The diminishing flier tread.
An unsung hero of staircase design.
When it comes to wreathing handrail design and a comfortable flow to the staircase, this is probably the least used tread designs and apart from the feature tread at the start of a staircase has the most impact on how the staircase looks and feels in use.
The diminishing flier tread or tapering tread, can drastically change the way the staircase flows underfoot and the visual impact of the handrail.
The diminishing flier will lead you gently into and out of the turn and the change the pitch, thus making the turn flow and helps remove any kinks in the wreathing handrail by divorcing the change of direction and the change of pitch.
Wreathing 180º without diminishing flier.
The most common design when coming to a corner is to just add winders, there design disadvantages to this.
- The treads become very narrow at the ends and do not always leave space for the spindles to sit on the treads without hitting the nosings of the tread above.
- getting even spacing that matches that or is close to that down the flight is hard to achieve.
- The handrail will change pitch and direction at the same point, often creating a visual kink the the flow of the handrail.
In the image here you can see the plan of this style of corner with the spindle spaced out evenly on the straight goings.
We use a typical setting out, with the handrail centre lines set at 1 goings distance apart, this will of course be exaggerated if we use ½ a going as the handrail centreline offset.
To keep the walkline goings correct around the winders: so they are the same as in the straight flight. The lead in and out riser lines are just outside the springing lines for the drum.
This will result in the wreathing 180º turn for the handrail to have a ramp built in at each end, which in turn will require thicker material when manufacturing the parts.
We can also see that the spindles are awkward to position, keeping constant spacing.
The nosings interfere with the spindle bases and will require either the spindles being housed around the tread or the treads being cut to accept the spindles.
When we carry the plan view up to the stretch out, you can see that the handrail changes direction and pitch in very close proximity, this is even with moving the lead in and lead out risers away from the springing points; to keep the walkline constant through the turn.
In the stretch out, the spindles look as though they have clearance in front of the nosing above, this may be the case at the handrail centre line, which the stretch out represents but not at the stringer face, as seen on the plan view, the spindles cross the nosings.
Wreathing 180º with diminishing flier.
When the plans are adjusted to include a diminishing flier, the tapered ends of the treads are increased in size by using a section of the straight flight.
This will increase the surface area of each tread, making it easier to position the spindles.
The number of diminishing treads used may vary to be able to space the spindles at the desired distance between each other, dependant on how close you want to keep the spacing around the turn in comparison to the spindle spacing on the straight fliers.
The layout of the staircase and the well size will also make a difference.
Normally using one flier either side of the winders will be sufficient.
The simplest way of adding the diminishing fliers is to use half the going of each tread either side of the winders, this will keep the spindle at that point in the same position on plan view but in the elevation it will lift it by one going at the start of the turn section and lower it by one going as it exits this new section.
The walk line will remain the same length divided by the number of treads required while the handrail centre line through the turn section will will increase by half a going at each end.
In this image you can see the spacing for the spindle is closer to that of the straight flighers and evenly spaced through the turn.
Reposition the riser lines.
With the new divisions along the handrail centre line through the extended turn length.
It is now possible to rotate the riser lines from their original position on the handrail centre line to the new one, while keeping the walk line position as it is.
The nosings may be drawn on again to confirm there is space for the spindles to be seated on each tread.
The change of pitch will now have moved to the riser line at the start of the first diminishing tread.