's details take priority where they exist.
- At the perimeter of the floor, where it meets the walls.
- At internal corners and changes of plane, such as wall to floor and around steps.
- Directly over structural movement joints in the shell.
- At intervals across larger floor and wall areas, where the design, tile and joint width call for them.
- Where tiling meets different materials, fittings or edges.
Signs of missing or failing joints
Problems can come from joints never having been formed, from joints being filled with grout during a previous repair, or from sealant reaching the end of its working life.
- Cracked grout running along corners, perimeters or straight lines across a floor.
- Tiles cracking in a line, or lifting into a ridge.
- Hollow sounding tiles clustered near corners and changes of plane.
- Sealant that is split, hard, shrunken, detached from the tile edge or missing.
- Joints that look like grout where flexible sealant would be expected.
Why grout must not fill movement joints
Filling a movement joint with grout, even a strong epoxy, defeats its purpose. The joint becomes rigid, the movement has nowhere to go, and the stress transfers back into the tiling.
It is a common mistake in pool repairs, usually made to tidy up a scruffy sealant line or during a regrout where nobody marked the joints first. We keep movement and perimeter joints flexible and renew the sealant where it needs it.
Renewing movement joints
When a pool is drained for regrouting or repairs, movement joints are worth assessing. Old sealant that has hardened, split or lost adhesion is cut out, the joint edges are cleaned and prepared, and a suitable sealant system is installed to its manufacturer's instructions, including any primer, backing and cure time before immersion.
Where joints are missing, forming new ones can be part of the scope, sometimes by cutting a line through existing tiling and sometimes by lifting and relaying tiles at a perimeter. That needs thought about positions, depth and how the joint will be finished, and sometimes advice from the pool's designer.
A maintenance item, not a one-off
Sealants in pools age with water exposure, chemistry and cleaning, so movement joints are worth checking at every drain-down. They are often among the first parts of the tiling system to need attention.
A simple record of each joint's condition at every drain-down, with photographs of any splits or gaps, makes it easier to plan renewal before small defects let water and stress into the tiling.
Common questions
Can I just re-seal over the old sealant?
Generally no. New sealant needs clean, sound joint edges to bond to. Old sealant is normally removed and the joint prepared before the new system is applied.
Are movement joints the same as grout joints?
No. Grout joints sit between individual tiles and are rigid. Movement joints are flexible joints at specific positions that let the tiled surface move without damage.
What colour should movement joint sealant be?
Sealants are available in a range of colours and are often matched to the grout so the joints read as part of the tiling. Suitability for immersion and the pool's chemistry comes first, and colour choice follows from what is available in a suitable system.
Will adding movement joints stop tiles cracking?
Properly placed movement joints reduce stress from movement, which helps protect the tiling. They cannot fix cracks caused by voids, impact or a failing background, so the cause of existing cracks still needs assessing.
