Manual of Purpose-Made Woodworking Joinery

Blockboard or plywood as shelves 135

For example, with a shelf- span of 900mm, the equa-

tion would be:

Thickness of shelf =^900 + 2 = 20mm

50

Two further examples, with shelf- spans of 1200mm

and 450mm would be:

Thickness of shelf =^1200 + 2 = 26mm

50

Thickness of shelf =^450 + 2 = 11mm

50

Note that calculated shelf- thicknesses usually have

to be slightly increased or decreased by a milli-

metre or two to suit commercial timber sizes and

manufactured-board thicknesses; but if a heavy load

of large hardback books or glossy magazines and the

like are anticipated for the shelves, decreases should be

avoided.

BLOCKBOARD OR PLYWOOD

AS SHELVES

Figure 7.2 (i) 9mm- thick, lipped face- edge of 18mm- thick

blockboard shelf; and (j) Similar lipping bonded to face-

edge of 18mm- thick plywood shelf. Note the sharp arrises

removed from the lipped edges.

(i) (j)

Figures 7.2(i)(j): Although I devised the shelf formula

for good quality redwood and hardwood shelves, I find

that it also works well on such material as blockboard

(when the core strips run parallel to the span) and

plywood (when the face grain of the outer plies runs

parallel to the span). Also, these materials have the

advantage of being more stable and will not suffer

from warping or cupping. Note, though, that as illus-

trated at (i) and (j), shelves made from these materials

need to be lipped on their face- edges to improve their

finished appearance.

First, though, having likened the shelves to floor
joists and because the formula for the shelves evolved
from the joist- formula, I give below an example of the
simple rule- of- thumb formula that exists for depth of
floor- joists related to span. The formula was traditionally
expressed in feet and inches, but was metricated by
technical authors in the 1970s.

Depth of floor- joists related to span

The formula for this is easily remembered as: span over
two, plus two, or:

Span

+ 2

2

This is expressed literally as: Depth of joists in centi-
metres equals span in decimetres divided by two, plus
two. Or, as an equation as:

Depth of joists

in centimetres

=

span in decimetres

+ 2

2

For example, with a joist- span of 4 metres (40 decime-
tres), the equation would be:

Depth of joists

in centimetres

=^40 + 2 = 22cm (220mm)

2

Note that The Building Regulations’ Approved
Document A1/2 Floor Joist Tables for a small house
recommends 195mm × 50mm C16 structurally graded
joists for a 4m clear span, which highlights a 25mm
difference between the more precise Joist- Tables and
the rule- of- thumb formula. This is because the latter
has to slightly overcompensate to make up for its
mathematical elementariness.

Thickness of shelves related to span

The formula for this is easily remembered as: span over
fifty, plus two, or:

Span

+ 2

50

This is expressed literally as: Thickness of shelves in
millimetres equals span in millimetres divided by fifty,
plus two. Or, as an equation as:

Thickness of shelf

in millimetres

= span in millimetres + 2

50