Sheathing Fastener Spacing for EIFS
Various types of sheathing boards, especially gypsum-based products, are the most common substrates for EIFS in North America. One question that frequently arises about substrates is, “How do I fasten them to the supporting framing?” Contrary to the all-too-common practice, attaching gypsum-based sheathing is not simply a matter of using the same fastener spacings as for interior drywall. There’s much more involved, and this article will give you some ideas about how to properly attach gypsum-based sheathings for use with EIFS.
Gypsum-based sheathings, like drywall, are a three-layer material. They consist of an outer facing, an inner core, and an inner facing. The facings are needed to give the boards strength against cracking due to bending. They also provide the gypsum-based core with some protection from water when on the wall prior to the application of the EIFS. Both paper and glass fibers are used for facings. Since neither the core nor the facings of gypsum-based sheathings are sufficiently strong to directly accept mechanical anchors such as screws or nails, attaching an EIFS to gypsum-based sheathings must involve either adhesives that bond the EIFS insulation to the sheathing, or mechanical fasteners that go through the sheathing into a structural material such as the studs, or both.
Using adhesives only is a very common way of attaching EIFS to gypsum-based sheathings, and this means that the sheathing, rather than being merely a spacer in the wall, actually acts as the medium through which the forces from wind (and other sources) are transferred from the foam to the stud framing. This is quite different from interior drywall applications, which have no wind forces to resist and are merely needed to keep the drywall in place in terms of flatness, fire resistance and so on.
Like a rockThis difference means that with EIFS, the sheathing is pulled-upon by wind suction loads, and that the sheathing fasteners must keep the sheathing from being pulled off the studs. Since the sheathing fasteners are tiny connection “points,” a lot of force is concentrated at these fasteners. The fasteners themselves may be strong and made of steel, but the gypsum-based sheathings are not, and hence a variety of factors determine what fastener spacing and fastener type to use.
Self-drilling screws, as well as nails, are commonly used to attach the sheathing, and their horizontal spacing is determined by spacing of the studs, such as 16 inches or 24 inches on center. Vertical spacing, however, can be varied, as can the fastener type. The vertical spacing of the fasteners, as well as the sheathing type and thickness, are key factors in determining the ability of the sheathing to resist being pulled off the studs. Other factors include fastener type (diameter, head type, etc.), proper “seating” of the fastener, stud gauge, stud stiffness (depth), stud spacing, thickness of the EIFS, and so on.
It’s important to realize that an EIFS-clad stud wall is, from a simplistic structural point-of-view, a series of links in a chain. The wind forces that tug at the wall pass through the layers in the following order: EIFS lamina/foam/adhesive/sheathing/sheathing fasteners/studs/building frame. The reason that the sheathing fastening system is important is that it is often the weakest link in the chain. I’ll bet you’ve seen photos of EIFS cladding, with sheathing still attached, lying on the ground after a hurricane. However, I doubt you’ve seen the wind suck the EIFS coatings off the foam. Being a weak link in the chain, what happens when the sheathing finally “lets go” is that the screws stay stuck in the studs, but a hunk of sheathing breaks away at each fastener. Hence, the EIFS, with the sheathing still attached, can separate as a unit from the wall.
EIFS manufacturers determine the wind pull-off strength for various combinations of the above factors by a number of methods. These include testing only, engineering calculations only, or a combination of the two. To determine the strength of the wall assembly by testing, EIFS manufacturers build large EIFS panels, with a stud frame, sheathing and the EIFS, and test them by applying a vacuum load to the face of the EIFS. This tends to pull the EIFS, and the sheathing, off the studs. These results are often available as test reports, and are published in the technical reports issued by the code agencies for the specific EIFS product.
Blown awayWhen trying to figure out if the EIFS assembly can resist certain wind loads, it often gets confusing because the load at which the sheathing separates from the studs is not the number you use to design the wall. A safety factor is applied to this number, thereby reducing it. This factor can be as high as 3 or more. Thus, for a wall that can actually withstand 90 pounds per square foot of force, the working or “design” load may be 30 pounds per square foot, or less. The determination of the best fastening pattern is further complicated by the fact that the wind loads to which the wall is attached vary with the height of the building, the portion of the façade, the site of the building, and so on. It’s a good idea to speak with the technical people at the EIFS manufacturer, or a knowledgeable local structural engineer, if you’re in a position to select the correct fastener system for attaching sheathing to studs in an EIFS wall.
If you’re curious to see what kinds of attachment strength you can get with various stud and fastener combinations, go to the Internet and log on to www.icbo.org. This is the Web site for the code group (ICBO) that writes the Uniform Building Code. Then go to the “Evaluation Report” section and look up an EIFS manufacturer. You can then display an exact copy of its technical report. Scan through the report and you will see tables or descriptions of various wall designs and information about what kinds of wind forces they can resist. You can even download and print out a free copy of this useful information.
Although adhesively bonding EIFS to solid substrates like concrete produces the highest wind pull-off resistance, it is possible to design EIFS-clad stud walls to resist very high wind loads. Some of the techniques that can be used to do so include using stronger sheathing types, such as cement boards, and placing wire lath over the sheathing and screwing the lath into the studs. With the wire lath option, the EIFS insulation is attached using adhesives, and the adhesive bonds to the sheathing, as well as grabbing onto the lath. This makes a very strong wall. It also is, incidentally, an easy way to increase the penetration resistance of the wall; vandals would have a much harder time digging through an EIFS wall with the lath in the way.
The point of all the above is that the selection of the proper sheathing fastener for EIFS-clad stud walls is an engineering matter, and there really are not, unlike with drywall, many rules-of-thumb. Luckily, there are plenty of data available that shows how various combinations of studs, sheathings and screws work together. As with most aspects of EIFS, which is still a proprietary product type, this means that sheathing fastener spacing is a product-specific matter, and you’ll have to check with the individual manufacturer for specific numbers.
On residential EIFS projects, where wind loads are low, this sheathing fastening issue is not normally troublesome. However, in windy areas and on tall buildings, it is definitely something that needs to be engineered. Luckily, on tall buildings, the design of the stud framing and the sheathing can be varied from floor to floor, thereby allowing the most cost-effective wall design to be used as the wind loads change from floor to floor.
The above discussion applies primarily to barrier-type EIFS. The same principles apply to drainage EIFS. However, the matter becomes more complex because wood-based sheathings are commonly used on residential drainage-type EIFS. The presence of building paper and similar sheet-type water barriers in drainage-type EIFS also makes the use of adhesive-only attachment methods impossible. This means that the foam is often structurally connected to the building via the sheathing itself using EIFS mechanical fasteners only. Thus, another structural element is added to the equation, namely the fastening strength of the foam fastener to the sheathing. We’ll discuss EIFS mechanical fasteners in another issue of Walls & Ceilings.