What does low modulus mean, and which sealant do I need for a joint that moves?

Most sealant failures in a moving joint are one of two things: the sealant could not stretch far enough, or it could stretch but pulled the surface off the substrate while doing it. Those are different problems with different numbers attached, and both are printed on the cartridge in small type that almost nobody reads.

Step by step

1. Work out how much the joint actually moves.A joint between two materials that expand at different rates with temperature — timber and masonry, a large uPVC frame in a brick opening, metal capping on render — moves several millimetres across a year. A joint within one material barely moves. A bath moves every time it is filled.
2. Understand movement capability.A percentage of the original joint width that the sealant can accommodate. Twenty-five per cent on a ten millimetre joint means about two and a half millimetres of movement. If the joint moves more than that, the bead will tear however well it was applied.
3. Understand modulus.How much force the sealant exerts on the joint faces as it stretches. Low modulus is soft and gentle; high modulus is stiff and pulls hard. Both can have the same movement capability, and they behave completely differently on a weak substrate.
4. Use low modulus on weak or friable substrates.Render, old lime mortar, soft brick, painted timber, plaster. A high modulus sealant on any of those will not fail itself — it will pull the surface off the substrate and take a layer of render or paint with it, which is the failure you see as a bead lifting with a crust attached.
5. Use high modulus where the substrate is strong and the joint is small.Glass, metal, dense concrete, glazed tile, and joints that move little. A stiffer sealant is tougher, more abrasion resistant and holds its shape better.
6. Match the joint width to the movement, not to the gap that happens to be there.A wider joint accommodates more absolute movement for the same percentage. A joint that moves a lot needs to be wide enough for the percentage to cover it, which sometimes means deliberately opening the gap up rather than sealing it as found.
7. Read the standard reference on the cartridge if there is one.Construction sealants are classified under a European standard by type, movement class and modulus, and the class appears on the cartridge of anything sold for building work. It is the most reliable comparison between products from different makers.
8. Get the geometry right or the numbers are meaningless.A bead bonded to three faces cannot stretch at all whatever its rating. Two-sided adhesion with a backer rod and a sensible width-to-depth ratio is what lets the stated movement capability be real.

Tips

• Sanitary silicones are usually low modulus for exactly this reason: a bath or a shower tray deflects when loaded and the sealant has to move with it without pulling the tiles.
• Acrylic decorator's caulk has very low movement capability. It is fine on an internal skirting line and it is the wrong product for a joint that genuinely works.
• If a bead has repeatedly split down the middle, the answer is usually joint geometry rather than a stiffer product. Check for three-sided adhesion first.
• If a bead has repeatedly peeled off with a layer of substrate attached, the answer is a lower modulus product, or repairing and priming the substrate.
• Painting a movement joint stiffens it, because the paint film cracks before the sealant does. On a joint with real movement, leave it unpainted or accept the crack line.

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