Tag Archives: lumber splits

Attaching Roof Purlins on Edge to Trusses

Attaching Roof Purlins on Edge to Trusses

Reader MIKE in MOUNT VERNON writes:

“If your purlins sit on top of your trusses in the middle of your building do they sit on top of the gable truss also or are they on the side of the gable truss with hangers?”

Most ‘West Coast’ (I use this liberally as it extends east into Montana and Utah) pole barn (post frame) buildings utilize a prefabricated roof truss on each side of widely spaced interior columns (most often 10’ or 12’ on center). At every roof purlin location, a short 2×6 block (known as a paddle block) is placed between these two trusses. Block length is equal to roof truss top chord thickness plus roof purlin size. With a 2×6 top chord and a 2×6 roof purlin, block length is 11 inches.

Paddle blocks are held in place by driving nails through truss top chords, into block’s narrow (1-1/2 inch) edge. Generally builders will use as many as three 20d (four inch long) nails from each side.

If your first thought was, “this is a lot of very large nails into a very small block”, you are absolutely correct. More often than not, nails will split paddle blocks, if not immediately, splits will appear over time.

Once in place, roof purlins are located uphill from the block. Purlins stagger at each truss, with first purlin attached to paddle block with two to four nails, then a second purlin is nailed to first, with nails extending into block as well.

Now, as many as 14 nails will have been placed into a single block, pretty well guaranteeing the paddle block’s inevitable failure.

Besides an obvious problem of splitting a paddle block with numerous nails, there are some other issues caused by use of these blocks.

Post frame building eave height is measured from pressure preservative treated skirt board bottom, to roofing underside at sidewall column outside. This means interior clear height, is reduced by thickness of any concrete slab, roof truss end thickness (heel height) AND (when purlins go over truss tops), roof purlin height. Having roof purlins over truss tops, costs usable space inside of building.

When using a structural design, where roof purlins overlap at each truss, roof steel cannot be pre-drilled. Pre-drilling has many benefits – perfectly straight screw lines and it makes it obvious to installers when a screw misses a roof purlin. With staggered roof purlins, all too often a leak occurs when this 1-1/2” offset at overlaps is not accounted for.

In paddle block scenarios, prefabricated roof trusses are spaced apart by 5-1/2 inches. Even though there are two trusses per column, they are not physically joined to each other. Paddle blocks are not creating a load transfer. As trusses do not load share, under extreme snow load conditions, weaker of these two trusses can fail, creating a collapse. When trusses are properly nailed together (face-to-face without paddle blocks), loads are carried by both truss pair members.

In your scenario, if (big IF) I was going to put purlins over top of interior (middle of your building) trusses, I would want to joist hang them into the side of end trusses, unless an end overhang was present.

Checks and Splits in Post Frame Timbers

Checks and Splits in Post Frame Timbers

Checks and splits in post frame timbers (wall columns) are often misunderstood when assessing a structure’s condition. There are two means where checks and splits can form in wood elements: during seasoning, or drying, and during manufacture.
Development of checks and splits after installation occurs after wall columns have dried in place. Usually these were installed green, especially after a recent pressure preservative treatment. Due to their size, it’s not practical for timbers to be kiln dried. Some are air dried for a period of time prior to installation, but mostly they are installed green, and therefore, are allowed to dry in place.
During the seasoning process, stresses develop in wood as a result of differential shrinkage often leading to checking, splitting and even warping. Wood fiber separation results in checking and splitting. Due to innate wood characteristics, it shrinks and swells differently. Generally wood shrinks (or swells) approximately twice as much tangentially to annual rings as compared to radially. Additionally, during initial drying process timber outside inevitably dries quicker than interior, causing differential stresses to develop.

Combined effects of these drying stresses in a post often, and sometimes inevitably, result in formation of a check or a split. Since wood’s weakest strength property is tension perpendicular to grain (similar to how wood is split with an ax), drying stresses can result in a check or split forming in a radial direction across annual rings. However, while these seasoning characteristics may initially appear as problematic, they likely are not. It is important to remember as wood dries, it becomes stronger. Furthermore, the development of these seasoning characteristics is, quite often, normal. Most importantly, both are accounted for in derivation of design values for timbers and are also accounted for in applicable grade rules.

A check is separation in wood fibers across annual rings of a piece of wood and a split is a separation of wood fibers across annual rings but through a piece of wood. A third type of fiber separation, known as a shake, occurs along annual rings and is generally a naturally occurring phenomenon in standing trees, not a result of seasoning. There are several types of checks and splits defined and handled in grading rules for timbers.
In evaluation of post frame timber columns normal checks and splits can often be interpreted as problematic by some design professionals with respect to allowable design values. However, in most cases they are not. There are instances, however, where a check or split may reveal an important issue or a problem. For example, a relatively large split across a severe slope of grain.

Still concerned? In many locations glu-laminated post frame building columns are available, usually at a slight premium. Individual members of glulams have been dried prior to fabrication and pose little chance of checks or splits.