A BOM is a list of all materials needed to complete a structure.
It is based on takeoffs and
estimates of the materials needed. It includes item number (parts and
description, unit of measure, quantity and, where called for, the stock
size and number, and
sometimes the weight. The carpenter uses it when ordering materials.
A BOM is usually made up by the draftsman when the original drawings
are prepared. However,
when no BOM accompanies field prints, it must be developed up by the construction
crew. For this
reason, a carpenter should be able to develop a BOM, as well as work with
one. Accuracy can best
be obtained by having a separate bill prepared by at least two estimators.
The bills may then be
compared and one copy corrected or both used to make up a final BOM.
Before a BOM can be prepared, a materials takeoff list and a materials
estimate list are
MATERIALS TAKEOFF LIST
The first step leading to preparation of a BOM is a materials takeoff
list. This is a list of all parts
of the building, taken from the plans, usually by tallying and checking
off the items indicated on
the drawings and specifications. Both architectural and engineering plans
provide the names and
sizes of the items that are to be listed.
For example, Figure 3-1 (page 3-2) shows a plan for the substructure
of a 20- x 40-foot TO
building. Table 3-1, page 3-3, is the materials takeoff list for this
building. This list identifies all
parts of the building, starting with its base and working upward. The
following paragraphs are an
example of computing the materials needed for the footers.
Look at the first and second columns of the materials takeoff list.
The first column gives the item
(footers); the next column gives the number of pieces (46) needed to make
up the item.
The 20- x 40-foot building shown on the plan requires 15 foundation
posts. Since three pieces are
needed for each footer, a total of 45 pieces is needed.
The length in place (1 foot 5 inches) is the actual length of the member
after it has been cut and is
ready to be nailed in place. The size (2 x 6) refers to the nominal size
of the lumber. The length
refers to the standard lengths available from the lumberyard or depot,
such as 8-, 10-, and 12-foot
pieces of stock.
Select the most economical length for the 15 footers. Convert the required
length to available
lengths for economical use. Seven 1-foot 5-inch-long pieces are cut from
each 10-foot piece of stock;
45 1-foot 5-inch pieces require seven 2 x 6 x 10 pieces. Leftover material
can be used for bridging.