Floor-line joints and why they are needed.

One of the questions that inevitably surface at my EIFS seminars involves the use of joints at floor lines in wood-frame buildings. This particular detail, although not difficult to construct, is so aesthetically objectionable to some people that they simply refuse to include them on the building. Not using floor-line joints is risky but sometimes you can get away with it. This month’s column discusses why they are needed and various options for using, or not using, them.

Horizontal joints are needed at the floor line in wood-frame buildings for the simple reason that wood changes its dimensions as its moisture content changes. The change in dimension of the wood causes forces to be developed in the EIFS that can lead to cracking. This issue does not exist when metal framing is used because the dimensions of the metal remain constant throughout its life.

With wood, the amount of movement depends on a number of factors, including the size of the wood (the movement is a percentage of the dimension of the wood), the species of wood, the amount of moisture absorbed, and the direction relative to the grain of the wood. The maximum water absorption of a given species of wood has a limit, and eventually comes to equilibrium if the nearby conditions stay constant. The problem is that the adjacent conditions do not remain constant for long. Seasonal climate changes, for instance, inevitably cause changes in the wood’s moisture content. Hence, the wood in a building is forever going through cycles of expansion and contraction.

From leak to shrink

Water ingress, via leaks, can also change the wood’s moisture content. But often, the largest single change occurs right after the building is completed. New lumber, which has been dried as part of the manufacturing process, has moisture in it. But it may continue to lose significant amounts of moisture after it has been installed, and thus shrink. The rim joists at the floor line of an EIFS-clad wood-frame building are also subject to the weight of the building above it, and tend to be compressed. The net result is a concentration of force at the floor line.

With EIFS, the insulation layer is attached to the sheathing. The sheathing, in turn, is attached to the wood framing, and rides along with it as the dimension of the framing changes. Although the foam plastic insulation in EIFS can absorb some movement, it sometimes cannot absorb all the movement that is being transmitted through it. This movement is then transferred to the EIFS lamina. If the movement is small, cracks do not occur in the lamina. However, it is definitely possible to overstress the lamina to the point of cracking. In fact, it can do more than crack. The EIFS can delaminate, causing a telltale bulge at the floor line.

To avoid such problems, EIFS manufacturers require full, sealant-type expansion joints at floor lines. These are not aesthetic joints, but are joints that are capable of taking significant movement up to 1/4 inch or more. Aesthetic joints have virtually no movement capacity and hence will crack if used at this location.

Keep in mind that floor lines occur not only between floors, but also where wood framing meets the foundation. It is quite common for the EIFS to continue from the wood framing onto the foundation. Joints are needed here too.

The issue of floor-line joints is especially relevant with EIFS because it has no seams. Other claddings, especially wood and vinyl siding, have lots of horizontal seams, and hence the wood’s movement is dissipated at the seams.

Complicated matters

With drainage-type EIFS the design of floor-line joints becomes more complex. The question arises as to whether to drain the wall at the floor line or to let the water continue to the bottom of the wall. Either way, the effectiveness of the weather-resistive barrier needs to be retained, while still allowing the joint to move up and down. Since the design of drainage-type EIFS varies considerably from one manufacturer to another, the way in which the system is terminated at floor lines also varies. Some systems use flashings while others use extruded trim. These details can get quite complex, especially where the floor-line joints run into vertical joints.

There are plenty of wood-framed EIFS buildings that do not have floor-line joints and that also do not have cracks. There are also plenty that do have cracks. Hence, people often ask if they are really needed. The answer is “yes,” if you want to be sure that you do not have problems. If you’re willing to take a chance, in dry climates, all you have to do—if cracks do occur—is to rebuild the EIFS at the floor line using expansion joints.

In damp climates, it’s a different story. If cracks occur, water may get in. This will make the wood even wetter, which will cause the wood to swell even more, which will make the crack open wider, which will let even more water in. The problem thus escalates until a lot of water gets in and serious damage occurs, such as bona fide leaks and structural damage. The immediate solution to this predicament is to seal the crack with a Band-Aid of sealant, and then add the expansion joints when conditions permit.

When inspecting an existing EIFS project, it’s a judgment call as to whether to add floor-line joints if they are not already there. If it hasn’t cracked yet, then why bother? If the cracking has not occurred after the first several years, it probably will not occur unless there is some significant change in the building structure, such as settling.

The unattractiveness of floor-line joints is one of the main reasons for not using them. One way to mask the presence of expansion joints is to add a band of foam over the joint. The upper piece of foam is part of the upper section of EIFS, and over hangs the lower section. The lower part of the band is not bonded to the lower section of EIFS. Hence, when the movement occurs, the bands simply ride over the EIFS. This detail is also nice because the joint behind the band is protected and will require little maintenance.

Another way to reduce the chance of cracking, if joints are not used, is to beef up the lamina by using high-strength EIFS reinforcing mesh. It’s also helpful to be sure that the EIFS insulation pieces do not have a horizontal seam at the floor line area. Having a solid layer of foam at the joint area helps make the EIFS stronger at that point.

In all, the use, or non-use, of floor-line joints is a decision that needs to be made mindfully. Some people simply will not tolerate their presence and are willing to take a risk by not using them. On the other hand, the EIFS manufacturers do require them. Some way of resolving this inherent movement matter does need to occur, if simply to ensure getting a warranty.