Manual of Purpose-Made Woodworking Joinery

Site measurements 103

• and relates to an upper and a lower ffl (finished
  floor level). To check the ffl at certain critical points
  like doorways and stairwells, a temporary bench mark
  (TBM) – a short horizontal line underscored with
  an arrow- head – is sometimes marked at doorways
  and stairwells, as reference points set at 1m above ffl.
  Carpets are discounted, but if a sand- and- cement
  screed is to be overlaid with a parquet floor or quarry
  tiles, etc, then the top surface of these is the ffl.

Determining the bulkhead position

Figures 6.29(a)(b): The bulkhead trimmer- or

trimming- joist is that part of the stairwell- opening

above a stair that – if wrongly positioned – can cause

head injuries. AD K1 refers to it with their Headroom

rule of NLT 2m above the pitch line.

Although forming the trimmed stairwell opening

in a timber- joisted floor is usually done by carpen-

ters, not joiners, such related work should be part of a

joiner’s knowledge – so certain points are covered here.

To determine the critical position of a stairwell’s bulk-

head (apart from taking scaled measurements from the

architect/designer’s drawing), a simple calculation of

the known step sizes, graphically illustrated below, can

be made.

Start with the headroom requirement of 2m above

the pitch line, add 40mm for tolerance and add the

bulkhead’s floor- depth of, say, 255mm (being 225mm

joists + 18mm chipboard + 12mm ceiling). This gives

a total of 2. 295m to be divided by a known rise of, say

180mm; which gives 12.75 divisions, i.e. 12¾ risers.

Nowadays, the need for accuracy is no less,
but the combined effect of simpler stair design,
factory production using CNC (computer numeri-
cal control) processes and CAD/CAM (computer-
aided design and manufacture), and advance
ordering to meet contractual commitments, has
resulted in the stairwell – more often than not –
being formed to suit the staircase, rather than the
other way round.
In either case, the following points should be given
attention:

Total rise of stairwell

Figures 6.28(a)(b): To obtain truly level treads, vertical
risers and newel posts, the ‘total rise’ of the stairwell
must be divided precisely by a number of equal divi-
sions for risers. As first illustrated in Figures 6.1(c)
and (d), increases or decreases to the ‘total going’ –
although also critical in most stair designs – do not
affect the stair being level or plumb. Figures 6.28(a)
and (b) illustrate the use of a timber batten known as
a ‘storey rod’ that was traditionally used for checking
the total rise of an established stairwell. Once placed
in position and marked at the top, it can then be
checked more carefully with a rule than a rule might
convey with in- situ measuring. The rod – if straight
and parallel – can also be used to plumb down the
face of the trimmer- or trimming- joist for the purpose
of checking the total going of the stair.
Note that the required storey- height for a pre-
designed stair usually depends on the blockwork
(or brickwork) being carefully controlled up to the
bearing- height for the floor joists or joist- hangers

Concrete ground-slab

Bench mark (TBM)

Storey rod

Take flooring thickness into account,

as seen here marked on storey rod

Check total going

Figure 6.28 (a) Using a storey rod to check the total rise
and the total going.

18mm chipboard

floor on rigid

insulation

TBM

FFL

FFL

18mm chipboard

floor

TBM

Figure 6.28 (b) When checking ‘total rise’, take the

different, finished floor- levels into account, if – as is likely –

the floors have not yet been laid.