The ratio between a crack's width and depth affect the performance of caulk. Properly preparing the work surface is critical if you want your 30-year caulk to reach its advertised lifespan. While the minimum and maximum widths for your caulking vary according to manufacturer specifications, the relationship between width and depth remain fairly constant: Your crack should be twice as wide as it is deep. For example, caulk performs best if a 1/2-inch-wide crack is 1/4-inch deep. Standard home improvement stores carry several task-specific and general purpose backer materials that will help you fill your crack before your apply caulk.
Arguably the most common crack filler material, backer rod resembles a soft, flexible foam rope. Backer rod is available in long rolls at standard home improvement stores, usually near caulking and weatherstripping products. Because of its flexibility, backer rod easily fits into irregularly shaped cracks. Additionally, the soft foam yields under pressure, allowing you to squeeze it into small cracks. You can cut the soft foam to length with standard hand tools, such as utility knives and household scissors.
Also called "spray foam," expanding foam is available in aerosol cans, similar to spray paint or canned drywall texture. The foam sprays from the tube as a viscous solid, expands to fill all types of gaps, and dries to relatively hard, spongy finish. Wet spray foam typically expands to at least twice its size before drying. Cured spray foam is suitable for tooling, such as cutting or sanding.
Common rigid filler materials include wood and rigid-foam insulation. Depending on the size of the crack and your project's circumstances, wood furring strips or shims are suitable for filling gaps. However, the wood should fit snugly into the crack. If the wood is loose, it might shift, expand or contract and break the caulking's seal. Rigid foam insulation is easy to cut and squeeze into larger cracks. Like wood filler strips, rigid foam should snugly fit into cracks for the best results.
If the crack is exceptionally wide, consider patching or filling it with a repair compound. Caulking manufacturers typically list maximum crack widths on product labeling. While precise dimensions vary according to manufacturer specifications, most caulking applies to cracks 2-inches wide or narrower. For wider cracks, fill the voids with a like material before applying caulking to the seams. For example, use a masonry repair compound for cracks in block or concrete surfaces and a plaster or spackling paste for repairs in drywall. If the joints between the repair compound and work surface contract, they'll still be small enough for a bead of caulk.
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