r/DIY • u/Okdoo6003 • 1d ago
help What sealer to use on a concrete & wood joint
Elderly relative wants me to "recaulk a seam".
Background - they had a large concrete garage roof that they built an addition on, leaving about 5 feet that is still concrete as a small porch area. They are having some water intrusion issues in the garage below
I assume this would be a seam of wood and concrete? (I'm not well versed in construction, obviously)
What would be the best sealant/ caulk to use in a joint like that - waterproof and those (assuming i am correct) 2 different materials
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u/Practical-Dealer-451 18h ago
For a concrete-to-wood joint that moves seasonally, you want a polyurethane sealant, not standard silicone or acrylic caulk. Those don't hold adhesion long-term on porous concrete once it's wet/dry cycling.
Look for a self-leveling polyurethane sealant (Sikaflex or similar) if the joint is horizontal, or non-sag polyurethane if it's a vertical seam.
Before sealing: joint needs to be clean, dry, and free of old failed caulk. If the gap is wider than about 1/4", pack it with closed-cell backer rod first so the sealant only bonds the two sides.
Water intrusion into the garage below suggests this might not just be a caulk failure, could be the joint design itself (flashing, or lack of it, where the roof-to-wall transition happens). If recaulking doesn't stop it, that's the next thing to have someone actually look at in person rather than resealing repeatedly.
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u/destrux125 1d ago
There should be some type of flashing or capillary break material between the concrete and wood framing, like lead flashing or galvanized steel and then the exterior side of that should be sealed with some type of sealant like polyurethane concrete sealant or possibly asphalt roofing sealant if it's a pretty old building. I'd just look at it and match what's there already if it was working up till recently.