Estimating in Building Construction

166 CHAPTER THIRTEEN

13–6 Roof Assembly

In this section, a quantity takeoff of the framing required for
the roof assembly of a wood frame building is done. The roof
assembly will require a takeoff of rafters, headers and trim-
mers, collar ties (or supports), ridge, lookouts, sheathing, and
the felt that covers the sheathing. If trusses are specified, a sep-
arate takeoff for roof and ceiling assembly will not be made,
and the trusses will be estimated as discussed in Section 13–10.

Rafters. Roof rafters should be taken off and separated
into the sizes and lengths required. The spacings most com-
monly used for rafters are 16 and 24 inches on center, but 12,
20, 32, and 36 inches on centers are also used. Rafter sizes of
2 6, 2 8, 2 10, and 2 12 are most common. The
lengths of rafters should be carefully taken from the draw-
ings or worked out by the estimator if the drawings are not
to scale. Be certain to add any required overhangs to the

For Length of

Rafter Multiply

Pitch of Roof Slope of Roof Length of Run by

(^1) ⁄ 12 2 in 12 1.015
(^1) ⁄ 8 3 in 12 1.03
(^1) ⁄ 6 4 in 12 1.055
(^5) ⁄ 24 5 in 12 1.083
(^1) ⁄ 4 6 in 12 1.12
Note: Run measurement should include any required overhang.
FIGURE 13.48.Pitch/Slope Rafter Length Factors.
Purchase Quantity Trimmer Joists
Size Length (lf ) Pieces
2  812 2
Purchase Quantity Header Joists
Size Length (lf ) Pieces
2  88 2
The ceiling finish material (drywall or wetwall) is estimated in
Chapter 16. 
FIGURE 13.47.Roof Pitch and Slope Calculations.
FIGURE 13.49.Rafter Length.
lengths of the rafters. The number of rafters for a pitched
roof can be determined in the same manner as the number
of joists. The principle of pitch versus slope should be
understood to reduce mistakes. Figure 13.47 shows the dif-
ference between pitch and slope, and Figure 13.48 shows the
length of rafters required for varying pitches and slopes.
Rafters, from the contract documents, determine the
following:

1. The size, spacing, and grade of framing required.


2. The number of rafters required (divide spacing into
   length and add one).
   If the spacing is the same as the ceiling and floor joists,
   the same amount will be required.


3. The length of each rafter (Figure 13.49). (Don’t forget to
   allow for slope or pitch and overhang.)

EXAMPLE 13-21 RAFTER QUANTITY

3 on 12 slope length factor is 1.03 (Figure 13.48)

Purchase Quantity

Size Length (lf ) Pieces

2 814 80



Use 40 rafters per side

Spaces 52 ¿0.75 spaces per foot39 spaces

Ridge length 50 ¿ building length 2 ¿ overhang 52 ¿

Use 14¿ rafters

Rafter length13.5¿ total run length1.0313.9¿

Run 12 ¿ run to center line 1 ¿ 6 – projection 13 ¿ 6 –