Expansion Joint Theory Explained


Expansion joints are, notoriously, the “weakest link” of acid brick floors.  No wonder…. they’re designed to be!

Acid brick have a property of “irreversible growth”.  That is, they “swell” in size after installation.  How much?  It will vary, but some rules of thumb have been developed, based on experiences learned over time.

The commonly accepted maximum amount of growth to be expected has been listed by various sources as .16%, or .0016.  Therefore, over a 20′ (240″) length, an acid brick floor may be expected to “grow” by as much as .38″, or about 3/8 of an inch.

Such growth is rare in our experience, but not unheard of.  Brick floors seem to grow more when they are subject to extremes of thermal (hot – cold) and moisture (wet – dry) cycling.

This “irreversible growth” of acid brick is the reason for expansion joints in brick floors.  If the irreversible growth of the brick in the floor is not addressed, the floor will eventually build internal stresses and “buckle” or “heave” off it’s original surface.

Expansion joints are actually open spaces between solid “panels” of the floor brick and mortar joints.  They are usually designed to be 1/2″ to 3/8″ wide, and extend the full depth of the brick.  In order to prevent damage in and to the open space, they are filled with closed-cell polyethylene foam rods or strips, and capped with a flexible joint sealant.

Because flexibility is the most important factor in formulating expansion joint sealants, they are usually neither as chemically-resistant or physically strong as the rest of the floor system.

Expansion Joint Practice

Well, since you have to have them, where should you put them?  This may be as much art as science, but in our experience:

  • At room perimeters
  • Drainage bay high points
  • Around fixed points in the floor (piers, columns, pipe penetrations)
  • NOT at floor drains
  • On 20′ to 40′ centers, depending on service conditions