How are your purlins? Sounds like a personal question, doesn’t it? Purlin is a curious word. While I don’t know its origin (o.k. I could look it up, but I am not going to), it does cause some confusion when it is being discussed regarding house construction. It is not uncommon for one to think of the horizontal members that are installed between wall studs. The term PURLIN actually refers to the horizontal structural member that supports the roof rafters in the attic. This is an important component. Without it, over spanned rafters would dip and sag, and be subject to cracking and collapsing. By supporting rafters with purlins, one can span the rafters further when building the roof structure.
Section R802.5.1 of the 2006 International Residential Code (IRC) requires that purlins be the same size as the roof rafters that they support. Earlier adopted code editions (by the State of Georgia), such as the 2000 and 2003 IRC’s, as well as the 1995 CABO One and Two Family Dwelling Code, had similar requirements. This means that a 2x6 rafter should be supported by a 2x6 purlin; a 2x8 rafter should be supported by 2x8 purlin, etc. The purlins are supported by 2x4 vertical members called struts. The struts are to be spaced no more than 4’ a part. Struts longer than 8’ in length are to be vertically braced.
The term “knee wall” is commonly used to describe the purlins and struts assembly. “In the old days” it was acceptable to support 2x6 rafters (a common rafter size) with 2x4 purlins. I still find houses built since 1996 with 2x4 purlins supporting 2x6 rafters. The purlins have normally been installed with their flat side supporting the roof rafters. It is not unusual to find that the 2x4 is bending or sagging from the weight of the roof rafters. It is also not uncommon to find that the struts are spaced greater than 4’ a part or are longer than 8’ in length and are not braced.
Undersized purlins, over spanned purlins, and unbraced struts are subject to sagging, bending, and structural distress. The roof rafters and the roof decking also subject bending, sagging, and dipping. This is indicated by the roof having an uneven or wavy roof appearance. These conditions can also interfere with rain draining from the roof, resulting in roof leaks. When the roof is replaced, one may also face the additional expense of replacing warped rafters and roof decking. A common “Best Practice” is to install the purlin with the narrow edge supporting the roof rafter (Such as found in the Journal of Light Construction Field Guide to Residential Construction, Volume 1). This means 5½” of wood supporting a rafter, versus 1½” of wood supporting a rafter. Which one do you think offers the most support?
If you have under sized purlins or purlins that a installed on their flat side, you may wish to consider having 2x6 purlins installed on the existing knee wall with the narrow edge butted up against and supporting the roof rafters. Provided that the present knee wall structure is accessible. Struts should also be properly spaced and braced. I suggest having a qualified contractor perform this installation since working in attics is both dangerous and precarious.