The Problems With Beam Repair Products
Let’s talk about epoxy, bondo and wood fillers; how well do these work for exterior beam repair?
If you walk into any home-improvement store inquiring about roof beam rot, you’ll likely be guided to the paint department to seek out a remedy. Here you’ll find a dizzying array of putties, pastes and preservatives for every occasion. Just like the many who came before you, you’re now faced with a predictable dilemma: which of these products (if any) will help you with your roof beam troubles? The following should help to flesh out this tricky topic.
High-tech putty-like wood fillers (under various trade names) are the go-to products for exterior wood repair. As you might expect, most exterior fillers share several desirable characteristics, such as good adhesion to solid wood, plenty of strength, and the ability to be tooled and shaped once they’re cured. When you add to this their resistance to water, pests and chemicals, you can really begin to appreciate their potential. Some of these fillers, like two-part polyesters (bondos) and epoxies, work quite well for some exterior repairs.
But long before deciding what elements will go into a repair, you must determine the nature and extent of the actual damage to the wood. Very minor decay might only require a little scraping, a spritz of paint-compatible preservative, and a thorough priming and painting. But an exterior beam can also get so badly decomposed that it can’t be repaired and must be extracted from a building altogether. By the time it’s discovered, most damage will fall somewhere between these two extremes.
Moderately rotted and cracked exterior beam ends and projections are more problematic, though; they usually don't lend themselves well to direct application of common wood repair products.
Good quality caulkings and coatings—which work well for routine maintenance — can also be short-lived over unstable or decomposing roof beam ends. In fact, a great deal of preliminary work must go into those weakened areas before attempting any restorative treatment. Without this preparation, it would be a lot like expecting sturdy new knee patches to actually hold on threadbare carpenter jeans.
From one product to the next, fillers tend to require the same course and degree of preparation. The affected wood must be dry and not contaminated with any other substances that might interfere with absorption or adhesion. The prep typically involves the gouging, treating and reinforcing of the rotted areas. Gouging is the removal of the soft or spongy decay (with hand or power tools) as needed to expose as much of the surrounding solid wood as possible. The resulting voids are then saturated with a liquid treatment designed to seal off, stiffen and arrest the further spread of any residual rot. For a more tenacious repair, the prep work is often supplemented with anchors (screws and the like) within the cavity to help hold the cured filler in place.
Even with meticulous preparation, the successful application of fillers on exterior beams is not without its challenges—particularly at those pesky end-cuts and projections. For example, the normal expansion and contraction of solid wood versus wood fillers occurs at different rates; over time, this uneven movement can lead to a separation at the joint between the two materials.
These openings—even hairline ones—will allow rainwater to channel back into a beam all over again. When you add to this the joys of working with harsh chemicals (including the unknown ones applied to a beam before your arrival), frustratingly variable temperature-related curing times, unanticipated rain events, and a host of other little job-site gremlins, one thing starts to become clear: it’s not always going to be easy patching your way to a lasting exterior beam repair.
If decay has compromised the structural integrity of a roof beam overhang (that is, the beam has disintegrated well back into the area that it was meant to support), the repair must also be sturdy enough to hold up the roof load above. But here’s the rub: fillers alone cannot reproduce the functional strength of solid load-bearing roof beams. Simply put, extensive deterioration can eliminate wood fillers as an effective solution.
This does not amount to a problem with any particular product, though; it’s more a matter of improper application. The bond at the joint between a filler and a withered load-bearing beam, for instance, will always have one inherent flaw: weakness. The two materials effectively form a ragged end-to-end butt joint; this is arguably the weakest kind of joint there is. Even with the application of chemical hardeners and reinforcing rods, the joint is unlikely to be strong enough to withstand a significant sustained weight—like that of an ordinary concrete tile roof.
In other words, without a firm and reliable bond, the joint is prone to creep open under load—and ultimately fail.
The best option is always prevention, which means regular beam maintenance. I've got two different pages on the website you can visit to learn more: Dry Rot Prevention and Detecting Beam Rot. If you want a bit of a how-to on roof beam maintenance visit Exterior Beam Rot Prevention.
If you decide you need to hire someone to work on your roof beams, be sure to visit Before Your Repair. Here, you'll find handy tips on choosing the right contractor and the right products to get the job done right.
parts of the above are excerpted from a roof beam dry rot repair book I'm currently writing