Understanding types of joints and the correct techniques to seal them

There's a lot more to sealing a joint than meets the eye. Do it right and you get many years of solid performance. Do it wrong and the sealant fails — fast.

How do you do it right? First, consider the types of joints you are most likely to encounter, and then read deeper to brush up on your technique for creating a lasting, high performance joint.

Butt joint

A butt joint is where similar or different materials abut each other, leaving enough space to insert a backer rod. In a butt joint, the backer rod is used to control the depth of the sealant and improve its ability to flex with the joint.

Bridge Joint

A bridge joint is when new sealant is applied over an existing, sealed joint. This is mostly used in jobs where a joint has failed or where the sealant is too difficult to remove, and usually involves a metal-to-metal joint. In a bridge joint, the sealant is applied to the face of each substrate.

Fillet Joint

A fillet joint is also known as a cove bead or inside corner bead in places such as windows, doors or the base of a wall. Fillet joints can be tricky, depending on the space between the substrates and how much a joint might move.

There are three different types of fillet joints:

  • Type 1 uses a backer rod between two panels that meet to form a 90-degree angle. Here, the backer rod performs the same function as in a butt joint.
  • Type 2 uses a foam block that fits into the corner of the joint to prevent triple-bonding. Use this type of fillet joint when there is not enough space to fit a backer rod. Both of these techniques are better suited for joints that have the potential to move.
  • Type 3, which is a good option for a joint that is less likely to move, uses bond breaker tape to prevent triple bonding when there is no room for more substantial backing material.

Joint backing

If you are filling a joint large enough to handle backing, you must always use it. Backer rods not only control the depth of the sealant — which should be half the width of the joint, up to half an inch in depth — but also ensure the sealant only bonds to the two sides of the joint. Proper backing material does not adhere to sealant.

The majority of the time a foam backer rod is used. Choose a backer rod that is roughly 25 percent larger than the joint, so that the rod must be compressed to fit in the joint and won't move while applying and tooling the sealant.

Using improper backing material will degrade the joint

Never use multiple backing rods to fill a joint. Always insert a backer rod deep enough to ensure good sealant depth.

If the joint is too shallow and the backer rod is flush with the substrates, the joint will not function properly. For situations like this, use bond breaker tape along the bottom and back of the joint to prevent the sealant from bonding to the back of the joint.

Minimum and maximum joint depth for best performance

  • Minimum joint depth: ¼ inch
  • Maximum joint depth: ½ inch
  • Minimum joint width: ¼ inch
  • Maximum joint width: 3 inches

Surface preparation

Proper surface preparation is critical in sealing a joint. Poorly prepared surfaces are the most common cause of joint failure.

Good surface prep gives the sealant its best chance to adhere to the substrate and perform optimally. Crumbling, flaking or dust-covered surfaces will not yield good results and must be reworked before sealing.

Begin by grinding, sandblasting or wire-brushing the surface to remove loose debris and any coatings that may prevent adhesion.

Next, clean the surface with a solvent. Avoid stains by choosing a solvent that's compatible with the substrate material and the sealant. Solvents remove oils, lubricants or waxes that may have settled on the surface. Change rags frequently to prevent smearing the contaminant across the substrate. Xylene, Xylol, MEK and Toluene are common solvents.

Priming is usually the next step, but there are instances where it's not required. It's a good idea to specify primer when quoting each job — you don't want to omit it only to realize you need it at the last minute. Work carefully and don't let the primer coat areas the sealant won't cover. It could stain the substrate, which causes a lot of grief and expensive rework.

Sealant installation

A worker loading a pro-pack into a bulk gun

After the surface is correctly prepared, it's time to install the sealant. Use either a cartridge gun, which holds individual sealant cartridges or a bulk gun, which is made to dispense larger pro-packs and multi-component sealants that are mixed in buckets at the job site.


If surface preparation is the most important factor in successful joint sealing, proper tooling is a very close second. This critical step forces the liquid sealant into the pores of the substrate and creates the hourglass shape that maximizes the joint's elasticity and strength.

Proper tooling, as pictured here, is one opf the most critical steps in making a good joint

Never use water or solvent when tooling sealant, a practice referred to as "wet tooling." While this may help produce a smooth, uniform finish, the liquid will interfere with the curing process and may stain the sealant. In fact, a well-tooled sealant bead should have a few visible tool marks along it — any joint that's too smooth is a dead give away it has been wet-tooled.

Put your work to the test

Now that you've followed these steps, testing should be a piece of cake — don't skip it. Not only does it give you a chance to check your work, but it serves as an accepted standard in case a client says there are problems after the job is complete.

There are a couple techniques for testing a sealed joint using ASTM C1521: the flap or tail test, which is destructive, and the dowel method, which is non-destructive.

The dowel method is a non-destructive adhesion test

The dowel method is a good way to confirm adhesion without actually cutting out the sealant. Press a dowel into the sealant to a depth roughly equal to the proven movement of the sealant (check the packaging for the specs). If the sealant remains tightly adhered to the joint, you have a good joint.

With the flap or tail procedure, make a horizontal cut across the joint and then a six-inch slice down either side of the joint. Now pull — if the sealant tears within itself, you have a good joint. If it pulls away from the joint face, the adhesion is insufficient.

Need help deciding what sealant is best suited for your next job? Download this brochure, which not only lists sealants by job type, but also gives a rundown of each sealant's properties.

Interested in learning more and earning AIA credits?
Take the Introduction to Caulks and Sealants Course on Hanley Wood University

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