Roofing & Masonry Department in Yonkers, NY

Cement is the binder, the grey powder that reacts with water. Concrete is cement plus sand plus stone aggregate, and it is a structural material — slabs, footings, posts. Mortar is cement plus sand with no large aggregate and additives for workability, and its job is to bond masonry units together, so it is deliberately weaker and more flexible than the brick or block it holds. Using concrete where mortar belongs gives you a joint harder than the brick, which then cracks the brick instead of the joint.

Some products are made for exactly that. Wet-patch roof cements are formulated to adhere to damp surfaces and to stay workable in cold weather, which is the whole reason they exist. They are a repair, though, not a rebuild: they buy time on a specific leak. Coatings and adhesives are different — most have a minimum application temperature and need a dry, clean substrate to cure properly, and going on outside that window is the usual reason a coating fails early. The label states the conditions.

Work in volume. Length times width times depth in feet gives cubic feet; divide by 27 for cubic yards. An 80-pound bag of ready-mix yields about two-thirds of a cubic foot, so a small slab adds up quickly. Setting a fence post is the common case: a hole roughly three times the post width and a third of the post length deep, with the post set on gravel for drainage. Mix a little more than the arithmetic says, since nothing ever comes out exactly level.