Why service life varies
A correctly applied external sealant on a well-prepared joint can last many years, but there is no universal lifespan. The service life depends on the sealant type and system, installation quality, surface preparation, joint design, the amount of movement the joint experiences, UV and weather exposure, building orientation, compatibility with adjacent materials, and how well the joint is maintained.
What shortens the life of external mastic?
Common reasons for early failure include applying the wrong sealant for the joint, poor surface preparation, applying new sealant over old, using no backing rod in deep joints, and ignoring building movement. Harsh weather and strong UV also accelerate ageing.
When should it be replaced?
Replace external mastic when it shows cracks, shrinkage, gaps, loss of adhesion, or when water is getting in. Regular visual inspections, especially after harsh winters, help catch problems early.
Frequently Asked Questions
Can you apply new mastic over old mastic?
No. Old mastic should be fully removed and the surface cleaned before new sealant is applied. Patching over old material will not last.