Installing Valley Sets
Question:
What are the requirements for installing “valley set” overlay roof trusses? I am interested in nailing and support conditions. Some engineers ask for the bottom chord of the valley truss to be ripped to match the roof pitch of the underlying trusses. Is this necessary?
Answer:
Valley sets are non-structural, but they do transfer roof dead loads and live loads to the underlying trusses. Each bearing point would have a tributary area of 4 sq. ft. assuming a 2-foot spacing for both the valley set and the underlying trusses.
The typical roof total load at the high end is 60 psf, which results in 240 lbs of load applied to the top chord of the carrier truss from the truss above. If we take 240 and divide it by the typical compression perp value for SPF, which is the worst case, we get 240 lbs/425 psi or 0.567 sq. in. If you divide this by 1.5 (which is the width of the supporting truss) you get 0.37 inches of width. The slope of the roof will determine whether there is less or more than 0.37 inches across the 1.5 inch thickness to support this weight.
It’s also worth considering that the live load is rarely on the roof, and the factor of safety prior to crushing for compression–perpendicular-to-grain is 1.9. With those considerations in mind, the 0.37 calculation can be reduced. For example, if the dead load is 20 psf then the width is 0.066 in. ([(20psf times 4)/425] divided by 1.5 divided by 1.9).
Even with all of this, we suspect that for the amount of load that is typically going to be seen there will not be crushing of any significance in terms of visibility and certainly this will never become a structural failure problem.
The key issue is probably having the proper nailing to hold the truss on the top in the proper place. If possible, nail through the face of the top truss down into the top of the top chord in the bottom truss as perpendicular as possible. Again, depending on the slope of the roof, using this nailing option should allow for a pretty good connection between the top and bottom trusses.