This document provides details on a roof truss design, including: The roof triangle has equal sides that extend from wall to wall at the base, with the two sides meeting at the center peak. The slope of the roof is stated as a ratio of rise to run, in this case 7/12, with a span of 6 feet and total run of 3 feet. The top chords are placed above the roof triangle and bottom chord below, with the span divided into thirds and quarters to locate the ends of the compression and tension webs. Triangular bearing blocks transfer the roof load between the top and bottom chords and ultimately to the walls. Gussets are applied to joints

1. ROOF TRUSSES
FINK TRUSS - W-TYPE
DRAWN BY
MICHAEL NAUTH

2. The roof triangle usually is isosceles (equal sides) and it extends from
wall to wall at its base. The two sides meet in the centre at the peak.

3. The slope or angle of the roof is stated as a unit rise (in ins.) over the
unit run (12”), in this case 7/12. The Span of the building is 6’, the total
Run is ½ of the Span or 3’ (36”), and the Rise is 7” x 3 or 21”.

4. The top chords of the roof truss are placed above the roof triangle and
the bottom chord is placed below. The Span in then divided into thirds
and quarters in order to locate the ends of the webs.

5. The centre of the compression webs run from the ¼ point on the top
chord to the ⅓ point on the bottom chord. N.B. The ¼ point is projected
up from the Span or bottom chord.

6. The tension webs run from the peak to meet the compression web at the
bottom chord.

7. Triangular bearing blocks are placed between the top and bottom chords to
transfer roof load to the bottom chord and then to the walls. Compression
webs longer than 6’ (1.8m) must be laterally braced.

8. Gussets (metal or plywood) are applied to both sides of the truss to keep
the joints together. The truss plant applies metal plates with a hydraulic
press. Plywood is ½” Douglas Fir and nails are 3” long and cleated over.

9. The components of the truss are labelled. The base of the truss triangle
is the sum of the Total Run and the Projection. The hypotenuse of the
unit triangle (unit length) and is calculated using Pythagoras’s theorem.

10. Using Similar Triangles, the length (X) of the Top Chord is determined.
X/42 = 13.89/12
X = (42 x 13.89)
12 = 48.615” or 4’ ⅝”.
Trusses are built on a flat surface and the pieces are cut to suit the
layout marks.