Gaps along your walkways/patios/stoops and home including where driveway meets garage should always be caulked with a polyurethane to prevent water penetration. What most people forget is the backer rod and waste their caulk, filling in large area with caulk. What is backer rod? So you filled that wide gap in the exterior trim with the best caulk you could buy, and the next year it had pulled away from one surface, leaving a large gap. Or you tried to fill a wider gap, and the caulk just fell in the hole. What went wrong? No backer rod.
Before filling a large gap with caulk, bridge the wide opening with a stiff foam backer rod. The backer rod is wide enough, so friction holds it just below the gap’s surface. The rod supports the caulk applied in an hourglass shape with a height-to-width ratio of about 1:2.
Why? Caulk needs to expand and contract as surfaces move. The hourglass shape allows the caulk to bond to only two surfaces; the narrower section easily expands and contracts with movement. Caulk should never completely fill a space. It should never be applied to three sides or an unbridgeably wide gap, or it will quickly fail. Caulk can’t expand and contract when it is pulled in three directions or when the cross-section is too thick.
You will find backer rods in larger paint and hardware stores. It is sold in lengths like rope, and it comes in various diameters. Choose a diameter that is wider than the gap to be filled, and force the rod into place with a blunt tool or putty knife.
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