EIFS – Attaching gypsum-based sheathings

In Canada, EIFS is normally applied to a substrate made of gypsum-based sheathing, which then must be fastened to the supporting framing. The fasteners and methods used to apply external sheathing differ significantly from those used to install drywall on the interior walls, and are an important element in the structural integrity of the whole wall.

What is gypsum-based sheathing?

Gypsum-based sheathings are composed of three layers: an inner and outer facing and a core. The facings can be composed of paper or glass gibers, which protect the inner core of the gypsum-based sheathing. Gypsum-based sheathing is not particularly strong, and cannot usually support EIFS being anchored into it on its own. Because of this, the EIFS must either be attached using adhesives, or by fastening the EIFS to the supporting wall through the sheathing. Sometimes, both of these methods are used.

Wind and sheathing: determining wind loads

If the EIFS is attached to the sheathing using adhesives, when the gypsum-based sheathing is hit with forces such as wind, it absorbs the wind through the foam and transfers it to the supporting wall. This can mean if it’s not attached adequately, the sheathing can be pulled away from the supporting wall because of the stress placed on the fasteners. In reality, the thin piece of sheathing is the only thing holding the lightweight EIFS to the supporting wall, and if the fasteners or adhesives give way, the EIFS and the sheathing can blow off in extreme winds, like during a tornado.

Various tests have been conducted and many reports are available that detail test runs of subjecting EIFS to various loads of wind. For safety reasons, walls are installed with a much lower wind load than the wall could actually withstand, and this difference could be as large as by up to three times as higher meaning that if the wall can actually withstand a certain amount of wind force, the sheathing is attached to the studs with a wind force bearing load of three times less in mind. The wind force can also vary depending on where the wall is on the height of the building, and these forces must be determined to choose the best form of fasteners and installation methods to ensure the sheathing is not ripped from the wall.

However, it’s important to note that the lower the building and less exposure to high winds the walls will have, the less of an issue fastener materials and spacing becomes, and in the case of a high building, the methods of installation used can differ from floor to floor if necessary.

EIFS with drainage

EIFS with drainage cannot be fastened like barrier EIFS because of the punctures it would create in the water-resistive barrier, and it is adhesively attached instead. In this case, spacing is not as important as the strength of the bond that ensures the substrate is not dislodged.

Fasteners

The most common types of fasteners used with gypsum-based sheathing are nails and self-drilling screws. Variables that can increase the strength of the sheathing to the supporting wall bond include the type and diameter of the fastener, its gauge, depth and stiffness, as well as the spacing (especially vertical) that is used.

Specifications for fasteners

The EIFS Council of Canada has written up a series of specifications for EIFS, which can be downloaded here: http://www.eifscouncil.org/pdf/kcd.pdf In this bulletin, various recommendations are given for fasteners, such as:

From the EIFS Standards and Specifications for Products:

Sheathing should be installed as per the manufacturers instructions, and installed with fasteners that are tight and flush to the sheathing, not countersunk.
Mechanical fasteners used should be supplied or at least recommended for use by the EIFS manufacturer, and some examples are: steel screws, impact-type fasteners that are corrosion-resistant that are of appropriate type and size, or polypropylene or polyethylene for harder substrates like masonry.
As a note to the above description, the specifications understand that all systems and substrates may differ and recommend consulting with the manufacturer.
The designer is responsible for determining the pull-out strength required by the fasteners.
Fasteners are commonly be installed spaced 300 to 400 mm on horizontal centers, and 200 to 300 mm on vertical centers.

Designing for stronger wind-resistance

Some ways to design an EIFS wall with higher wind-resistance in mind include:

Bonding the EIFS with adhesives to more solid substrates, creating a high pull-off resistance.
Using stronger sheathing and wire lath over the sheathing (which is fastened to the sheathing) will also create a much stronger wall that is not only wind force-resistant, but much more impact-resistant.

The manufacturer will usually be able to provide information and specific numbers with regards to the fastener spacing for their particular product, as each EIFS system is different in this regard and the manufacturer should be consulted to find the best possible combination of materials, spacing and methods.