Old log cabin (C) Daniel FriedmanRoof & Floor Trusses

  • TRUSSES, FLOOR & ROOF - CONTENTS: Photo guide to types of roof & floor trusses, wood and metal. Common roof truss defects, ID by visual inspection. Common floor truss defects, ID by visual inspection. Definition of Howe truss and Fink Truss - design sketches. Broken roof truss bottom chord: inspector falls into toilet. Common truss spacing intervals.
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Floor & Roof Trusses:

Photo Guide to Residential Roof & Floor Trusses. This article describes and illustrates the different types of roof and floor trusses used in residential construction. We describe common roof or floor truss defects that can be observed by visual inspection. This article series describes building framing materials used in different epochs of residential construction. Knowing when certain materials were first or last in common use can help determine the age of a building. Our page top photo shows modern floor framing details for a modular-constructed house.

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Floor & Roof Truss Materials used in Building Construction

Wood Floor & Roof Truss Photos

Truss example (C) Daniel Friedman

Our photos show an attic view of modern roof trusses (above) and floor trusses (below).

Truss example (C) Daniel Friedman

[Click to enlarge any image]

Illustration of Howe Trusses & Fink Trusses

Howe Truss example (C) Daniel Friedman

These illustrations illustrate two common wood roof truss designs: the Fink Truss and the Howe Truss.

Fink Truss example (C) Daniel Friedman

Common defects observed when inspecting wood roof trusses in residential buildings

Roof truss plate (C) D Friedman

Some common defects we find when inspecting wood truss construction in residential buildings include:

Broken Truss Chord, Inspector Falls into Toilet

Watch out: the bottom chord of the roof truss is often used to support the ceiling below. But the bottom truss chords, especially with pressed-on gusset-plate wood roof trusses, may not support the weight of an inspector who walks on them. Worse, the bottom chord of the truss is often hidden from clear view, buried in attic floor insulation.

Hybrid roof and floor trust combination (C) Daniel Friedman

The author [DF] standing on a truss chord while installing an attic fan, suddenly found himself one floor down, in the bathroom of an unsuspecting condominium owner who had no idea that a contractor was working in the attic over her head while she was using the toilet.

As the dust settled from this painful and embarrassing incident we investigated the broken roof truss bottom chord.

A large knot through the bottom 2x4 truss chord had not interefered with the truss function while the chord was in tension - it's usual load. But when walked-on and when the contractor stepped on to the chord right over the knot, it simply broke in two, leading to an accident.

Our wood roof truss photo at above left shows a design that would have been less likely to collapse when walked-on. The bottom chord of the roof truss is formed by a wood floor truss. The two trusses are bolted together at the truss ends using plywood gusset plates.

(History & dates of the design and use of site-built and factory-made roof trusses, king trusses, A-trusses, and web trusses are in process, contributions invited,CONTACT us)

Below our photos show details of finger-jointed wood truss web connections.

Truss example (C) Daniel Friedman Truss example (C) Daniel Friedman

Metal Floor & Roof Truss Photos

Our photos below illustrate the use of steel roof trusses below a flat poured concrete roof. At below right you can see that steel roof trusses are not completely immune to damage from water, leaks, and rust. We don't consider surface rust to be of structural significance on these trusses, but when the rust has advanced to flaking exfloiating steel rust then the integrity of the truss and thus the structure is questionable.

Truss example (C) Daniel Friedman Truss example (C) Daniel Friedman

Below our photograph illustrates a steel I-joist also used in roof and floor construction. Also see  I-JOISTS, Wood Roof Floor.

Steel web Truss example (C) Daniel Friedman

See Framing Methods Age for the history and date ranges of various building framing methods.

Also see NAILS & HARDWARE, AGE and SAW CUTS, TOOL MARKS, AGE for additional building age clues likely to be available when examining building framing materials.


Reader Question: when did building codes change on roof truss spacing?

(Dec 11, 2014) SHYANNA said:

Hi, do you happen to know when the building code for roof trusses changed from 24in to 16in. What year?


Let's get this roof truss spacing information a bit more accurate:

Roof truss support systems are designed by engineers and architects and generally comply with AS 1720.1 that includes various loads and forces such as wind loading, nailin g schedules, etc. Trusses are typically spaced on intervals of 600mm (23.6" or 24"), 900mm (35.4" or 36"), or 1200mm (47.2 or 48") apart or "on center" depending on the live and dead loads of the roof.

Roof truss spacing may be on a variety of spacing intervals depending on truss design, size, type, loading and span, and for roofs, roof pitch may also be a factor. Some common spacings using floor trusses as an example, include 12" on center (o.c.), 16" o.c., 19.2" o.c., and 24" o.c. - Excerpt from Span Tables for Joists and Rafters, (C) 1993, American Forest & Paper Association, Washington D.C.

See FRAMING SIZE & Spacing, Age, Types too.


Continue reading at I-JOISTS, Wood Roof Floor or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see TRUSS UPLIFT, ROOF causing interior ceiling & wall cracks

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