Canada Builds


The roof of a building functions to protect the occupants from snow and rain. It also functions as the plane where heat tries hardest to escape. In a design sense, it functions as a symbol of shelter and protection. A roof not only needs to function, it needs to play a role in completing and enclosing a space. When the roof is too low, the occupants become stressed. When the roof is very high (like a church) the occupants become more calm.

Roof Edge Details

Quickly sketch the roof edge to scale by showing wall, roof slope and overhang. The approximate distances for these pieces can be obtained by measuring from elevations or building sections already drawn or started. With this sketch complete, another drawing can be started with a better idea of how the pieces of a roof edge assembly will come together. The aesthetic considerations of this detail are also important and the geometry or ‘proportioning’ of the elements is key to creating a roof edge that does not look too heavy or ill proportioned.



Figure 1                Roof Edge detail studies


Figure 2                Roof Edge Detail

With a sketch completed, the next drawing can begin to resolve and indicate portions of the assembly with a little more clarity. The scale of the drawing is important. The level of detail that will show on a detail at a scale of 1:5 (3” = 1’-0”) is very different than the level of detail that will show at a scale of 1:50 (1/4” = 1’-0”). Detail sections such as the one shown above are usually drawn at a scale of 1:10 or 1:5 in order that the design assemblies can be understood in a clear and legible fashion on a piece of paper.


Figure 3                Roof Edge Detail being resolved

If the interior ceiling is flat with an attic above, the insulation can be placed directly above the horizontal roof plane.

Figure 4                Roof edge detail being resolved with quick notations

During the process of sketching a detail, add notes and questions and comments. These can be helpful to make determinations about the work and to bring clarity to the reader in terms of design intent. Placing notes and dimensions on initial sketches also helps to organize and edit the text for the final drawing. Line weights and hatching should also be considered at this stage in order to emphasize portions of the detail.

Figure 5                Roof edge detail with various pieces proportioned to scale

Draw the pieces to scale and pretend you are in the act of building the detail. What would be built first? Think about that question as you draw and the detail will become more meaningful as a process.

Figure 6                Roof edge detail

Add dimensions were necessary to clearly describe the intent. If a dimension is not useful to someone putting the detail together, then don’t show it. There will be some discretion involved in this kind of decision making process and the author of the drawing should ensure that the information provided is clear and easily understood.

Figure 7                Roof edge detail

In the detail above, the top of the window frame (head) is showing too close to steel lintel (beneath the brick). The bottom of steel lintel might be better aligned with the underside of the rough opening.

A  3/8” (10 mm)  or 1/2” (12 mm) space will be required between the window frame and adjacent substrate in order to provide for backer rod and sealant. See figures 2,3,4.


Figure 8                Roof edge detail studies for roof overhang with cathedral ceiling

If the interior space adjacent to the roof edge is vaulted or raised (also referred to as cathedral ceiling), the conditions of the roof edge and roof assembly will change. Without an attic space to ventilate the roof, it is still necessary to provide venting in the roof cavity in order to ensure that any trapped moisture is allowed to evaporate. With cathedral ceilings, this is accomplished by way of adding ‘purlins’ or cross pieces perpendicular to the rafters. The 1/2” exterior plywood sheathing is then place upon the purlins and the resulting detail is a roof plane that will satisfy the requirement for cross ventilation.