I’ve been in the prefabricated roof truss industry in one fashion or another (sawyer, salesman, truss builder, forklift operator, crane truck driver, truss designer, general manager and owner) for 36 years. Yes, it is a very long time.
Over this time span, the roof truss industry has evolved from trusses which were pretty much designed long hand by engineers who knew how to work a slide rule, to today’s sophisticated products.
Designs for roof trusses always dealt with fairly straight forward things like span, overhangs, roof slope and basic loads (snow and dead loads). The new building codes have made things a bit more complex for the average truss designer. Snow loads are now calculated by taking into account ground snow load, the importance of the building, will the building be heated or unheated, exposure to wind, roof slope and roofing materials. Roof trusses must be designed to withstand wind loads (more trusses fail due to wind than snow), which will be influenced by whether the building is open, enclosed or partially enclosed. Wind loads are also greater near the endwalls of buildings, than in the middle, which must be accounted for. Roofs must also have the ability to withstand unbalanced loading and drift loads, as well as the increased loading on overhangs.
Luckily, technology and programming have been able to leap the ability to design roof trusses with all of these complex factors to never before dreamt about accuracy.
In most cases, prefabricated roof truss designs are using higher grades of lumber than what can be found in the great majority of lumber yards and home improvement warehouses. Much of the specified lumber is now either machine stress rated (msr) or machine evaluated lumber (mel) and can be more than twice the strength of the #2 grade material at the local lumber dealer.
A roof truss failure, for whatever cause, often comes with catastrophic results – damaged or ruined property, livestock or human injuries or fatalities. For these very reasons, Building Officials want to see roof truss designs which are sealed by a RDP (registered design professional).
Found a barn truss design somewhere on the ‘net? Don’t even bother thinking about using it. Winging something together? Not only is in not safe, it probably will not save money, and the Building Official – isn’t going to like it.