Does a concrete joint need a sealer, a filler, or both? The answer depends on the joint type, the traffic that crosses it, and whether you plan to coat the floor later.
A sealer is soft and flexes with the slab’s expansion and contraction, keeping water, ice and dirt out of a joint. A filler is rigid and supports the joint edge under heavy traffic, but only in saw-cut joints. Install either one only after the slab has shrunk as much as possible. Check it at the end of the first year.
Sealer or Filler: Two Jobs, Two Materials
A sealer and a filler both go into the joint, but they do different jobs. A sealer is the soft material that moves with the slab. A filler is the rigid material that holds up the edge of the joint when heavy traffic crosses it.
The table below uses the distinctions made by Concrete Network, a vendor page. Read the joint type first, because it decides whether a filler is an option at all. The table is a starting point, not a substitute for the product’s instructions.
| Attribute | Sealer | Filler |
|---|---|---|
| Material | Soft | Rigid |
| Main job | Accommodates the slab’s expansion and contraction | Supports the joint edge when heavy traffic crosses |
| Joint it needs | A sealed joint with a backer rod or bond breaker in the bottom | Saw-cut joints only, since rounded tooled edges cannot support it |
| When to install | After the slab has shrunk as much as possible | After the slab has shrunk as much as possible |
| First-year check | End of the first year of service, then repair or replace as needed | End of the first year of service, then repair or replace as needed |
A filler is not a stronger version of a sealer. It is a different tool for a different joint. To confirm the spacing and saw-cut depth before you cut or seal, use the Control Joint Spacing Calculator.
When to Seal or Fill a New Joint
Seal or fill a joint only after the slab has had a chance to shrink as much as possible. Concrete Network says both sealers and fillers should be installed only after that shrinkage.
Fillers need more patience. The same page notes that a filler can take a year or more to be effective, so plan on a long timeline rather than an instant result. No wait time is given for shrinkage in this guide, because none was verified.
Whichever material you choose, check it at the end of the first year of service. Repair or replace the sealer or filler as needed. The source does not list signs of failure, so this guide offers no defect checklist. The short list below keeps the timing in order.
- Wait until the slab has had a chance to shrink as much as possible.
- Prepare the joint: dry, clean, vacuumed, with a backer rod or bond breaker in place.
- Expect a filler to take a year or more to be effective.
- Inspect at the end of the first year, then repair or replace as needed.
A timing point attributed to ACI 302.1R, on deferring a fill, was seen only in a second-hand snippet. It is not verified, so this guide does not use it.
How to Prepare a Concrete Joint for Sealing
Prepare the joint by making it dry, free of dust and debris, and vacuumed thoroughly. Then set a backer rod or bond breaker in the bottom before the sealer goes in.
Concrete Network says the joint must be dry and free of dust and debris before sealing. Vacuum it thoroughly before you seal. The same page says sealed joints should have a backer rod or bond breaker in the bottom.
The backer rod keeps the sealer plug thinner, which reduces the amount of sealant you need. For larger joints, ArmorPoxy also describes foam backer rod as support for a joint filler, a point covered in the coating section below.
Concrete Network also discusses the shape of the sealant plug. It cites ACI 504R-90, which the page calls “Guide to Sealing Joints in Concrete Structures,” and says that guide recommends a 3:2 shape ratio over 1:2. The page adds that newer sealers may work at lower ratios. This article did not read ACI 504R-90 itself, so treat the ratio as Concrete Network’s reading of it. The dimensions behind the ratio were not verified here.
Why Rigid Epoxy Fills Can Crack
Filling a control joint with epoxy, or with a nonflexible epoxy joint sealer, can crack the joint. ArmorPoxy, a coatings company, makes that warning in its help article on coating over control joints.
The same article describes three approaches to a coated floor with joints:
- Leave the joints empty until the coating is dry, then fill them.
- Use a flexible joint sealant before coating.
- Leave the joints open.
Both sources point the same way for joints that move. A flexible sealer is the match for movement, while a rigid fill is the match for traffic on a saw-cut edge. Decide the joint plan before you apply a coating, because the order of work is part of the choice.
Worked Example: Size the Joints Before You Seal
The calculator sizes the joint layout, and sealant needs a separate check. In the calculator’s own example, a 4 in slab with a factor of 2.5 gives a maximum spacing of 10.0 ft and a saw-cut depth of 1.00 in. A live check of the tool on 2026-11-08 returned those results.
Apply the same numbers to a 20 by 20 ft slab, one step at a time.
- Read the spacing. The maximum spacing is 10.0 ft, so joints should sit no farther apart than 10.0 ft.
- Count the panels. A 20 ft side divided by 10.0 ft gives 2 panels, so one joint runs each way.
- Place the joint. It sits 10.0 ft from each edge, which leaves two 10.0 ft panels in each direction.
- Read the depth. For this 4 in slab, the saw-cut depth is 1.00 in.
Treat the example as a layout check, not a sealing plan. The sealant is not part of this calculation. The tool sizes spacing and saw-cut depth, so check sealant details against the product’s instructions. This guide gives no sealant depth or cure wait, because neither was verified.
For why joints crack and where they belong, read why concrete cracks and where joints go. That guide covers joint depth and spacing, so this article does not repeat those numbers.
A Checklist Before You Seal or Fill
Run through this list before you buy material or start the work.
- Joint type: Is the joint saw-cut? Only saw-cut joints can take a filler.
- Traffic: Does heavy traffic cross the joint? If so, a filler supports the edge, but only on a saw-cut edge.
- Shrinkage: Has the slab had a chance to shrink as much as possible?
- Coating plan: If the floor will be coated, pick one of the three approaches before you fill.
- Joint prep: Is the joint dry, free of dust and debris, and vacuumed?
- Backer rod: Is a backer rod or bond breaker in the bottom of a sealed joint?
- Year one: Will you check the joint at the end of the first year and repair or replace as needed?
For more concrete topics, browse the GeniesMag guides.
Enter your slab thickness and factor in the spacing and saw-cut depth tool, then plan the sealant against the product’s own instructions.
FAQs About Sealing Concrete Joints
What is the difference between a sealer and a filler?
Think of a sealer as the flexible one. It is soft and accommodates the slab moving. A filler is the rigid one. It holds up the joint edge where heavy traffic crosses.
Can I fill a control joint with epoxy?
Not with a nonflexible epoxy. ArmorPoxy, a coatings company, says fills like that can crack. Its options are to fill after the coating dries, use a flexible sealant before coating, or leave joints open.
When should I seal or fill a new concrete joint?
Wait until the slab has had a chance to shrink as much as possible. Concrete Network notes that a filler can take a year or more to be effective. Check the joint at the end of the first year.
What is a backer rod, and does a sealed joint need one?
A backer rod is a foam support set in the bottom of the joint. Concrete Network says sealed joints should have a backer rod or bond breaker, which keeps the sealer plug thinner.
Does the Control Joint Spacing Calculator size the sealant?
No. The calculator sizes joint spacing and saw-cut depth only. Check sealant details against the product’s own instructions, since this guide gives no sealant depth or cure wait.
Do I need to check sealed joints after the first year?
Yes. Concrete Network says fillers and sealers should be checked at the end of the first year of service, then repaired or replaced as needed.
Can a filler go into a rounded, tooled joint?
No. Concrete Network says a filler is effective only with saw-cut joints, because rounded tooled edges cannot support it.
Sources and Further Reading
References Used in This Article
- Concrete Network, “Sealing Joints in a Concrete Slab” (vendor page), accessed 2026-11-08.
- ArmorPoxy, “Coating Epoxy Over Control Joints: Best Practices and Recommendations” (coatings company help article), accessed 2026-11-08.
ACI 504R-90 is cited by the Concrete Network page. It was not read for this article.
This guide draws on a vendor page and a coatings company’s help article. Sealant depths and cure times were not verified, so check the product’s own instructions. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-11-08.
Shakeel Muzaffar is the Founder and Editor-in-Chief, with over 15 years of experience in digital publishing and online tool development. He oversees editorial quality and formula verification to ensure accurate, practical, and easy-to-use calculators.