If your driveway went in a few decades ago, there's a good chance it's divided by strips of wood or black fiberboard. When the concrete was new, those strips sat flush and tight.
Now you can push a screwdriver through them, dandelions grow out of one, and the slab on one side sits just low enough to catch the snowblower.
You don't need a new driveway, and maybe not even a contractor. You do need to know which joints you have and what goes back in them, because the wrong fill makes things worse.
Two Kinds of Joints, Two Different Jobs
Control joints are straight grooves sawed or tooled partway into a slab, usually about a quarter of its thickness, roughly every 10 to 12 feet on a driveway. Concrete shrinks as it dries and will crack somewhere. The groove makes it crack there, in a straight line, out of sight.
Expansion joints go all the way through. They separate two pieces of concrete, or a slab from something that doesn't move with it, like your foundation, garage floor, or front steps. (Where a slab meets a fixed structure, the industry term is isolation joint, but most people call them all expansion joints.) The gap is usually about 1/2" wide, though old wood dividers can be a full 1-1/2", and it was filled with a compressible strip at the pour: asphalt-impregnated fiberboard, wood, cork, rubber, or foam.
Current guidance says a driveway rarely needs expansion joints between its own panels, but plenty of older driveways around here got wood strips across the slab anyway, sometimes just for looks. More strips means more wood to rot.
Which One Are You Looking At?
Grab a flat screwdriver and walk your driveway, walks, and patio.
| Clue | Control joint | Expansion joint |
|---|---|---|
| Width | Narrow saw cut, or a tooled groove with rounded edges | Usually about 1/2" or wider |
| What's in it | Nothing, or a hairline crack along the bottom | Fiberboard, wood, foam, old caulk, or dirt where the strip used to be |
| Screwdriver test | Hits solid concrete a short way down | Sinks into soft filler or goes straight down to soil |
| Where it is | In a grid across the slab | Where concrete meets the house, garage floor, steps, sidewalk, or street; on older driveways, across the slab too |
| What it's for | Decides where shrinkage cracks form | Lets two pieces of concrete move independently |
Both kinds are worth sealing outdoors. Expansion joints are where the rotted wood lives, so that's most of what follows.
What an Open Joint Does Over a Wisconsin Year
A rotted-out strip is an open channel into the base under two slabs at once. Follow it through the calendar:
- Winter. Water that ran in during fall freezes in the gap and the soil below. Ice pries at the slab edges, wet soil can heave one slab above its neighbor, and salty slush off your car speeds up the edge damage.
- Spring melt. Snow piled along the driveway melts at once, and every open joint funnels it to the base. Much of Brown County sits on red, clay-rich glacial soil, which stays soft long after the snow is gone.
- Summer storms. A downspout or sump line emptying onto the driveway sends a stream down the nearest joint, washing soil out from under both slab edges. That's how one slab ends up lower right at the joint. (More in voids under concrete.)
- All season long. Sand, grit, and weeds move in. A joint packed with material that won't compress can't close when the concrete expands on a hot day, so the slab edges take the pressure. Keeping debris out is a big reason joints get sealed at all.
Your Options for an Old Wood or Fiberboard Strip
Leave it alone. If the strip is firm, flush or slightly below the surface, and tight to both slabs, it's still working. Check it every spring.
Trim the top and seal over it. If the strip is mostly sound but gaps are opening along its edges, cut out the top half inch to an inch, lay bond-breaker tape on the wood, and fill the pocket with flexible sealant that grips only the concrete sides.
Pull it and seal the joint. The usual answer once the wood is soft, crumbling, or mostly gone. With closed-cell backer rod and a flexible sealant, there's nothing left in the joint to rot.
Replace it with new wood. Doable if you want to keep the look of wide wood dividers: ground-contact treated or rot-resistant lumber, cut for a snug fit. But new boards in old gaps often work loose, and driveway wood here stays wet much of the year. Expect to redo it someday.
Fill it with mortar, grout, or concrete patch. Don't. The joint exists so the concrete can move. A rigid plug can't squeeze when the slabs grow in July or stretch when they shrink in January, so it usually cracks loose and lets water back in. When it does hold, the stress goes elsewhere: chipped edges or a new crack in the panel.
Joint Condition and What to Do About It
| What you see | What to do |
|---|---|
| Control joint, tight, slabs level | Seal it if it's about 1/4" wide or more and collecting grit; a thin saw cut can be left alone |
| Any joint packed with sand, weeds, or cracked old caulk, slabs level | Dig it out, backer rod if it's deep, flexible sealant |
| Wood or fiberboard firm, flush, tight to both sides | Leave it and check each spring, or trim the top and seal over it |
| Strip soft, crumbling, sunk, sprouting weeds, or gone | Pull what's left, clean the joint, backer rod, sealant |
| Strip sticking up above the slabs | Trim or pull it, then check whether the slabs around it settled |
| One slab lower than the other at the joint | Lift the low slab first, then seal |
| Slab at the house tipped toward the foundation | Lift first, then seal (see caulking where concrete meets the house) |
| Joint edges crumbling or broken off | Repair the edges first; sealant needs solid concrete to grip |
| Old mortar fill cracked loose | Chip it out carefully, then seal it properly |
Pulling Old Wood and Sealing the Joint
The high-level version. Sealant types and backer rod sizing are in our concrete crack caulk guide.
- 1Check the slabs first. Lay a straightedge across the joint. If one side is lower, read the next section before you seal.
- 2Wait for mild, dry weather. Most joint sealants want at least 40°F, a dry and frost-free joint, and no rain until the sealant skins over. Sealant makers also recommend installing when the joint is near the middle of its movement range, so mild days beat heat waves and cold snaps. Late summer and early fall are the sweet spot here.
- 3Get the wood out. Soft rot comes out with a stiff putty knife, a chisel, and a flat bar. For firmer wood, split it down the middle and pry the halves toward the center of the gap, not against the concrete, so you don't chip the edges. Wear eye protection.
- 4Clean down to bare concrete. Pull weeds and roots, wire-brush both joint faces, and blow and vacuum out the dust. Let it dry completely.
- 5Set backer rod near the top. Closed-cell rod a bit wider than the gap. You don't need to fill the whole depth, just set the rod where it gives the sealant the right shape.
- 6Seal it and keep it low. A flexible polyurethane sealant rated for traffic, self-leveling on flat joints, finished slightly below the surface so plow blades skate over it. On a wide gap, check the label's maximum joint width; an old two-by board leaves about 1-1/2", near the top of the range many self-leveling sealants list.
- 7Keep off it for as long as the label says.
When to Lift Before You Seal
If the slabs no longer meet flush at the joint, the low one dropped because water got under it, probably through that open joint, and carried soil away. Sealing it where it sits leaves the trip lip and the hollow underneath, and puts a fresh bead right where a snowblower will catch it.
The better order: clean out the joint, lift the low slab, patch the holes, then seal. Pulling the rotted wood first also frees the slab to rise without dragging on its neighbor. We lift with high-density structural polyurethane foam through a few 5/8" holes, about the size of a dime. The foam fills the void and raises the slab in small, controlled increments. See our concrete lifting page.
One limit: foam only raises concrete. If frost or roots pushed a slab up, the fix is lifting its neighbors to meet it, or a different repair entirely.
When You Don't Need Us
Do it yourself if the slabs are level, the old strips come out without a fight, and you can get a dry, mild weekend.
Get help if:
- A slab has dropped at the joint, or the slab by the house slopes toward the foundation. That's a lift, then a seal.
- You've got a lot of footage, like a grid of wood strips across a driveway and patio, and you'd rather it get done once.
- The joint edges are crumbling. That needs a concrete repair contractor first. It's not work we do, and we'll tell you so from your photos.
To see how we handle a sealing job start to finish, read our concrete caulking process.