2d timber frame drawing with reference plane

Choosing Reference Faces

Part four of Introduction to Square Rule joinery series

Note: Due its length this post has been divided over more than one page

Controlling what matters

Focus on what matters and let the rest go!

Now lets apply some of the key concepts of earlier posts. In practical terms, if the imperfect timber we are using has two relatively square surfaces it is easier to imagine that the perfect timber inside begins on these two surfaces(these are the reference faces) and the remaining perfect faces lie within the remaining imperfect timber. If we align the reference faces of the timber with the reference planes of the building then the important dimensions and functions of the building/structure will be "fixed".

Clearly there is a direct correspondence between reference planes and reference faces. The art and craft of timber framing begins by properly pairing the reference planes to the appropriate timber surface and thereby creating a reference face. Pick two reference faces and you have a reference edge along which you can locate station marks where the joinery is laid out and cut.

Drawing & Frame Requirements

Building Requirements

Recognizing the reference planes in a building is the first step. In drawings produced specific for timber framing; i.e. frame or shop drawings, the reference planes will be those designated(i.e. either by notation or by dimension strings to timber faces (not to timber center-lines). When producing your own drawings or when using architectural drawings that are not specific to timber framing you will have to derive the reference planes.

2d timber frame drawing with reference plane
Reference Planes of a Simple Frame
Common Building Reference Planes
  • Exterior Walls
  • Roof Plane
  • Floor Plane(s)
  • Top of Foundation (especially useful for sills)
  • Window & Door Openings
  • Interior Partitions
  • Stairways
3d timber frame with reference planes
Reference Planes in 3d

Structural Requirements

Many of the structural requirements of the frame correspond directly or indirectly with building components. Joists have floors/sub-floors attached to them which is a direct relationship; the top of the joist is a reference face to which the sub-floor reference plane is attached. The joists will then bear on a beam which creates an indirect relationship; the top of the beam is a reference face because it supports joists which in turn support the reference plane. When the joists are installed flush with the top of the beam this relationship is obvious, but it is not obvious when the joists rest on top or lap over the beam.

A simple way to identify structural requirements is to recognize the force(s) that the timber is trying to resist. The surface closest to the force it is resisting is the reference face. Typically structural requirements dictate that the reference face correspond to the upward and outward faces of the timber (but not always).

Conventions

Most of the time building and structural requirements dictate reference face selection, but when these are absent then the rules for choosing reference faces becomes more subjective. Under these conditions a consistent convention should be used. An example would be reference faces are "towards the nearest outermost reference plane". But what if the timber or assembly(either wall or bent) is dead-center in the building. Then a convention such as "undesignated reference faces toward the south and west" will be more useful. Any convention (or combination of conventions) can be used so long as it covers all circumstances and is consistent throughout the frame.

Best practices would be to use frame drawings that dimension to reference planes/faces, rather than to center-lines, but if they are missing establishing and writing down the conventions should be done before calculating any joinery locations. Defining every reference plane and knowing where every reference face is a crucial step before locating joinery.

With reference planes identified and the conceptual reference faces visualized, then the next step is bringing that information to the timber. Part two of choosing reference faces focuses on understanding the characteristics of the timbers themselves.

On to part two …. Timbers have needs too!!….Next