2. Types of Straight Wooden Stairs

Table 1: (see Fig. 6) Such stairs may be designed with risers or without risers

Saddle stairs

Saddle stairs are staircases where the steps are set on the sawn-out or attached rise triangles or on bearing blocks fitted at the strings.

Figure 4 - Saddle stairs - (1) sectional view of saddle stairs 1 stair string, 2 step, 3 stair foot (bottom step), 4 stair head (head step) - (2) sawn-out stair string s - height of rise, h_u - height of string bottom, 5 sawn-out rise triangle, 6 string bottom

For stairs with sawn-out or attached rise triangles the string bottom must have a sufficient height. The height of the string bottom is calculated by means of the following formula:

	(Formula 1)
hu = height of the string bottom
s = height of rise
Wd = string thickness

Table 2:

Perpendicular height (h_u) of the string bottom for wooden saddle stairs

Stair flight width	Stair string thicknes	Perpendicular height of the string bottom
B	Wd	hu
in mm	in mm	in mm
up to 1200	60
from 1200 up to 1250	65
from 1250 up to 1500	70
from 1500 up to 1750	75
from 1750 up to 2000	80

Saddle stairs can be designed as pure saddle stairs or as combined saddle stairs. There are three types of pure saddle stairs.

Figure 5 - Types of saddle stairs

(1) stairs with sawn-out rise triangles
(2) stairs with attached rise triangles
(3) stairs with bearing blocks nailed onto the strings

Stairs with attached rise triangles and stairs with bearing blocks fitted at the strings are mainly built as temporary stairs. Such stairs are used temporarily only, such as in the preparation of a building site as access to site accommodations, as access to working platforms, etc.

They are removed and disassembled later. Thus the material can be reclaimed and reused.

When the sawn-out rise triangles of the strings are cut-off from the string bottom, such wood can be reused as posts (see Fig. 4/2).

Similarly the squared timber of stairs with attached rise triangles can be reused.

Combined saddle stairs can be built into apartments or smaller dwelling houses as floor-to-floor staircase.

Such stairs are not removed but remain permanently in place until they are worn out.

Figure 6 Combined saddle stairs - 1 sawn-out stair string (outer string), 2 mortised stair string (wall string), 3 screw rod with rosette, 4 wood screw (countersunk head), 5 step (mortised at left-hand side, saddled on right-hand side)

Figure

The steps (treads) are mortised into the wall string and set on the outer string.

Such stairs may also be designed with risers.

The visible end-grained wood of the outer string is to be lagged by veneering.

What are "temporary stairs"?
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Inserted stairs (skeleton-step stairs)

Inserted stairs (or skeleton-step stairs) do not have any risers. Therefore, they should be used as access to ancillary rooms or be built into ancillary rooms only, such as stairs to or in the attics.

The steps (treads) of such stairs are inserted (slid) into the mortised step supports from the front.

Figure 7 - Inserted stairs (skeleton-step stairs) - (1) sectional view of inserted stairs - 1 stair string, 2 step, 3 stair foot (bottom step), 4 stair head (head step), 5 lower (bottom) stair-apron, 6 upper (top) stair-apron, 7 string bottom, screw rod - (2) (3) Insertion (sliding-in) of the step into the insertion slot 1 stair string with mortised insertion slots, 2 step, 3 insertion slot, 4 line of inclination of the stair flight. 5 step with facing board to cover the insertion slot, 6 lower (bottom) stair-apron, 7 drilled hole for the screw rod, 8 stair string with arris for insertion, 9 stair string without arris for insertion

The steps may be designed with or without arris for insertion. In order to cover the mortise, the front of the step can be provided with a facing board (see Fig. 7/2).

The string height depends on the angle of inclination of the flight and can be measured from the drawing floor (Fig. 16).

Figure

However, it should not be less than 260 mm.

If no arris is applied for insertion, the stair strings must be drawn together by means of screw rods to fix the steps.

Figure

Screw rods are round metal rods with threads at both ends and with nuts.

The nuts at the outer string can be covered by rosettes. Screw rods are also recommended for stairs with arris for insertion.

What are "inserted stairs" (skeleton-step stairs)?
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Mortised stairs

Mortised stairs (stairs with mortised treads) have risers. They are typical floor-to-floor staircases in dwelling houses.

The riser serves a double purpose. It supports the step and prevents deflection of the step under load and it also protects the user of the stairs from objects falling down from other flights upstairs. The string height depends on the angle of inclination of the flight and can be measured from the drawing floor. However it should be at least 260 mm.

The step thickness depends on the flight width.

Figure 8 - Sectional view of mortised stairs - 1 stair string, 2 step, 3 stair foot (bottom step), 4 stair head (head step), 5 lower (bottom) stair-apron, 6 upper (top) stair-apron, 7 string bottom, 8 string top, 9 screw rod

Table 3:

Step (tread) thickness of solid wood in planed condition for wooden stairs

Stair flight width	Saddle stairs and inserted stairs with a tread width		Mortised stairs
	up to 240 mm	up to 300 mm
	in mm	in mm	in mm
up to 800	35	32	30
from 800 up to 1000	40	35	30
from 1000 up to 1300	45	40	35
from 1300 up to 1500	50	45	40

The riser thickness should be within 15 to 20 mm. The strings of mortised stairs should be drawn together by at least two screw rods. The nuts in the outer string can be covered by rosettes.

What types of stairs can be built?

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