The gap between two driveway slabs is full of grit, a few weeds and a crumbling strip of black fiber. Water runs into it every time it rains, and the slab edges are starting to chip. Shopping for an “expansion joint filler” turns up three different kinds of product that do three different jobs, and buying the wrong one is how a joint is back to this state by next winter.
This guide separates those jobs, then picks a product by location: a driveway or sidewalk, a pool deck, and a joint that is too wide for sealant alone. It also covers the part most guides skip, which is how many cartridges a real run of joint takes. The facts come from the manufacturers’ own data sheets for the products named, and it is general guidance, not an engineering specification. A joint that moves a lot, or a slab that has settled, needs a contractor’s eye.
Filler, backer rod, sealant: three jobs, not one
An expansion joint is a deliberate gap that lets a slab grow and shrink with the weather without cracking. Three different things sit in it, and the word “filler” gets used for all of them.
The first is the joint filler proper, a compressible strip that is set in the form before the concrete is poured. Sika’s SikaCem Concrete Expansion Joint is a typical example: a closed-cell polyethylene foam in 4 inch wide rolls that is cut to length and fixed between the forms, with a tear-off strip you can leave facing up if you plan to caulk the top after the concrete cures. If your slabs are already poured, this is not what you buy. The existing strip is already in there.
The second is backer rod, a round closed-cell foam cord that is pushed into an existing joint to set how deep the sealant sits and to stop the sealant bonding to the bottom of the gap. The third is the sealant itself, the flexible cap on top that keeps water and grit out. For a joint that is already poured, the repair is almost always the second and third together: clean it out, set backer rod, pour sealant.
If what you are looking at is a random crack in the middle of a slab rather than a built-in joint, none of this applies. Start with the concrete crack filler guide instead.
Driveway and sidewalk joints
Horizontal joints that cars and feet cross need a sealant that flows level by itself, so it settles flat instead of mounding. Sika’s Self Leveling Sealant is a one-part polyurethane in a grey cartridge, classified ASTM C920 Type S, Grade P, Class 25. Class 25 means it is rated to handle plus or minus 25 percent movement, and Sika’s sheet adds that the total expansion and contraction should not exceed 25 percent of the average joint width. In practice that means a joint can open and close by a quarter of its width, which is generous for a slab, but it is not unlimited.
The data sheet sets rules that decide whether the job works:
- Depth: the sealant must be at least 1/4 inch deep and no more than 1/2 inch in a moving joint. For horizontal joints that see traffic, the sheet gives a minimum of 1/2 inch.
- Shape: the proper design is a 2:1 width to depth ratio, so a 1/2 inch deep bead belongs in a joint about 1 inch wide, and a 1/4 inch bead in a joint about 1/2 inch wide. The sheet says to use bond breaker tape or closed cell backer rod at the bottom of the joint to prevent bonding on three sides.
- Temperature: apply between 40 and 100 degrees Fahrenheit, with the sealant conditioned to 65 to 75 degrees first. It is tack-free in 1 to 2 hours and takes 3 to 5 days to cure fully.
- Not on slopes: the sheet says it is not designed for sloped surfaces, so a steep driveway section can need a different product.
- Not on asphalt: the sheet says not to use it in contact with bituminous or asphalt materials. A concrete slab that meets an asphalt driveway needs an asphalt-compatible sealant at that edge; the driveway sealer guide covers where each surface fits.
Check price on Amazon: Sikaflex Self-Leveling Concrete Expansion Joint Sealant (10.1 oz). This is an affiliate link (see our disclosure for details).
Backer rod: the cheap part that decides if the sealant lasts
Skipping backer rod is the most common reason a poured sealant tears. Without it the sealant bonds to the bottom as well as both sides, and when the slabs move the bead is pulled in three directions and splits. The rod also fills most of the gap, so you are not paying to fill a deep joint with sealant.
Choose a closed-cell rod, which does not soak up water. M-D Building Products’ listing describes a closed-cell foam rod that is pushed into the joint until it fits snugly, which is the point: it should be a little larger than the joint so it holds itself in place. Push it down until exactly the depth you want for the sealant is left above it, using a screwdriver or a putty knife with a flat edge as a gauge. In a 1 inch wide joint that depth is 1/2 inch.
Check price on Amazon: M-D Building Products Foam Backer Rod (1/2 in x 20 ft). This is an affiliate link (see our disclosure for details).
How much sealant a real joint takes
Because the shape follows a 2:1 rule, doubling the width of a joint does not double the sealant. It roughly quadruples it, since the depth has to double as well. Sika’s yield table for the 10.1 ounce cartridge shows it: a 1/2 inch wide, 1/4 inch deep bead runs about 12.1 feet, while a 1 inch wide, 1/2 inch deep bead runs only about 3.0 feet. Here is what that means for 60 feet of joint, roughly a 12 foot wide driveway section with five cross joints:
| Joint width and sealant depth | Feet per 10.1 oz cartridge (Sika’s table) | Cartridges for 60 feet |
|---|---|---|
| 1/2 in wide, 1/4 in deep | 12.1 | 5 |
| 3/4 in wide, 3/8 in deep | 5.4 | 12 |
| 1 in wide, 1/2 in deep | 3.0 | 20 |
All three rows follow the 2:1 shape. The first suits a narrow joint that no vehicle crosses. A joint on a driveway that carries cars needs the 1/2 inch minimum depth, which at 2:1 means a joint about 1 inch wide and the bottom row. Buy for the depth the data sheet requires, not the depth that looks adequate, because that is the number that decides how many cartridges you need.
Pool deck joints
A pool deck is the one location where the standard sealant above is not the answer. Sika’s data sheet caps chlorine exposure at 5 ppm, and a deck joint is splashed with pool water, so it is worth comparing your own pool’s level against that cap. For these joints, W. R. Meadows’ Deck-O-Seal is a two-part polysulfide sealant made for the purpose. Its data sheet lists ASTM C920 Type M, Grade P, Class 25, NT, the same plus or minus 25 percent movement rating, and says it is not affected by today’s pool chemicals.
It is a different job to apply. It comes as a 96 ounce kit of base and setting agent that must be mixed for at least 10 minutes in the proper ratio, and once mixed it has about 1 hour of working life at 77 degrees. It is tack-free in about 4 hours, sets firm and rubbery within 24 hours, and the sheet says to protect it from rain for at least 24 hours. Application temperature runs 40 to 122 degrees. The same 2:1 width to depth ratio applies, with 1/4 to 1/2 inch of sealant depth, and it is not for joints narrower than 1/4 inch.
Two further points from the sheet are easy to miss. It is not for use underwater, so joints below the waterline need a different Deck-O-Seal grade. And for bigger joints the filler behind it must be non-asphalt: asphalt-impregnated fillers are not compatible, which means the traditional black fiber strip some older pool decks were built with can be a problem for a sealant that goes on top of it.
Check price on Amazon: Deck-O-Seal Pool Deck Sealant 4701033 (96 oz kit). This is an affiliate link (see our disclosure for details).
Wide joints and the old asphalt strip
A joint much wider than an inch is too big to fill with sealant at the 2:1 shape. The fix is a deeper backer, either a larger-diameter rod or a compressible filler strip, so that only 1/2 inch of sealant is poured on top. The Deck-O-Seal sheet suggests a cork expansion joint filler for large joints, again with the warning against asphalt-impregnated board.
That warning is worth applying to a driveway too. If you are pulling out old black fiber board before resealing, check whether it is asphalt-impregnated, because Sika’s own sheet says its sealant should not touch asphalt. Pulling the old strip out entirely, then setting rod and sealant, takes the question away.
Before you pour
- Clean the joint down to bare concrete. Dust, old caulk and curing residue stop the bond, and Sika’s sheet asks for clean, sound, frost-free joint walls.
- Check the width, then the depth available, then decide the sealant depth from the 2:1 rule.
- Set the backer rod at the right depth, so the space left above it matches the sealant depth you chose.
- Pour at the middle of the joint’s seasonal range, not on the hottest day of summer. Sika says to apply the sealant when the joint is at the midpoint of its designed movement.
- Use opened cartridges the same day, and keep traffic off until the sealant has cured, which the sheet puts at 3 to 5 days.
Checked for accuracy on October 10, 2026, against the manufacturers’ data sheets for the products named.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.