The stair carriage is support built into the underside of the staircase.
The carriages sit inside the stringers, under the treads and risers.
The carriages assist in a few ways, they remove a large amount of movement that may otherwise be noticeable, by removing this movement the life of the staircase is extended and the chance of treads or risers coming loose is reduced, thus lowering the chance of squeaky treads.
There are quite often 3 under a staircase, for one that is around 3′ or 900mm wide.
The timbers used are 2 1/2″ – 3″ in thickness, 3′ – 5′ in depth. This will depend on the length of the flight the depth of the well string below the tread and the depth of the landing on which it is supported.
On straight flights, the carriages are set between the trimmers at either end of the flight, with one against each stringer and one centrally through the flight.
When landings are built into the flights, the carriages will be fitted to the trimmers that border the mid flight landings.
The timbers will vary in size depending on the space between the bottom of the tread riser juncture and the bottom of the stringer, they may also be notched to house the back of the treads and bottom of the risers, this will allow for a deeper carriage to be used.
The addition of tread support plates may also be used, these plates are cut to fit up under the bottom of the treads and are attached to the sides of the carriages.
These plates are held into place under the treads by the means of glue blocks.
How the carriage meets the trimmer at the top of a flight.
The top riser will be set in front of the landing trimmer, the distance in front of the trimmer will leave enough room for the drum that sits between the strings or string and landing fascia, this distance will be determined by the handrail flow.
Allowance must also be made for the thickness of the drum.
The carriages will run up under the top riser until it leans against the landing trimmer.
How the carriage meets the trimmer at the bottom of a flight.
At the bottom of the flight, the trimmer is set a similar distance away from the face of the bottom riser as the top trimmer is set away from the face of the top riser.
the main difference being the bottom trimmer is set in front of the bottom riser and the top trimmer is set behind the top riser.
The bottom face of the carriage.
The bottom face of the carriage will finish flush with the bottom of the well string, the plaster bead will be set below this.
This will allow for the laths or plasterboard to be set onto the bottom of the stringer and be supported onto the bottom of the carriages.
The tread support plate.
The tread may be supported in the middle of the flight by the use of a plate that it fitted to the side of the carriage.
This plate fits up to the underside of the tread and is fixed by means of glue blocks as used between the treads and risers.
The bottom of the plate is cut at the same pitch as the bottom of the carriage, the bottom may be cut just above the bottom of the carriage so as not to interfere with the lath and plaster or plasterboard.
The plate may be screwed or the use of cut nails is common in fixing to the sides of the carriages.
The tread plates are typically 2/3 of the going in length.
The grain in the timber which is normally 1″ sawn will be vertical to reduce the chance of shrinkage across the width, reducing the efficiency of the supporting plate.
Find out more about the stringer drums.