What is Creep?

Posted by in Structural terms, Wood | March 10, 2014

Creep is a permanent sag in the wood member (it may also occur in concrete or masonry) caused by the stretching and adjusting of the wood fibers to the sustained weight on them, that is, the weight of the member itself, the wood deck, the other building components, and/or the stationary building contents.  Creep usually develops over years.

A common example of creep is the permanent sag in the shelf of an inexpensive bookshelf.  When books are set on the wood shelf, it may sag slightly.  If the books are removed after a short time, the shelf returns to a level position.  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 sag is from creep.  Studies show that the amount of sag from creep is proportional to the sustained weight on the member and the number and magnitude of the cyclical changes in the moisture content of the wood material.  That is, the greater the number of the magnitude of change in the moisture content, the greater the sag from creep.

Creep is usually considered during analysis and design with calculations for deflection of the subject joist or beam.  Industry standards generally recommend a 50% increase in the calculated sag or deflection due to the sustained weight.  However, a 100% increase may be recommended when surface green lumber is used.  In conditions where there is a significant swing in the magnitude of the moisture content or there are a great number or magnitude of the cyclical variations in the moisture content, an increase of 100% may not even be enough.

Even though creep usually develops over a period of years, we have observed where over stressed  wood beams developed a significant permanent sag during a fire event.  This significant permanent sag was due to the excessive heating and drying during the fire, the excessive wetting during the extinguishing operations or being open to the weather after the fire, and the subsequent drying during remediation and/or restoration.

The most common solution to creep in residential framing is to shim, re-set, or adjust the affected building components or contents.  This may include adjusting door latches or frames, shimming furniture or cabinets, re-leveling floor finishes, patching wall or ceiling cracking, etc.  Most home owners do this without a second thought about the cause of the sagging in the floor.  In cases of significant creep a portion of the structure may need to be temporarily shored and the sagging structural member replaced.