Part two of Introduction to Square Rule joinery series
A reference plane is any geometric plane that is an important aspect of the building/structure; e.g. building exteriors, roof planes, floor levels, window and door openings. These are the critical points that the frame interacts with and are the dimension the frame must match to achieve the goal of a square, level and plumb building.
A simple way to identify reference planes is to consider where the frame interacts with other building components; floors, roofs, sheathing, windows, doors and interior partitions.
In timber drawings a reference face is noted with a filled in triangle on its surface. The point of the triangle indicates the direction of the closest reference face. On a reference face the point indicates the reference edge. An empty triangle indicates a non-reference face.
A primary reference face is a reference face that coincides with reference plane. A timber may have two primary faces e.g. a corner post has two exterior walls, an exterior beam supports a floor and has an exterior wall attached to it.
A secondary reference face, is a reference face that is designated by a convention and not its relationship to a reference plane; e.g. drawing note states that references faces are to the south and west. In some circumstances the secondary reference plane may not be an actual surface but a theoretical plane; e.g. the central axis of a post in a king post trust. An important aspect of the secondary reference face is that if a convention is used then it must be consistent throughout the frame.
On non-reference faces, joinery is laid out by pulling measurements from the nearest reference face.
A reference edge is the edge of a timber where two reference planes meet. This a true edge that can used accurately for layout. Station marks should be laid out/cut-in along the reference edge. Joinery is always laid out in reference to the reference edge when possible.
A station mark is a location along a the reference edge that indicates the start of the joint. It is the intersection of the reference face of the adjoining timbers. Station marks on drawings are depicted with a filled in carrot. Note: station marks are not typically the bearing surface of a joint
If you have any conventional construction experience you will know that a 2 x 4 stud does not actually measure 2 inches by 4 inches. Instead the stud actually measures 1 1/2" x 3 1/2". However before the the stud was "dressed" it likely did measure 2 inches by 4 inches; ie the rough-cut dimension. As you see, the nominal dimension is simply the name for the size of a timber and not the actual dimensions used. In conventional construction the actual dimensions are always less than the nominal dimensions. In rough cut cut timbers, the actual dimensions can vary from slightly less to slightly more and sometimes both. e.g. an 8 x 8 that measures 7 7/8" by 8 1/4".
The actual dimensions are whatever the timber measures in reality; as opposed to it nominal dimensions and ideal dimensions.
The ideal dimensions are dimensions that joinery is cut to. Ideal dimensions are almost always less than the nominal dimensions; occasionally the idealized dimensions will equal the nominal dimension.