Greg posted a very useful piece on calking.
snip
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Caulk seals and spans. Providing the surface is carefully cleaned1, and an appropriate chemistry identified, sealing is often trouble-free. But "spanning" is another issue entirely if the spanned gap moves with time and temperature.
How far can you stretch caulking before failing? Well, a bead of fresh silicone caulk might stretch (and recover) by 300%, and latex by nearly 100%. That's in the lab. In the real world, performance is actually closer to 50% and 20%. Why? Well, caulking ages and dries out and may shrink. It stiffens in the cold and softens in the heat. It's exposed to cleaning solutions, sunlight, water and scratches. When stretched, those scratches explode into cracks, in the same way the nick on the top of a ketchup packet concentrates forces so you can easily rip open the foil.
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