Outdoor Adhesives

Think "outdoor adhesive" and the word "waterproof" may come to mind. But joints made with these adhesives need to survive more than mere rain. Climate extremes in parts of the U.S. go from subzero to 100°F-plus—challenging for any adhesive.

To discover which adhesives stand up to Mother Nature, we tested four types of products—a type-3 waterproof glue, a type-2 water-resistant glue, epoxy, and polyurethane—on dozens of half-lap and mortise-and-tenon joints in cedar. Why these joints? The half-lap exposes glue lines to the elements, as shown below, while mortise-and-tenons prove vulnerable to checking on the end grain closest to the mortise. And outdoor projects frequently call for one or the other of these joints.

Half the test group bore the brunt of nine months anchored to the roof of our Midwest offices, enduring snow and freezing rain, shirt-soaking humidity, and 39" of rainfall—9" above normal. To provide a baseline for comparison, we created another set of identical joints that suffered only a little sawdust inside our workshop.

Then came crunch time. At the Iowa State University Structural Materials Testing Facility, we fed both groups to a machine capable of applying and measuring thousands of pounds of force. We applied downward force against the short leg of each joint until the glue or the wood failed. If the break exposed more than half the joint, as measured with a 100-grid scale, we considered that a glue failure. Less than half signaled a wood failure.

So which adhesives worked best? We thought you'd never ask.

How type-3 glue works: Water in yellow glues carries microscopic polymer strands into the cells of the wood parts being joined. As the water evaporates, the strands harden to give the glue strength to hold parts together. In type-3 glues, a chemical reaction between the strands also locks them together to withstand severe moisture.

Test results: Outdoor half-laps and mortise-and-tenon joints tested nearly the same as both groups of indoor samples [photo below] as measured in both wood transfer and break strength.

■ Paper-thin layers of wood transferred between the outdoor half-lap parts, but that doesn't signal lack of strength. The force required to break the outdoor type-3 half-laps was greater than that to break the outdoor epoxy joints, suggesting they'll perform about the same in real-world projects. Cracks on the ends of the half-laps left the joint unaffected.

■ On the outdoor mortise-and-tenon samples, two tenons broke completely off before the glue joint failed [photo below]. That was despite moisture reaching the mortises through end checking.

■ Conclusion: Despite peeling off thin layers of wood in places where the half-laps touched [photo below], it also tore off large chunks where the glue bond outlasted the strength of the weathered wood. Type-3 adhesive rivals the epoxy's strength in both types of outdoor joints without the mixing mess.

How type-2 glue works: Polymer strands penetrate the wood cells and intertwine like type-3 glues, but these strands don't interact with each other to become thoroughly waterproof. The glue finishes bonding when the water in it evaporates as the polyvinyl acetate formula penetrates the wood cells.

Test results: As with the type-3 glue, outdoor half-laps made using type-2 glue nearly equaled the strength of the joints left indoors for nine months. On outdoor mortise-and-tenon joints, wood transfer dropped only slightly from that of the indoor joints.

■ Wood transferred in thin layers on both indoor and outdoor half-laps, but that signaled no lack of strength. Type-2 joints required as much or more force to break than joints using epoxy, which pulled off larger chunks. On some type-2 half-laps, most of the half-lap on one part remained bonded to the mating piece [photo below]. And type-2 glue works without mixing or messy cleanup.

■ Outdoor mortise-and-tenon joints using type-2 glue retained more of their original strength than any other type of adhesive tested. Despite checking on the mortise piece ends, glue around the tenons still pulled off considerable material from the mortise sidewalls [photo below]. The glue bonds around the mortise shoulders also held up, despite glue-line exposure to the weather.

■ Conclusion: Although type-2 glue isn't marketed as waterproof, outdoor half-lap joints made with it proved as strong as joints bonded with type-3. If the phrase "water-resistant" still leaves you uncertain about how it will weather, switch to a type-3 adhesive—which often has a longer working time—or thoroughly seal the project with multiple layers of primer and paint.

How epoxy works: Epoxy resin and a curing agent, or "hardener," react chemically when mixed. That reaction forms a bond between the epoxy and the surface so strong that it displaces air or moisture, the reason some epoxy can cure underwater. The shape of the epoxy molecules also helps them form strong bonds with each other. For this test, we used an extended-time epoxy suitable for assembling large projects.

Test results: Epoxy joints left indoors tested the strongest of all four adhesives. On two of the three indoor mortise-and-tenons, the tenons snapped before the glue bond broke. But after nine months outside, joint strength tested about equal to type-2 and -3 glues. Of the three outdoor mortise-and-tenon samples, two tenons broke [photo below].

■ End-grain checks on the outdoor mortise-and-tenons allowed water to reach the tenons, but apparently didn't affect the adhesive bond [photo below].

■ The edges of one half-lap piece remained bonded to the end grain of the mating piece on both the indoor and outdoor samples [photo below], suggesting epoxy makes a good choice for outdoor half-lap joints partly because it creates water-repelling joint lines.

■ Conclusion: Epoxy proved stronger than the wood in all but one outdoor half-lap. Use it for outdoor projects with less-than-perfect joints or for joining unlike materials.

How polyurethane works: Polyurethane adhesive cures in reaction with moisture, but water doesn't carry it into the wood cells as with type-2 and type-3 glues. Polyurethane sticks to a wide variety of surfaces, including wood and metal. Carbon dioxide released in the curing process causes foaming.

Test results: Indoor half-lap samples tested far stronger than indoor mortise-and-tenon samples, but both dropped in strength after exposure. All three outdoor mortise-and-tenon joints suffered glue failures.

■ Although the tenon on one indoor mortise-and-tenon sample tore away chunks of the mortised piece, all three tenons from the weathered samples experienced glue failure [photo below].

■ On indoor half-lap samples, polyurethane outlasted the wood where the faces met [photo below]. On the outdoor half-lap samples, half-lap shoulders also tore out mating wood on the opposite pieces, confirming test results suggesting they're stronger than mortise-and-tenon joints.

■ Among the outdoor joints, mortise-and-tenon joints broke easier than the half-laps, which are simpler to make.

■ Conclusion: Polyurethane does an excellent job of joining unlike materials, such as a metal cap on a wooden post, but you can find weather-hardy, less messy wood-to-wood joint choices.

If you doubted type-2 and type-3 yellow glues would last if used to build an outdoor chair or garden arbor, worry no more. They might be a little shy of epoxy in initial strength, but exposure to the elements leaves type-3 glue with about the same holding power as epoxy, especially in mortise-and-tenon joints. Type-2 glue didn't equal epoxy mortise-and-tenon joints, but it held wood equally well in half-laps. What's more, these yellow glues offer easy application and cleanup, no mixing mess, low cost, and familiarity for most woodworkers.

For occasions when your joints are less than perfect, epoxy has the advantage of filling small gaps better than yellow glue, which likes a tight-fitted joint. It also bonds metals and plastic to wood.

And while polyurethane didn't retain as much of its original strength as the others and suffers foam-out problems, it's a lower-cost alternative to epoxy for joining wood to metal.