What are ceiling collars?

Collars, also called ceiling collars, are timber components that are installed within a roof structure to create a flat ceiling surface. They also act to provide additional stability to the roof - providing restraint to the rafters under the load applied to the roof. These components are typically installed within a loft conversion/room to provide a flat ceiling surface. In these instances, the pitch is fairly steep in order to provide adequate height within the room. If the pitch was shallow, the usable space would be severely limited towards the edges of the room. As such, steeper pitches of 45 degrees or more are commonly used for loft conversions. That being said, whilst we gain space within the room, the height from the floor to the apex of the roof becomes uncomfortably high in terms of both construction, and the finished aesthetic of the roof. The tight apex can be unhomely, and also a nuisance to dust. These increased pitches also create longer rafters that require additional support. A solution to all of these issues is the introduction of ceiling collars within the roof. The collars are structural timbers that are installed horizontally between the rafters to create a flat ceiling surface. This height is typically around 2.4m off of the joists to replicate regular ceiling height. In structural applications, these collars are bolted to the rafters to create stability in the roof assembly by providing resistance to the roof spreading.  

How are ceiling collars installed?

To install collars, the height to the underside of the installation off of the floor must first be marked on the rafters. This can be easily achieved through the use of a rod cut to the height of the ceiling. This can be marked on the underside of every rafter, or on the underside of the end rafters, with a chalk line, string line, or laser level being used to pull a consistent line along the underside of the rafters. A good tip at this point is to temporarily nail battens to the underside of the rafters on these markings to help support the collars during the installation. In a similar format to the roof joists we studied earlier, the collars span between the outside edges of the rafters with an angled cut on them, slightly shy of the surface of the rafters to prevent them from protruding over time. The collars can be measured, marked, and cut once these layout marks are present. 

With the collars cut, they can be slid into place on top of the supporting battens and temporarily screwed into the sides of the rafters with coach screws. With the collars in place the underside of the ceiling surface can be checked for flatness with a straight edge to ensure that there are no serious deviations in the ceiling. We want to ensure that they are as flat as possible before the holes for the bolts are drilled, as the installation of the timber dogs between the timber components will cause an ever so slight deviation - such is life. Once the positions of the collars are true, a hole for the bolts can be drilled through each intersection as shown in the diagram, with the bolts being installed as required. Once the bolts are installed and tightened up, the supporting battens can be removed, and we are left with a perfectly flat ceiling ready to be insulated and receive the plasterboard.  

If collars are being installed, the location of their installation can be marked on each rafter via the pattern through the process of stepping out. Whilst not 100% accurate due to the discrepancies in the straightness and shape of each timber, these reference marks can help a great deal to get the collars situated before the final adjustments are made.