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Residential Roof Structures with Trus Joist® products

April 8th 2016 in Wood Products - #tech talk

By Andy Teasell

Traditionally, the roof structure in a house consists of a flat ceiling and a large, empty attic space framed with either wood trusses or stick framing. Recently, a trend is emerging where some builders are making use of this valuable space by either “vaulting” the ceiling, or offering extra levels and living areas, and at the same time creating a more “contemporary” look with their roof profiles. On these projects, the normal “pitched roof” is giving way to horizontal or sloping roof planes, decks, and even large double-cantilevered overhangs.

For many designers, TJI® joists are the structural framing material of choice in these projects, due to their strength, lightweight and shallower depths. In addition to TJI® joists, there are areas of a roof structure where a TimberStrand LSL joist is a very effective, user-friendly option, and we’ll show you where.

One of the many challenges facing a builder with a roof like this is the design, framing and cost of the elements and connections – especially the large double cantilevers. A “double” cantilever is one that overhangs from a wall in both the “North-South” direction and the “East-West” direction. Some of these homes have double cantilevers extending 4 feet or more in both directions – generating significant uplift and downward forces that require careful connection design.

Often the architect does not want to see dropped beams to support double cantilevers, since they can interfere with sight lines or the overall architecture, so the challenge for the designer is to keep all the framing within the plane of the roof. Some longer double cantilevers simply require a dropped beam to meet strength and code requirements – and can look appealing if the right material is chosen, but when the architect insists, and the overhangs are reasonable, there are methods with Trus Joist® beams and TJI® joists that have demonstrated effective performance and are worth reviewing with your structural engineer.

The “traditional” approach of sending a cantilevered flush beam at an angle off the corner of the building to pick up both overhangs is effective, but requires a large number of hangers and angled connections. There are some alternative and economical ways to frame these structures without resorting to an angled beam, however an accurate design (and a structural engineer for the project) is critical to the successful performance. Your Weyerhaeuser representative can put you in touch with a Trus Joist® product engineer or an experienced designer for more detailed information and economical solutions for this type of framing.

No matter what method you choose to frame your cantilevers and other roof framing, TimberStrand® LSL joists can be a useful choice in select areas, such as when angles get tight and spans get short. Because it can be cut at angles and (for short spans) nailed or toe-nailed to other members without hangers (just like conventional lumber), TimberStrand® LSL joists can save the builder a lot of time.
Here are some examples of non-conventional connection options where builders have come to appreciate TimberStrand® LSL roof joists and beams:

Sloping, angled joists can create challenges in selecting and installing hangers. A field-cut TimberStrand LSL roof joist can make the job easier.

Roof structures usually need to resist uplift forces from the wind in additional to the gravity forces of snow, rain or occupancy. Trus Joist® beams enable easy connections that can manage large forces in both directions.

For more information about innovative and cost-effective framing techniques for the new style of roof framing, contact your Trus Joist® representative.

Andy teasell