Dormers

What is a dormer?

A dormer is a structural assembly that protrudes vertically out of a pitched roof structure. The purpose of a dormer is to increase the usable/habitable space within a pitched roof assembly. This is achieved by removing the height/space limiting rafters and installing structural timber walls to create a more conventional living space. The walls of the dormer are assembled in accordance with the common structural stud wall practices that we’ve already looked at. The top plates of the structural walls act as the wall plates for the roof assembly of the dormer to be pitched from. There are many different styles of dormers and dormer roof types that can be implemented within a larger roof structure. Dormers are very common within the scope of modern loft conversions, due to the additional living space that they offer. Some examples of these styles have already been included in some of the previous diagrams. Here we’ll look at some different varieties in more detail.  

What are the criteria of a dormer?

The primary criterion of a dormer is that it does not interrupt the criteria of the primary roof structure. It's critical that the finished structural and aesthetic criteria of the roof are not compromised by the installation of the dormer. For all intents and purposes, the geometry of the existing roof should remain unaffected by the dormer. It goes without saying that all of the standard criteria such as plumb and square must be met.  

How are dormers installed?

Whilst there are a handful of different dormer styles, the basic construction procedures remain relatively unchanged between them. A common style that most of us will have seen is the gable end dormer - a perfect example to examine in depth.  

The first step in the process is to determine the location of the front facing vertical dormer wall. By definition of a dormer, this wall protrudes out of the finished roof plane, and so this front facing vertical wall will be sat either on the wall plate above the masonry below, or further back into the roof space on the roof joists. The wall is assembled in accordance with the standard procedure of constructing a structural stud partition and is commonly faced with structural sheet materials to provide stability to the assembly. For this regular shaped dormer, the front wall will sit parallel to the wall plates and the ridge board. It will also typically sit parallel to other internal components that form the habitable loft space, such as an internal partition or stairway opening. The height of the wall off of the floor joists will determine the height of the room, as the ceiling joists of the dormer will sit on the top plate of the partition. If the dormer is installed as a part of a habitable space, then this wall must be full height, to provide a vertical surface all the way to the floor to allow for the installation of plasterboard inside the room. If the dormer is installed to provide light to an attic space, and is not being faced internally with plasterboard, then the upper portion of the dormer wall that protrudes out of the face of the roof can be installed upon a double beam that is installed between the two outside rafters that frame the dormer.  

With the stud wall assembled and situated in place, the dormer cheeks can be installed. Depending on the exact criteria of the dormer, this can be achieved in a few different styles. If the dormer itself is forming the boundaries of a room within the roof space, then it's typical for the cheeks/returns of the dormer to be formed in full height structural studwork also. If the dormer is small, fronts only a portion of an internal room, or is relatively far up the rake of the roof in relation to the finished floor level, then it may sometimes be the case that the dormer cheeks are pitched off of a double/triple rafter. In this instance, a sloped stud is installed on top of the rafter, with the possibility of additional slope studwork underneath the rafter, though this is not necessarily always the case.  

The dormer cheeks in this scenario are installed perpendicular to the front facing wall, tied together structurally at the corners with overlapping plates to provide stability to the total assembly. In this instance, it's beneficial for the structural sheathing to extend past the cheeks and onto the side of the front facing wall. The plate height of the dormer cheeks will run round level with the front facing wall of the dormer. Once these walls are established and terminated within the roof space, then the ceiling joists can be installed. These joists are installed in accordance with the regular joist procedures.

From this point, the rest of the roof and gable rake wall can be installed in accordance with the procedures we’ve already looked at, as well as the specific roof shape procedures that we’ll look at shortly. In this instance, the pitched roof of the dormer sails back into the main structure of the roof and terminates in two valleys - framed via lay boards.   

Now that we understand the basic construction process behind the assembly of a dormer, here are some diagrams of the other common dormer variations, with annotations highlighting the primary differences.  

Gable

Shed

Hip

Eyebrow

Flat roof