Choosing a Truss Design Package
– New construction in CZ3 (Southeastern NC).
– 52′ x 40′ exterior stud to stud dimensions.
– 2×6 exterior walls @ 24″ OC with dense pack cellulose.
– Gable roof with raised heel trusses, a vented attic, and a 7′ deep wrap-around porch on two sides.
– Porch ceiling height same as interior (9′)
– Soffit vents will be moved to the porch eaves.
I’m including the porch roof as part of the truss package, and I have a couple of different options from the truss manufacturer. I’d like input on which is the better option. In both, the short side porch on the gable end will be trussed with small mono pitch trusses attached to the sheathed gable wall.
The difference is in the long side porch roof. It can be done as single piece 47′ trusses or as 40′ trusses that include the change in pitch and then 7′ mono pitch trusses to complete the porch roof that would attach the same way as those on the short side gable wall. See attached PDF files. I included the basic outline of the trusses for simplicity. I can attach more detail if necessary.
Using the separate mono trusses would allow continuous exterior sheathing all the way up to just below the top chord. Some have said that doing this creates a stronger connection against shear and uplift. This seems logical and it does apparently eliminate the need for blocking between the trusses.
Using the single piece trusses, I would sheath up to the top of the double top plate and then I would need to block and sheath the area between the trusses due to the height of the heel. The back of the house (no porch roof) will have a 15″ heel height, and in the front, due to the change in pitch, the top chord is just over 3’3″ above the exterior wall top plate. I’m not sure if this would still be considered the actual heel height since the porch extends 7′ out past the exterior wall. I understand this can be done using pre-made sheathed boxes or like this…
https://res.cloudinary.com/engineering-com/image/upload/v1484694181/tips/Strap_dnc1nk.png
One benefit of sheathing to the top plate vs. continuous sheathing past the top plate is that I could using a tape/adhesive membrane attached to the exterior of the sheathing up and over the top plate and then attached to the back of the ceiling drywall for air sealing. I really like this idea.
I would greatly appreciate any thoughts, input, and preferences. Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
See what the Truss yard can build - I think the two piece truss is a better plan. The shearwall connection is better as is the air sealing plan.
Tim,
Thank you for the reply. The truss manufacturer has already given me a design and estimate for both options. I get what you mean about the shear wall connection, but I sort of felt like the air sealing option was better with only sheathing to the top plate. I don't see how I could use the adhesive membrane idea with the continuous sheathing up past the top plate.
Some hurricane straps have a pretty high shear resistance like the Simpsons H1 or H10. That could be an option instead of running the sheathing past the top plate but I guess you would need to ask an engineer.
The two piece truss can use the air sealing plan of installing a strip of WRB over the top plate prior to truss placement. Lap down over the outside shear panel. On the inside leave a flap to seal up to the ceiling drywall. Use Simpson truss screws to secure the trusses to the top plates. Then install the shear panel up the outside of the trusses to the roof.
The hurricane H1 or H10 need blocking to transmit the shear force from the roof sheathing into the block which is captured between the trusses. The height of the truss end at the porch would not transfer the shear down to the top plate due to the heel height.