Ladder frames

This method of constructing a boxing sees two ladder frames being assembled at right angles to form a solid timber structure over which the plasterboard can be installed. The advantage of this style over the others is that the carpenter can create the frame and insulate the inside, without requiring any plasterboard to assemble the structure. This means that the boxing can be formed and left for the tackers to board as they see fit. Additionally, the solid timber frame of the ladders provides more stability to the boxings and the plasterboard, making it a more durable choice.

To begin constructing this component, we must mark out the desired dimension of the boxing on the floor and the surrounding walls. For the most part boxings are square in their plan view, though rectangular boxings are required in some instances. When constructing these corner boxings, we seek to minimise the overall size so as not to impede on the living space within the room. Therefore, the inside face of the ladder frames when installed should be around 10mm away from the pipe to keep the overall size down, without creating an acoustic bridge. To mark out the dimensions on the floor, a square is registered on the surrounding walls to mark a tangent that touches the pipe. Using a square, check that these two perpendicular marks meet at a 90 degree angle on the outside corner of the boxing. The chances that everything here is perfect is slim, and so slight adjustments should be made in order to minimise any extreme defects. Measure away from these marks 10mm, forming the registration lines for the back of the ladder frames.

Using an offcut of batten, mark the thickness of the ladder frames on the floor off of the registration marks. Typical ladder frames are constructed from 2”x1” batten. Depending on the exact orientation of the ladder frames, which is based on discretion, the overall dimensions of the boxing can be seen. In the diagram displayed here, we can see a plan view of how the finished ladders will appear within the layout marks. With the floor suitably marked out, these lines can be plumbed up the walls. It's important to check here that the walls themselves are plumb and not leaning inwards or outwards, as this will affect the overall width of our frames. If they are leaning out of plumb, the frames should be made to the shortest width, with the remainder being packed. To measure the width for the frames, simply measure the layout lines on the floor. In this assembly technique, one of the ladders receives into the side of the other, with the two ladders being different widths. The widest ladder can be measured from the wall to the outside registration mark. The smaller frame can be measured from the wall to the inside face of the other ladder.

The ladder frames themselves are constructed of two long outer battens, with smaller battens fixed in between them to create a specific width extrusion. To determine the length of the small battens, simply take the overall width of the ladders and deduct twice the thickness of battens from both of these measurements. The remaining length is the length at which the smaller battens should be cut at. Setting up a stop block on the chop saw can make quick work of cutting these short battens. The vertical lengths of the batten should be set out with a 400mm layout for the smaller studs, to accommodate the plasterboard that will face the ladders. With the battens marked out and all of the short battens cut, the two ladders can be assembled. This process is much the same as assembling a soffit or gable ladder, with a nail gun making quick work of the assembly.

Once the ladders are assembled, they can be fixed to the walls on the layout marks. An SDS can be used in combination with plugs and screws to achieve a solid fixing into both of the masonry walls. Packers may need to be used here to plumb up the outside edges of the ladders. Once fixed to the walls, the two ladders can be screwed together so that the outside corner is flush. At this point final checks can be made to ensure the internal corners are square and the boxing is plumb. With the boxing assembled, insulation can be fitted around the pipe to dampen any noise that is created within the boxing and insulate any cold spots that could occur within the pipe.

Ladder frames in straight runs

In this scenario two equal sized ladder frames are installed either side of the pipe. Another ladder can be made to fill in the gap between the two, or a single small batten can be used to tie them together. Alternatively, plasterboard can be used to bridge the gap. Attention should be paid when measuring the width of these ladders to keep the boxing square.

Singular timber grounds in corners

This style of boxing is faster to install and requires less timber in order to achieve the desired outcome. The drawback of this though is that the overall boxing is not as rigid, with less support behind the plasterboard. Additionally, the plasterboard itself provides rigidity and structure to the boxing, requiring the carpenter to cut and install the plasterboard during the construction of the boxing. This isn’t a hard process but requires plasterboard to be on site - which may conflict with the schedule. The process for constructing this style of boxing is as follows. Single timbers up the wall and cut the plasterboard to the corner point. Another batten is attached to the edge of the first plasterboard, in order to provide a pickup for the secondary piece of plasterboard.

Singular timber grounds in straight runs

The same principle as before, with single battens up either side of the pipe. 2 battens in the corners where the plasterboard faces intersect. This is not rigid at all though and relies entirely on the plasterboard for its strength.

It's important to note that the overall construction of horizontal boxings is exactly the same, though differences do occur in terms of fixings, as the front facing ladder/timber will typically receive into the timber joists that run above the boxing.

RSJ Boxings

In modern residential construction, RSJs are a common structural component that are used in a variety of applications. In scenarios where they project within a room, ladder frames or pegs must be installed in combination with plasterboard to provide an aesthetic surface finish. The process is as follows.

What is an RSJ boxing?

An RSJ boxing is a timber assembly that is fitted within or around a steel in order to provide a timber surface for the plasterboard to be installed upon. This concept may be applied to both horizontal load bearing beams, or vertical load bearing posts.

What are the criteria of an RSJ boxing?

The timber framework that encases an RSJ must be installed square, plumb, and level in each of its components. All of the components must be fitted tight within or to the steel in order to minimise the overall size of the boxing. Insulation must be installed within the steel where applicable before the plasterboard is installed. Appropriate fire rated plasterboard must be used to encase the steel.

How are RSJ boxings formed?

When boxing in steels there are a handful of different approaches. Here we will look at the overall assembly of the different variations.  

Solid blocking

This approach involves installing solid timber blocking that is wedged tight between the two leaves of the steel. By cutting these pieces of blocking ever so slightly oversize, the two leaves will pinch the solid blocking in place. These timbers can then be faced with plasterboard. The plasterboard should be installed to the appropriate vertical height. When wedging the timbers in this method, it's not impossible for these timbers to come loose over time, though with the plasterboard installed with appropriate beads this won't affect the stability of the assembly. In some scenarios it can lead to cracking in the finished plaster, though this is very rare. This method is the fastest to box in the steel, and results in the thinnest boxing, a consideration when space is tight, or the desired aesthetic is to be sleek and modern. To fix the bottom board, modern adhesives can be used. If the side faces are cut flush to the bottom of the steel, a board can be cut for the bottom that is just under the total width of the boarded steel. This method requires props though to hold the board in place whilst the adhesive dries. It does result in the most minimal boxing size for the steel though. Alternatively, 2 battens can be attached to the underside of the steel via an overhang on the side face pieces. This provides a solid timber pickup for the bottom board to be screwed into. This method is faster, though adds additional height to the finished depth of the boxing.

Ladder frames

This method of boxing is very similar in concept to that of regular utility boxings, utilising ladder frames for its construction. 2 ladder frames are created to be installed either side of the steel, fixed directly into the ceiling joists above. Alternatively, timber pickups can be installed in the web of the steel for additional fixing points for the ladder frame. The two frames can be fixed plumb hanging from the ceiling, with the bottom piece of plasterboard providing the necessary spacing between the two. Another method involves the implementation of another ladder, or single timber that spreads the two vertical ladders apart at the bottom. Whilst this method is debatably more durable over time than just wedged timbers, it is far more time consuming to assemble. It also creates a much larger profile around the steel that forms the finished boxing. That being said, these characteristics can be manipulated to create a boxing that marries into other components around the room.  

It's important to note that in all scenarios, any voids within the steel should be insulated with fibreglass insulation where the steel supports cavity masonry or is otherwise in a cold bridging implementation. This is to avoid cold spots.