Assuming you use some object (and not a wall) as a joist, did you try:
- Use Modify furniture (joist), set co-ordinates accurately for the first joist.
- Move joist, and while moving press CTRL+SHIFT to duplicate along a straight line.
- Repeat as many times as you need joists.
- Set the last joist co-ordinates accurately.
- Select all joists and use Furniture-Distribute horizontally (or vertically).
- Use Furniture-Group if you need to move later (or duplicate/rotate or whatever for a new wall).

Uploaded an example to SF 3d models 373:

joists_etc.sh3d

ok