Humps and Sags in a Roof Framed with Sawn Lumber
We are sometimes contacted to evaluate humps and/or sags in roof slopes, as shown in the featured photo and Figures HS-1. Most often we are asked to evaluate residential roofs which are commonly framed with 2×6 or 2×8 rafters.
Examination in the attic often discloses that the sagging is due to the undersizing of the common rafter for the span. That is the depth of the common rafter is not adequate for the length of the span. Generally, the actual depth of a common rafter in inches should be about 1/20th of the span measured in inches. For example, the rafter depth for a 12 foot span (144 inches) should be at least 7.25 inches deep, that is, a 2×8. If the depth is less than 7.25 inches, the rafter will sag immediately from what is considered an elastic deflection.
Elastic deflection is the temporary downward flexing of a rafter under its own weight, the weight of the construction on it, and the weight of ice and/or snow. It is considered temporary because if the weight is removed, the rafter pops back up to its straight, unsagged condition. That is, the condition is elastic, similar to the elasticity of a rubberband.
Sagging in a rafter also commonly occurs from creep. Creep is a permanent or plastic sag in the wood member caused by stretching and adjusting of the wood fibers to the sustained weight on them (shingles, sheathing, drywall, insulation and rafters). Creep develops over years.
The permanent or plastic sag of creep remains after the weight is removed. A common example of creep is the sag in a cheap wood bookshelf. When books are set on the wood shelf, it sags slightly. If the books are removed after a short time, the shelf returns to a level position. This is elastic. However, if the books remain on the shelf for a long time, the sag increases, and when the books are removed, the shelf does not return to its original, level position. The permanent or plastic sag is from creep.
Studies show that the amount of sag from creep is affected by the cyclical change of moisture content in the wood and the magnitude of the sustained weight on the rafter such as self-weight and weight of construction. The same studies show that a temporary load from weight of ice and snow (up to four months) has no affect on creep.
When all the rafters in a slope sag relatively uniformly, the sag is usually not noticeable. However, if there is a hump in the roof surface (see featured photo and Figure HS-1 ), it may become very noticeable.
Humps are commonly due to intermediate walls that extend up to underside of the roof deck (Figures HS-2). Since walls generally do not sag with the rafters, the sagging rafters around them results in the roof surface over the wall being higher than the adjacent surfaces. The high area is commonly described as a “hump.”
On some occasions a hump is due to one rafter not sagging as much as the others. This may be due to an unusual stiffness in the wood material of one or more rafters or may be due to one or two rafters being set properly with the crown up and the surrounding rafters being set improperly with the crown set downward.
A crown is the natural bow seen when sighting down the side of the sawn lumber. A sawn lumber floor member or roof member should be set with the crown (or bow) upward. This allows the weight on the member to “push” the crown down and bring the sawn lumber member closer to level.
Sometimes a hump may appear when one unbroken rafter is surrounding by adjacent fractured rafters (Figure HS-3). The cause of the humping and the sagging can often be determined easily by direct visual examination of the roof framing or when the framing is not visible be determined by a structural engineer with a little more analysis through visual examination of the building components under the humping and sagging combined with a good understanding of common roof framing.