Controlling Rafter Deflection: Knee Walls, Purlins and Collar Ties (as Braces)

The implications of having deflecting (sagging) rafters may be simply cosmetic or may lead to a roof collapse. In some cases, the roof moves to a given position and is stabilized, perhaps by intermediate supports. Although the roof continues to show deflection, it may be quite stable.

Inspectors should look for a “dishing” in the roof surface. This is easiest to see when looking along the plane of the roof surface. Be careful not to confuse sagging sheathing with rafter deflection. Sagging sheathing is a repetitive pattern between rafters, whereas sagging rafters will be visible on a large scale across the roof surface. Keep in mind that you cannot draw conclusions by only looking at the roof from the outside.

With the addition of intermediate supports, knee walls, purlins and collar ties (braces), the effective rafter span can be reduced, stiffening the framing and controlling rafter deflection.

Methods of Reducing Rafter Spans
A well-constructed roof will not show visible deflection. When you identify rafter deflection, look for evidence that corrective action has been taken. If you do not see evidence of corrective measures, you can recommend improvements, but without writing specifications. 

Here are some common solutions that help prevent rafter deflection; these may be part of original construction or retrofits to control a problem.

Knee walls and purlins. Knee walls reduce the rafter span and prevent rafter deflection. They carry the dead loads from the rafters, sheathing and roof covering down to the soil through walls, floors, foundations and footing systems. They also carry the live loads from wind, snow, water, equipment, foot traffic and so on.

Knee walls also may support interior finishes and insulation. These walls also are called dwarf walls, struts or strongbacks. They are the same as conventional 2×4 stud walls elsewhere in the house.

Vertical knee wall with living space on interior

There is a top plate, studs and a bottom plate. Knee walls may be vertical or up to 45° off vertical. If they are offset, they often are called struts or stongbacks.

 
When studs line up with the rafters they are supporting, there is often a single top plate. Think of knee walls as small bearing walls that support rafters.

Typically, purlins are 2x4s or 2x6s. The purlin should be the same size as the rafter, at least. Think of purlins as small beams that run under the midpoint of all the rafters.

Purlins are supported by 2×4 posts or struts. If the purlins are 2x4s, the posts usually are spaced every 4 feet. If the purlins are 2x6s, the posts usually are installed every 6 feet. If the posts are longer than 8 feet, they should be braced to prevent buckling. Purlin posts also can be up to 45° off vertical.

Knee walls and struts ideally rest on bearing walls. Sometimes they bear close to, but not directly on, bearing walls. In situations where the walls or posts rest on joists, the joists should be the next size larger than they otherwise would be.

The concentrated loads exerted by knee walls or purlins and posts may cause sagging and cracking of ceilings below. When dealing with house structures, think vertically. Follow loads up or down through the house from one floor to the next. Often, the reward is
a very clear explanation of a pattern of movement, cracks or both that has puzzled others.

Collar ties as braces (a Canadian solution). Unlike the traditional (US) collar ties that are mounted in the upper third of the rafter span to prevent rafter uplift under wind forces, collar ties installed at the midpoint of the rafter span can act as compression members to control rafter sag.

To effectively reduce the spans of the rafters, collar ties (braces) run horizontally across the mid-height of attics and are typically 2x4s. They are primarily compression members that carry the live and dead loads that rafters receive. They are being squeezed, rather than pulled, from each end.

The advantage of collar ties over knee walls or purlins is that, often, they are less expensive to install and do not require a bearing wall below.

Ideally, collar ties are placed halfway along the rafter span. You get the most benefit by making the rafter spans equal on either side of the collar tie. If the collar tie is close to the peak, the span of the rafter below the collar tie is much longer than the rafter span above the collar tie. This means that the long rafter span on the lower
section still might deflect under load.

Similarly, putting the collar ties close to the bottom leaves a long rafter span from the peak down to the collar tie.

A collar tie can perform the same task as a knee wall or a purlin. It keeps rafters from sagging. The collar tie uses the opposing rafters to prevent sagging, whereas purlins and knee walls use the bearing walls or ceiling joists below to resist rafter sag.

Because the collar tie mainly is used as a compression member in the application shown in the photo below, the mode of failure is buckling. The buckling will occur in the narrow dimension. A 1×8 is not nearly as good at resisting buckling as a 2×4. Generally, a 2×4 is the minimum size for a collar tie.

Longer collar ties are more susceptible to buckling. It’s easier to push on each end of a yardstick and have it bend than to push on each end of a 6-inch ruler and have it bend. If the collar tie is longer than 8 feet, lateral bracing should be provided near the midpoint. This is most often accomplished with 1x4s being nailed across all the collar ties near their middle.

Collar ties are provided on every pair of rafters to stiffen the roof framing and to control rafter sag. Collar ties (braces) are effective only on steep roofs. If the slope is less than 4 inches by 12 inches, collar ties are not the best solution.

Collar ties (braces) are different from ceiling joists and rafter ties. Rafters can spread apart at the bottom under load. Ceiling joists or rafter ties are tension members that prevent the bottom of rafters from sliding off the top of the wall or pushing the wall out. Their role is not related to collar ties (braces).

Canadians are amusing in many ways. The use of mid-height horizontal braces to strengthen roof framing and reduce rafter size or span is clever, but their insistence on calling these braces collar “ties” is just funny.

Other Rafter Inspection Tips
In addition to looking for deflection, including the bowing of tension members, be sure to inspect the roof framing and look for adverse conditions such as notched, cracked, split or rotted wood members. Rafters that pull away from ridge, hip or valley rafters are common issues, often caused by poor fastener installation. In some cases, knots in rafters create enough weakness to allow them to crack or split. As discussed, roof framing inspection includes looking from the outside as well as the inside. Your report should document any limitations that prevented a full inspection.  

More details about roof structures can be found in the ASHI@Home Training Program (https://www.homeinspector.org/ASHI-HOME-Training-System).