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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?

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    It may be possible but you should get an engineers stamp on your design. This is one area I have done and don’t guess the engineer will charge for the stamp but it would be better than taking advice off the internet had having your roof collapse.
    – Ed Beal
    Commented Jun 20, 2020 at 14:58
  • Can you? Sure! Will the roof sag or collapse? Only a structural engineer can, will, should tell you with any certainty. Splashing some cash for his services will be a much smaller expense than rebuilding your house on your own dime - you can be sure that insurance won't pay for damages after a modification like this without an engineer's stamp!
    – FreeMan
    Commented Jun 20, 2020 at 16:29
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    IMM, the weak point with homemade trusses are the joints. Factory made trusses use those big splice plates that are pressed into the wood under high pressure. I think many DIYr's would have a hard time coming up with a joint that's anywhere near as strong. Possible, but takes lots of effort and attention to detail.
    – SteveSh
    Commented Jun 20, 2020 at 16:52
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    I live in the mountains and the few structural engineers in the area are booked for months. Do you think this is something a 'remote' SE could design without doing a site visit?
    – John
    Commented Jun 20, 2020 at 22:50

1 Answer 1

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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.

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  • I'm not sure I understand your suggestion. The current roof system is exactly the same on both sides - 3x8 timber rafters 4' on center with 2x8 tongue and groove boards above. The difference is one side of the house currently has cathedral ceilings with 3x8 timber rafter ties 8' on center (every other rafter), and the other side has 2x8 ceiling joists 16" on center.
    – John
    Commented Jun 20, 2020 at 22:47
  • @John Ahhh...I see. Well that makes it easier. You still can’t change the loading on the footings, but if both sides of the ridge beam have 3x8’s at 4’ on center then it’s just a matter of removing the joists and verifying the ridge beam, post spacing and footing size is still adequate. Do the existing rafter ties connect to the 3x8’s on both sides of the ridge beam in the same manner?
    – Lee Sam
    Commented Jun 20, 2020 at 23:50
  • One side of the house has 15' timber rafter ties that span from the exterior wall plate to the center wall plate. They are nailed to the plates and the rafter. The other side of the house has 15' ceiling joists that span from the exterior wall plate to the center wall plate, and are nailed to both plates. There are no timber rafter ties on this side of the house.
    – John
    Commented Jun 21, 2020 at 14:57
  • So the ties are at the top of plate height...I see. I think you can splice and extend the ties to the other side and connect into top plate. Then add a vertical brace at center span from the splice up to the ridge beam. Then remove the joists. This will give you the “open” feeling you’re looking for without transferring so much load to the existing exterior walls and footings.
    – Lee Sam
    Commented Jun 21, 2020 at 15:59

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