3

I am remodeling a small room 12x12 and want to vault the ceiling by inserting a scissor truss alongside the existing conventional truss. The existing and proposed trusses are both 2x4 members 2'-0" o.c and the span is 12'.

The desired result is to sister the scissors truss onto the existing along the top chord and then remove the vertical web and bottom horizontal chord of the existing truss. This approach is being taken to preserve the existing roof.

What would be the recommended fastener type, size, spacing/pattern and construction adhesive needed to properly attach the two chords. Can you site a code or similar design reference that will satisfy a local building inspector?

The image attached shows the desired result. The blue top chord of the existing truss are to remain and the red new scissors truss is to be sistered along the top blue chord. The image also indicates 2x4 blocking between the trusses (white elements) for drywall. Any code or design guide references for this type of connection are appreciated.

roof truss layout diagram

3
  • When you say that you have an "existing truss", is that the blue element in the picture? If so, is that all, or have you not drawn the parts you're going to remove? My suspicion is that your AHJ is going to want to see engineering drawings for a standalone scissor truss (which shouldn't be an issue), so it kinda doesn't matter how you attach to the old wood up there because the old wood is being completely superseded. May 12, 2023 at 22:23
  • Are your existing blue trusses currently tied together near the bottom? Do you have engineering that shows the red truss will meet local bldg code? Is your roof asymmetrical?
    – Huesmann
    May 13, 2023 at 12:33
  • Thank you for your comments Yes, the blue parts are to be retained. Since I cannot attach the new top chords to the roof sheathing, connecting the top chords seemed necessary. I have contacted a truss manufacturer and they will fabricate the truss in halves so they can be placed into position and joined together with pressed plates using a mobile press. I cannot get them to specify the type and spacing of fastening of the new and old two top chords. I guess I will need to hire an engineer to prescribe this using the truss manufacturers specs and drawings as a basis of design? May 13, 2023 at 16:32

2 Answers 2

1

If the new truss is completely replacing the old truss there really is no requirement that you robustly connect the two. You mostly want to make it so that the new truss can't move out of position under the load of the roof. Since you can't fasten the sheathing to the new trusses you'll do this instead.

I would use pairs of 3 in framing nails or screws every 16 to 20 in. This will be more than adequate. Don't sweat it if you can't reach all areas. If you get the majority of it nothing's going anywhere.

Construction adhesive is not to be considered structural and is generally only used as a sound prevention mechanism, such as under subfloors and in stair systems.

Don't forget to install a new "rat run" along the bottom cords of the new trusses. This would usually be a 2x4 laid flat near the center of the span, or two runs at the 1/3 points. For your short span one is plenty.


I don't have any official references for this answer. I base it on a couple decades of legitimate home construction experience under well regulated conditions.

1
  • Thank you for your input. I will ask the city if this approach is acceptable or if they need the engineer to prescribe the fastening spacing and type. May 13, 2023 at 16:38
1

I have done exactly what you are proposing. A big question is are the new trusses pre-engineered?

If they are, and the company building them understands what you are doing, they will provide you with an anchoring schedule. That will tell you the where and with what on how to nail/screw the new trusses.
The inspector will want to see that and verify that you did as the truss company instructed. (Easy way)

If you don't have that. Or if you made the new trusses yourself, be prepared to need an engineering certificate. That verifies that the work you did is structurally sound and you can cut away the undesirable parts of the old trusses. (harder way)

It is far easier to get approved if you get pre-engineered trusses with a schedule. (and save the cost of an on site engineer assessment)

3
  • I agree, and had thought about mentioning fastener schedules, but engineered trusses generally won't have been designed specifically to attach to existing trusses. They'll simply be designed for a given span and pitch. We'll have to see what OP says.
    – isherwood
    May 13, 2023 at 14:35
  • Thank you for your comments. This confirms what I anticipated needing an engineer to prescribe a fastening schedule. May 13, 2023 at 16:34
  • You may not need a engineer, if you had the trusses built, ask the company for a fastening schedule.
    – RMDman
    May 13, 2023 at 18:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.