Epoxy adhesives can help provide great solutions on the jobsite these days, but there are some key points to keep in mind that will limit your frustrations and keep the local building officials happy with your work.

Epoxy 101

What is an epoxy adhesive?

An epoxy adhesive is a type of glue created by combining a sticky, water-insoluble substance called resin with a hardener—a curing agent that hardens the adhesive for a durable finish.

What can epoxy adhesive be used for?

Epoxy adhesive can be used for something as basic as joining two pieces of concrete together for a broken step. Epoxy adhesives are also useful for something more complex, such as retrofitting an anchor bolt into concrete so the structure meets the International Residential Code (IRC) or International Building Code (IBC) requirements.

What are some common types of epoxy?

Simpson Strong-Tie makes SET Epoxy Adhesive, which is designed for anchoring and doweling threaded rod and rebar into unreinforced masonry; they also make SET-XP High-Strength Epoxy Adhesive, which is designed for anchoring and doweling in cracked or uncracked concrete and in masonry applications.

The product you choose depends on the project at hand, but both the SET and the SET-XP are two-part epoxies that meet current IRC/IBC requirements.

Dunn Lumber What is Epoxy and How to Use It Seattle WA 1

What is a two-part epoxy?

The resin and the hardener are packaged in a co-axial cartridge—essentially, a tube within a tube. The hardener is held in a red tube in the center, which is revealed once you cut the cartridge. The resin and the hardener are dispensed simultaneously and mixed within the attachable mixing nozzle.  When properly mixed, the SET will cure to a gray color, and the SET-XP will cure to a dark teal color for easy post-installation identification. 

How do I fill a hole with epoxy?

Before you fill, clean the hole of any dust and dirt. Cleaning can be accomplished by blowing the hole out with oil-free compressed air for four seconds, then brushing inside the hole with a nylon brush for four cycles before finishing the cleaning by blowing compressed air into the hole again for four seconds. If you have a deep hole, use the nozzle extension to reach the bottom. The key is to be as void-free as you can during installation—air is not your friend when you’re trying to set adhesives. Fill the hole with adhesive by starting from the bottom and working your way up to prevent any air pockets. Then, insert the threaded rod (or rebar) by slowly rotating the rod counter-clockwise and working it to the bottom of the hole.

What’s great about the Simpson Strong-Tie products is that they come with two mixing nozzles, so you can reuse the epoxy—just take off the old nozzle and screw on a new one for your second use.

What are optimal temperature and curing times?

Weather and the base material temperature of your concrete will determine how long it takes the epoxy to harden, but a general rule of thumb is 24 hours in 70° weather. This means that, in 70-degree weather, you can “bolt it up” and fully load the anchor after 24 hours.

For example: SET-XP will take 72 hours to cure at 50 degrees.

Also keep in mind that epoxy will not cure in temperatures below 50°. (For colder adhesive installations, consider using an acrylic-based adhesive such as Simpson AT or AT-XP.) AT-XP and SET-XP are similarly packaged, so be sure you're choosing the right epoxy adhesive for your project.

Common Problems with Epoxy

Waiting too long

Every so often, we’ll get a call on a nice sunny day and an installer will say, this stuff’s not working! It’s hardened in the tube. We soon find out that they had been working on a project, sat down to answer the phone or eat lunch, then returned 10 or 15 minutes later to a hard, unusable mixture. The rest of the cartridge can still be used but will require a new mixing nozzle. The moral of the story: When you’re working with epoxy and threaded rod and holes, have everything ready to go and lined up. You don’t have minutes to spare, which means you don’t want to be drilling holes one at a time. Have all your holes drilled, clean, and ready to go. That mixed adhesive will harden fairly quickly.

Not cutting deep enough

Sometimes, people won’t cut the tube deep enough, meaning the red tube that holds the hardener isn’t showing. They’ll crank and crank and all of a sudden, the cartridge explodes—because they didn’t cut deep enough. We advise cutting about halfway down until you have visual confirmation of the red tube.

Dust or dirt

If you don’t clean the dust and dirt out of the holes and off your drill, you’ll significantly reduce the epoxy’s holding power. It may not always make a huge difference, but if cleaning dust and dirt is specified and will be inspected by a third party, not cleaning could get you into trouble. To clean, you’ll want to use the “blow-brush-blow” technique—you blow out the dust and dirt with oil-free compressed air for a minimum of four seconds, use a nylon brush inside the hole for a minimum of four strokes, then blow the hole out again for four seconds. As I mentioned earlier, clean-out brushes should be located near the epoxy. As an alternative, you can use a vacuum drill system such as Simpson Strong-Tie Speed Clean™ DXS, which vacuums the concrete dust as it drills, eliminating the need for further hole cleaning.

Past expiration

Epoxy has an expiration date, which is printed right on the tubes. Dealers should be rotating their stock, just like you would rotate milk in a grocery store. It’s always important to check the date. Using expired epoxy may result in potential safety issues.


Epoxy can damage fabrics and other surfaces, so be sure to keep it away from clothes and carpet—there’s no undoing an epoxy spill.

We hope this overview of epoxy basics has been helpful. If you have any additional questions on this topic (or other Simpson Strong-Tie products), feel free to reach out to me at solutions@dunnlumber.com.

Bruce MacPherson is an employee of Dunn Lumber with a working knowledge of building materials and a strong background in Simpson Strong-Tie products including connectors, mechanical anchors, fasteners, and epoxies. Bruce has given presentations on deck safety related to IRC and IBC building codes, with a focus on building safer structures.