Even the best caulking will fail if you don’t understand what happens to the joint after the caulking cures

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Caulking formulations have never been better, so why does real-world caulking performance often fail to live up to the claims on the tube? Exterior caulking, especially, has a reputation for cracking, peeling loose and generally turning into useless blobs of hardened goop long before it should. 

It doesn’t have to be this way. The problem usually isn’t always caulking itself, so some are certainly better than others. But even the best caulking will fail if you don’t understand what happens to the joint after the caulking cures. 

Exterior gaps are rarely stable. Wood, brick, siding, metal and other materials expand and contract as temperature and moisture conditions change. Caulking has to stretch and compress along with this movement, and this is where things get interesting. 

Strangely enough, a narrow gap can actually be harder to seal permanently than a somewhat wider one. 

Suppose you have a 3/32-inch gap between a wooden door frame and surrounding brick. If movement causes that gap to grow to 3/16 inch, the caulking has been asked to stretch almost 100 per cent. Even a highly flexible product probably won’t survive. 

But suppose the original joint is 3/8 inch wide. If the surrounding materials move the same absolute amount, that movement represents only about 25 per cent of the width of the caulking bead. That’s much easier for caulking to handle. 

This doesn’t mean you should fill enormous cracks with caulking. Wide, deep joints waste product and can actually reduce flexibility. That’s where backer rod comes in. 

Backer rod is flexible foam, usually round, that’s pushed into a deep gap before caulking is applied. Besides saving caulking, it controls the proportions of the finished bead. Ideally, caulking depth should be roughly half the width of the joint, with a minimum depth of about 1/4 inch and a maximum of about 1/2 inch. A bead that’s unnecessarily deep has more trouble stretching without pulling away from the sides. 

There’s another important principle that almost nobody talks about: avoid a three-way bond. 

Caulking should stick firmly to the two sides of a joint, but not the bottom. If it bonds to all three surfaces, the cured caulking can’t thin out naturally as the joint stretches. Eventually something gives, often the bond between the caulking and one side of the gap. 

Backer rod prevents this in deep joints. For shallower ones, where the bottom of the gap is a hard surface and not far enough down to fit backer rod, bond-breaker tape laid along the bottom does the same job. Even electrical tape can serve as a reasonable substitute in some situations. 

Product choice matters too. For demanding exterior work, my two main choices are polyurethane and silicone. I favour polyurethane because it remains highly flexible, adheres extremely well and can be painted. Fully exposed exterior joints I did with polyurethane caulking in 1996 are still in great shape today. Silicone offers excellent ultraviolet resistance and remains flexible over a broad temperature range, though it doesn’t stick as well as polyurethane. I never use cheaper caulkings because they simply don’t last as well.  

Finally, neatness matters. Apply masking tape along both sides of the joint, inject the caulking, then tool the bead within about five minutes using a plastic spoon or short piece of dowel dipped in a mixture of 500 ml of water and a tablespoon of dishwashing liquid as a lubricant on your tool. Pull off the tape while everything is fresh. Tooling improves appearance and also forces the caulking into close contact with surrounding surfaces. 

Yes, all this takes more effort than simply squeezing a tube into a crack. But there’s only one thing more annoying than caulking carefully: having to do the same job over and over again. 

Steve Maxwell always favours doing home repairs only once whenever possible. Visit him online at BaileyLineRoad.com and joint thousands of people who get his free email newsletter each Saturday.