Oakum vs. Backer Rod: What’s the Difference?

If you’ve ever restored an old window or tried to repoint a brick wall, you’ve probably run into a choice that every preservationist faces at some point: do you stay faithful to the traditional materials or lean on modern convenience?

Few debates capture that tension quite like the question of oakum vs. backer rod. One is as old as wooden ships; the other, a product of the space-age sealant boom. Both get the job done—but how they do it, and when to use which, makes all the difference.

What is Oakum?

Long before there were caulking guns and polyurethane tubes, there was oakum. The word itself comes from Old English roots meaning “what’s combed out,” referring to the loose fibers pulled from worn-out rope.

For centuries, sailors and shipwrights used those tar-soaked strands to seal the seams between planks on wooden ships. They’d drive the fibers into the joints, soak them with pitch, and—voilà—the ship stayed afloat. It was messy, hot work that often fell to prisoners or workhouse inmates who spent their days “picking oakum” from old ropes by hand.

Sure it looks like something you’d pull out of the shower drain, but this wasn’t some mistaken or haphazard material. It was an incredibly effective sealing material for joints both large and small.

As time went on, oakum made its way from the shipyard to the building site. It found new life sealing cast-iron pipes, chinking log cabins, and packing joints in stone and brickwork. Depending on the need, it could be made from hemp, jute, or cotton and soaked in pine tar, resin, or left plain for what’s called “white oakum.” When done right, it flexed with the material around it, resisted water, and breathed just enough to let moisture escape—a perfect match for the old-world craftsmanship it accompanied.

The Rise of Backer Rod

Then came the 20th century, and with it, the age of synthetics. Builders needed something faster, cleaner, and more predictable than oakum. Enter backer rod, the unassuming foam noodle that changed joint sealing forever. Developed in the latter half of the 20th century, backer rod gave builders a way to control sealant depth, prevent three-sided adhesion, and save on expensive caulking materials.

The early versions were made from extruded polyethylene foam—lightweight, compressible, and easy to use. Over time, engineers improved the design, introducing open-cell and bi-cellular types to solve issues like outgassing, where trapped air could cause unsightly bubbles in sealant.

By the 1980s, backer rod had become the industry standard. It was tidy, reliable, and perfectly suited for the expanding world of synthetic sealants like silicone and polyurethane.

What They’re Made Of—and How They Work

Oakum and backer rod couldn’t be more different in their composition. Oakum is all natural—fibers and tar, the kind of material you can smell before you see. It’s pliable, irregular, and forgiving, which makes it ideal for the rough, unpredictable joints found in old masonry or timber. You tamp it in by hand or with a caulking iron, packing it tight before sealing over it with putty, pitch, mortar or even a modern sealant. It fills the space, breathes with the building, and has that satisfying, time-honored texture that reminds you you’re working with history.

Backer rod, on the other hand, is engineered perfection. Picture a uniform foam cylinder—light, flexible, and consistent from one end to the other. Instead of stuffing the whole joint, you push it in about a third to halfway down, creating the perfect substrate for your sealant. It compresses predictably, rebounds when needed, and helps your sealant perform exactly as intended. Closed-cell versions block moisture, while open-cell varieties let air escape to prevent blistering. It’s not romantic, but it’s efficient—and efficiency has its place.

When to Use Oakum

Oakum earns its keep in settings where authenticity matters. If you’re restoring a 19th-century window, working on a historic masonry façade, or redoing the joints of an old log structure, oakum gives you the right look, texture, and movement.

Where oakum really shines is with irregular gaps where foam might not fit evenly. Personally, I have found it incredibly useful filling the gap between brickmolding and masonry on old windows and doors. That gap is incredibly varied in size from as little as no space to upwards of a full 1” in some places across a single window. Backer rod can’t compete here.

White oakum (untarred) works well when you want breathability, while tarred versions add extra weather resistance. Just don’t expect oakum to handle the kind of expansion and contraction you get in modern structures—it’s tough, but it doesn’t have the elasticity of today’s materials.

When to Use Backer Rod

Backer rod shines in modern restoration where performance and precision take the lead. Use it when you’re applying flexible sealants, dealing with larger or uniform joints, or when movement is expected—like expansion joints.

It’s also the go-to choice for high-performance applications, including fire-rated joints and large-scale commercial restorations. The key is sizing: choose a rod that’s slightly larger than the joint so it compresses snugly without tearing. It’s less messy, faster to install, and it keeps your caulking looking professional and clean.

The Pros and Cons

Oakum is like a good pair of leather boots—it takes more effort, but it’s built for the long haul. It conforms beautifully to rough or irregular surfaces and lets moisture breathe out instead of trapping it. But it’s labor-intensive, messy, and can harden or degrade over time, especially if the tar dries out. It also doesn’t play nicely with every modern sealant, so test before you mix old and new.

Backer rod is your dependable workhorse. It gives clean results, consistent compression, and minimizes the amount of sealant you need. It’s also much easier to install in quantity. The downside? It’s not breathable, and closed-cell types can sometimes outgas, causing bubbles in sealants and it’s doesn’t do well with gaps of varying size. Still, for most restoration work where you’re using modern caulks and sealants, it’s the right tool for the job.

You don’t have to pick sides in the oakum vs. backer rod debate. In fact, a hybrid approach can give you the best of both. Use oakum for those wandering joints of varied size where breathability is importantly and install backer rod where modern sealants and uniform joints are needed. It’s a fitting metaphor for preservation itself—honoring the past while embracing the best tools of the present.

The Bottom Line

At the end of the day, oakum and backer rod are two sides of the same coin. They both do what craftsmen have always needed—fill a gap, seal a joint, keep the elements out, and make the building last a little longer. Whether you choose the tar-scented nostalgia of oakum or the tidy efficiency of backer rod depends on your project, your priorities, and maybe a little on your personality. One’s for the purist; the other’s for the pragmatist. And in the world of historic restoration, there’s room for both.