Cantilever Projections
Utilizing the principles of leverage, in combination with the standard practices of the formation of structural studwork and joist systems, cantilever projections can make interesting feature within modern construction. Whilst not as common in modern residential construction, these principles are often seen in larger scale commercial, industrial, and architectural statement pieces.
What is a cantilever projection?
Cantilever projections, also referred to as bay dormers, or cantilever bays are self supporting assemblies that are stitched onto the side of a building, hanging outside of the lower footprint of the building. They might also be installed as a feature inside larger homes. The purpose of these structures is to create additional floor space on the upper floors of a building, without the necessity for a load bearing support directly underneath it. Instead, through carefully engineered specifications including considerations of downwards load in combination with the fulcrum of the assembly, timber or steel beams are projected away from a vertical structure to support the assembly on top. Whilst some form of vertical support is required, the end of the projection is not directly supported, with the load being distributed back inwards towards the vertical support. As we can see in the diagram of a typical cantilever projection, the joists are positioned and formed in a fashion to support the assembly outside of the building. Inside the building, the joist ends are bearing on an internal structural wall, with a structural stud wall on top of them, bearing the load of the roof joists. Two double joists are situated at each end of the projection to support the external timber walls that will form the assembly. The joists are supported at their final point by the internal skin of the external masonry, before they protrude out of the building. In this scenario, the projection is only a small portion of the total length of the joists. Based on the principles of a leverage, a very large force would have to be applied downwards to the projection joists in order to push them downwards. This factor, coupled with the downwards load applied to the other ends of the joists by the load bearing wall means that the projection is very well supported. Here we shall look specifically at how these projections are formed.
What are the criteria for the formation of a cantilever projection?
There are many mechanical principles that are involved in engineering and assembling a projection in a safe and structural manner. Alongside those considerations, there are additional criteria such as weatherproofing, fireproofing, and insulating. The external wall that defines the projection is typically formed from timber, and as such must abide by the standard timber framing principles in terms of insulation. A full fill insulation system must be installed between the studs, with insulated plasterboard typically being required over the face of the studs once the primary insulating has been completed. The outside face of the studs is faced with sheathing plywood to provide stability to the assembly, preventing racking. A waterproof membrane must be installed and counter battened over the face of the sheathing plywood to correctly weatherproof the assembly. In terms of meeting fire regulations when assembling the structure, all faces of the projection must be compliant. As we would expect, a fire rated cladding system is installed on the battens to provide adequate fire protection to the structure. In addition to this, some form of fire rated cement board is applied to the underside of the exposed joists to provide adequate fire protection. Besides these factors, the bay will typically support part of the roof assembly, and so a layout consideration must be accounted for here.
How are cantilever projections formed?
The formation of a cantilevered projection is straightforward, utilising many of the techniques and procedures that we’ve already looked at. In essence, the majority of the bay assembly is formed through carefully placed joists and structural studwork, with the finishing steps being the roof structure installed overhead. Here we shall look at the procedure of forming a cantilevered projection.
The first step as always begins in the layout stage. By consulting the drawings, we can determine the critical dimensions of the projection that will aid us in our layout. Importantly, we need to ascertain how wide the structure is to determine the overall layout of the joists and the position of the outside beams, as well as the total depth of the projection as it leaves the building to determine the length of the joists. In a new build scenario, the joists for the bay can be freely rolled before the next floor of masonry is erected, giving the bricklayers a hard edge to finish to. In a renovation scenario, the layout is often much more involved, having to accurately remove existing masonry to allow for the joists to sail through. Carefully placed holes may need to be made at the correct height on the internal masonry to provide an adequate bearing surface for the joist ends. Although the full criteria for these joists are expanded somewhat, the basic O/C layout and solid blocking installation principles must still be adhered to. The two outside beams may fall off of the layout of the rest of the regular joists that run parallel with them. It's important to continue the intermediate joists between the two beams on the same layout as the regular joists.
With the layout of the joists considered, the joists can be prepared by cutting them to an approximate length (slightly longer than required to be accurately trimmed later), and any doubles or triples can be formed as specified within the drawings. All of these procedures are in accordance with the standard practices that we’ve already looked at. With the timber prepared, the installation can begin in accordance with the determined layout. Typically, the outside edges of the timber studs that form the side of the projection denote the location of the outside edges of the beams, ensuring all of the components are kept flush for the installation of the structural sheathing material. As these beams will dictate the layout of the stud walls, it's imperative that they are perfectly parallel with one another, at a fixed distance apart along their entire length. This ensures that the projection remains a consistent width in its entirety outside of the main structure. It's also preferable for these beams to exit at exactly 90 degrees from the main structure, though this is dictated by the layout of any studwork to be erected internally. Between these two fixed locations, the remaining single joists are installed at 400mm centres, continuing from the layout of the regular internal joists.
At this point in the process, the exact termination point outside of the building for the joists is not critical, as they will be cut off neatly to a line for the installation of the front trimming timber/s once the installation of the joists is completed. Once the joists are located in place and battened for stability, as well as blocked with solid timber, the ends can be cut off to a line for the reception of the facing timber. Typically, another double timber is mechanically fastened to the ends of these joists to provide a full bearing for the outwards facing studwork at the front of the projection. The thickness of this beam is allowed for when marking out the joist ends, before cutting them off straight to a line. Once the joists are cut off, the double timber can be installed as required. With the joist assembly fully installed for the reception of the timber walls, the framing for the projection can be undertaken.
The structural stud walls for the bay area are formed in much the same way that regular structural studwork is formed. The bottom and top plates are doubled up to provide stability to the over assembly. Joints in the plates are staggered with supporting studs underneath the joints. The studs are placed on specific centres to receive the external sheathing. The internal corners of the studwork are formed to create pickups for the internal finished sheathing.
Any window openings are formed in accordance with the procedures that we’ve also studied. The bottom plates of the studwork can be nailed directly into the beams, with mechanical fasteners being used to secure the outermost studs to the masonry.
Once the bay area is formed, the final step is for the roof to be pitched onto it. This roof may be a continuation of the main roof of the building, or a separate roof structure that sits against the external wall of the building.