2024-25_NTCA_Reference_Manual_dig_dwnld

Underlayment Cracking

Problem

Cause

Cure

Mixing product with too much water or latex additive.

Excess water or latex additive creates excessive voids in the cured underlay ment matrix that leads to high shrinkage, cracking, soft chalky surface, lower com pressive, tensile, and flexural strength and possibly debonding. Bridging joints and cracks that expand and contract will cause the underlay ment to crack. Underlayments contain aggregates to control shrinkage when applied within proper thickness. If the thickness limita tions are exceeded, the aggregate does not control the additional shrinkage ex perienced by the larger mass of material. Over-working will bring the water and/or latex to the surface of the underlayment creating excess voids in the top layers. When the excess water evaporates, the cement particles try to occupy the voids left behind causing excessive shrinkage and cracking. Exposure to high air movement, heat or sunlight will cause rapid water loss on the surface causing incomplete hydra tion of the underlayment in this area. As the lower levels of the underlayment cure and experience normal shrinkage, the top layer will crack.

Always follow manufacturer recom mended mix proportions. Always care fully measure water for each mix during installation per manufacturer’s instruc tions. Honor or properly repair all moving cracks, expansions and control joints. Provide perimeter joints to accommo date moisture or thermal movement. Check manufacturer instructions to de termine if additional aggregate may be added for thicker applications or if mul tiple pours are allowed. If not, a different type of underlayment may be required.

Bridging expansion, control joints, or slab cracks.

Application greater than specified maxi mum thickness.

Over-working surface.

Do not over work the underlayment any more than is essential for getting a smooth surface.

Exposure to excessive wind or direct sunlight during initial curing stage.

Prevent direct sunlight or wind exposure to freshly applied underlayments. Do not forget flowable/self-leveling underlay ments.

Not using a primer.

Rapid absorption or moisture in the un derlayment.

Follow manufacturer’s recommenda tions for priming.

96

NTCA Reference Manual | 2024 / 2025