King post trusses

King post

King post

A king post is a central vertical supporting post used in architectural, bridge, or aircraft design applications.-Architecture:...

 truss

Truss

In architecture and structural engineering, a truss is a structure comprising one or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. External forces and reactions to those forces are considered to act only at the nodes and result in...

A roof

Roof

A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....

 truss that is mostly wooden with two principal rafter

Rafter

A rafter is one of a series of sloped structural members , that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.-Design:...

s, a tie beam and a central vertical King post. The simplest of trusses. Mainly used with two angled struts.

The King post roof truss shown in the gallery was typically restricted to smaller roofs.
As an example a roof truss of this type spanning 20 ft would use the following timber sizes.

• Tie beam - 4" × 8"
• Principal rafter- 4" × 5"
• King post
  King post
  A king post is a central vertical supporting post used in architectural, bridge, or aircraft design applications.-Architecture:...
  
   - 4" × 4"
• Strut
  Strut
  A strut is a structural component designed to resist longitudinal compression. Struts provide outwards-facing support in their lengthwise direction, which can be used to keep two other components separate, performing the opposite function of a tie...
  
   4" × 4"
• Purlin 4" × 7"
• Common rafter 2" × 4"
• Ridge board 1½" × 9"

A variation of this truss is known as a King bolt truss, in which the king post (which is normally under tension and required quite sophisticated joints with the tie beams and principal rafters), was replaced by a metal bolt. (Usually wrought iron.)

Queen post trusses

Queen post truss. A roof truss mostly wooden with two principal rafters and two vertical Queen posts with a restraining tie beam at the bottom and a straining beam at the top.

The Queen post truss extends the span and combined with spliced joints in the longer members extends the useful span for trusses of these types. As an example say spanning 35 ft the member sizes would be in inches:-

• Tie beam - 5" × 11", with two pieces spliced.
• Principal rafter- 5" × 7"
• Queen post - 5" × 5"
• Strut 5" × 5"
• Straining beam 5" × 8"
• Straining sill 5" × 3"
• Purlin 4" × 7"
• Common rafter 2" × 4"
• Ridge board 1½" × 9"

These sizes are of course generalisations as the pitch, strength grade of the timber all come into the equation.

External links

• Illustrated roof glossary