Chimney waterproofing in Sunbeam by local experts

Siloxanes and clay brick

Siloxane molecules are larger than silanes, and they link chemically to the silica and alumina in fired brick and in mortar. In BIA's assessment they give good penetration and repellency and have worked well on certain brick walls, which is why the note suggests a siloxane, or a siloxane and silane blend, for exterior brick. Some siloxanes tolerate a slightly damp surface. The one we apply on chimneys is a modified siloxane, and its maker describes it reacting with the minerals in the masonry, in the presence of ultraviolet light and moisture, to form a water-repellent zone just below the face. Nearby glass needs protecting for the same chemical reason, since glass is mostly silica.

Waiting after the joints are repointed

New mortar in a joint is still hardening and still giving off water for days after it is placed. Coating it too soon can interfere with that cure and lock the moisture in. BIA's pre-coating checklist sets a floor of 72 hours for freshly repointed mortar and repaired sealant joints, and the maker of the repellent we use sets the same interval after tuckpointing or other repairs.

On a chimney that has just been repointed, treatment is therefore booked as a separate visit. Writing the cure period on the estimate keeps anyone from spraying on the same afternoon the trowels are put away.

Visibly dry brick and a rain-free window

For a penetrating product to soak in, the pores need to have room for it. The maker's directions call for clean, structurally sound masonry that is visibly dry, and they rule out wet or saturated surfaces. They also ask for a six-hour window with no rain expected, unless the treated wall can be sheeted with plastic. After a run of wet days, brick can look dry at the face while still holding water further in, so on the day we check the brick itself, pressing and looking at freshly scraped mortar, rather than relying on the forecast and the look of the surface alone.