First of all I think your design will have to be properly reviewed by an engineer to make certain it is safe.

My first thoughts are that the dovetail isn't the best for the joint between the tie beam and the plate, as the dovetail will shrink and it may pull out if it does:



When the dovetail drys out the distance shown above with red will get smaller and the dovetail pocket will not. Therefore it maybe possible for the dovetail to withdraw.

And with this being somewhat of a truss the location of the rafter foot to tie beam will have to be correctly done to make it strong.

Would it be possible to have three trusses one at each post location and then have purlins run between the rafters?

That way you can make each truss a good full length tie beam and connect them with an interrupted plate.

This would eliminate two trusses, and the problem of these two trusses joining the plate.

The purpose of a plate is to support the rafters. In a truss situation like this the bottom cord, the tie, is more important to supporting the rafters then the plate. This plate is then secondary as it just joins the trusses together.

In my opinion then the plate can be interrupted, and not continuous.

I'm very interested in others comments about your design, as well as my comments about it.

Jim Rogers