Help! Roof confusion

Hi folks. I'm currently renovating the Minshuku house from the Sakura DLC as my new office, and I'm having trouble with the decorative beams in the upstairs (Pic 1.) They're made of roof pieces, and I either accidentally deleted some of them or the house just doesn't come with them all, so there are beams on one side of the roof but not the other. I like the look of the beams and want to keep them, as well as add the ones that are missing, but I genuinely cannot figure out how to recreate them exactly. In general, I have trouble with the roof tool, but I especially can't get it to cooperate now.

Part of the trouble is that I can't tell where the roof exactly starts, and I want the beams to be identical. The other difficulty is that I sometimes can't get the roof tool to work, and I don't know why; I'll drag the roof size, hit enter, and just... nothing will happen. No roof. I've been sure to use the same roof style settings (Pic 2.) It's driving me crazy, and I'm considering giving up and removing all the beams. Anyone who's good at roofing, can you try and help me figure out how to place these?

1. What the beams look like

2. Beam style

3. Beam shape