Back in the days before Autoset C®, Matchpoint®, and Wizard®, truss set-ups and the truss profile were dependent on the truss builder’s ability to accurately jig the truss being built using cross reference information and data. For example, a heel to peak measurement was taken to ensure the truss was square and symmetrical. Multiple measurements were made to adjust for variations with the design dimensions. Back in the day, deviations to the design were less important because normally roofs were a set of single profile trusses. Often, they were a set of commons with matching gable ends. We used to have a short hand for orders, no need for cutting sometimes, because the orders were repetitive and predictable. In those days, as long as the trusses were consistent (consistent error) with the order, it didn’t really matter if the trusses were a little higher than design or a little lower. Today, that is not the case. It’s not uncommon to have a run of trusses that have zero matching profiles. To aid in manufacturing, sometimes the same projects are built on different tables, relying on set-ups to be accurate and within tolerances. We can no longer depend on “consistent error.”
What are those tolerances? According to ANSI/TPI 1–2014, the tolerances for finished truss units are listed in Table 3.5-1 [see PDF or View in Full Issue]. When these tolerances are not met, the results in the field are undulations in the roof sheathing, telegraphing to the roof materials and resulting in an unattractive product.
This first photo [see PDF or View in Full Issue] shows a roof system undulating from truss to truss, visible as the shingles are not level but rather ripple up and down like waves in the roof. Attempts were made by the framer to adjust for this, but that caused additional problems and variations. While this can be caused by improper installation of the trusses where the trusses are not placed following the peak or pitch breaks, this example was caused by up and down crowns of 2 x 6 top chord materials.
In the second photo [see PDF or View in Full Issue], these humps in the roof are caused by two roof trusses that were repaired after the truss was built and outside of the jigging. The profiles do not match the rest of the trusses on the house, causing serious variations resulting in a very poor roof plane. In each of these cases, the trusses did not meet design code.
What can the fabricator do to prevent this from happening?
- Truss to truss variations are very important and should be analyzed and understood.
- Consider investing in computerized or automated jigging. Lasers and wizards contribute a great deal to quality improvements, especially when dealing with truss to truss variations.
- Refrain from plating trusses without the aid of jigging. Simply positioning the truss over another truss is not adequate to ensure accurate profiles.
- While the TPI weekly inspection procedure doesn’t require comparisons between like trusses, a good in-house quality program should.
- Accurately stack your finished products so variations are more evident.
- Particularly if you are not using automated jigging systems like lasers and computerized puck systems, use caution and cross reference information to make sure trusses are square and true.
- On occasion, make field inspections of installed trusses to verify your plant’s ability to hold truss to truss dimensions.
- Learn about installation mistakes that can cause otherwise perfect trusses to appear inconsistent. These can be shingling, sheathing installation or material quality, weather barrier installation, misplacement of trusses, or crooked, twisted, and uneven bearing walls.
- Crown is very important, especially with wide boards. Automated material loading to the saw does not address crown.
Understanding the end result is key to understanding construction tolerances.
Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. While he is a TPI 3rd Party In-Plant Quality Assurance Authorized Agent covering the Southeastern United States and performs 3rd party safety auditor services, these articles represent his personal views, knowledge, and experience. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis