Convert timber roof rafters to trusses?

Question

We want to open up the floor plan of our house by removing the center wall.
The roof is 4:12 pitch with 3x8 timber rafters 4' on center, a 3x10 timber ridge beam, and 2x8 tongue and groove boards above. On one side of the house, there are 3x8 timber rafter ties 8' on center (every other rafter) that connect to the rafter at the exterior wall plate and span 15' to rest on the center wall.
The other side of the house has 2x8 ceiling joists with hung ceilings that also span 15' and rest on the center wall. There are no 3x8 timber rafter ties on the ceiling joist side of the house. The total span is 30' (15' to the middle of the center wall).
We would like to remove all the hung ceilings and ceiling joists from the one side of the house to expose the timber rafters and tongue and groove cathedral ceiling above, and remove the center wall to open the floor plan.
Question:
Can we make trusses out of the existing timber roof structure by splicing new 15' rafter ties to the existing rafter ties and adding 'W' support timbers mid-span fastened together using typical 5 finger timber truss base fan plates?

Lee Sam · Answer

No, you can’t change the framing from beams at 4’ on center to trusses 4’ on center, because you’ll overload the exterior wall footings.
Currently, one-half the roof load is supported by the center beam and footings with one- fourth the roof load transferred to the exterior walls and footings. By changing to trusses, you’ll transfer one-half the roof load to the exterior walls and footings.
I’m sure the exterior footings are not designed to accept double their current load.
Why not add beams at 4’ on center to match the one side with beams? This will keep the basic roof system the same. (You should double check the size and span of the ridge beam to insure it’s still acceptable.) You can still remove the center wall, just add posts and footings where needed.

Convert timber roof rafters to trusses?

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