Roof Trusses Used in Construction: Is A 4/12 Pitch Roof the Ideal?

Roof trusses: Types and Variations- Pros and Cons

Roof truss
••• Photo J Rodriguez

Selecting the best type of roof truss for a house is quite challenging. Functionality and aesthetic conditions will lead the considerations of the type of roof truss that you will be selecting in the construction of houses and buildings. Weather conditions must be also taken into consideration when choosing the roof truss for your construction. Most often, variations of the common truss are named for their web configuration, such as the King Post, Bowstring, Fink and Howe trusses, with the chord size and web configuration typically being determined by the load, span, and spacing.

A common truss is recognizable by its triangular shape and is most often used in roof construction. Each roof truss has pros and cons, and it must be designed to suit specific conditions and purposes. It is important to have in mind what type of look you want to have in your finish building when selecting the type of roof truss. A parallel chord or flat truss is so named for its parallel top and bottom chords and is often used to construct floors.

There are many roof truss types and variations, including Arch Frame, Belgian, Dropped Chord, Gambrel, Hip Step-down, Howe, Parallel Chord, Pratt Bottom Chord, Pratt Top Chord, Raised Heel, Scissor, Semi Howe, Slope Frame and Warren.

Pros and Cons of Roof Trusses

Parallel Chord Roof Truss

Commonly used in cathedral ceilings. Its cost is higher because it requires the use of steel members to serve as bracing. The complexity and numerous wood members used is another disadvantage. Thermal bridging caused by the steel braces can decrease energy efficiency.

Raised Heel Roof Truss

This method allows having greater energy efficiency. This roof truss reduces the condensation problems and mold conditions since they create a vapor barrier. It requires a soffit siding and additional insulation works, which adds up to elevated construction costs. It can be designed to span an area and provide full depth attic space.

Scissor Roof Truss

Also commonly used in cathedral ceiling for its convenience, this type of roof truss reduces the needs for a bearing beam. It uses lower chords, sloped inward, instead of being horizontal. One of the disadvantages of using this type of truss is the problems to complete insulation works, also elevating construction costs.

Dropped Chord Roof Truss

Composed of a convention truss, with a secondary chord truss suspended below to help reduce truss uplift. Similar to the Raised Heel Truss, this type also creates a vapor barrier creating great insulation characteristics. Vapor barrier needs additional blocking and siding in the intersection of walls and ceilings, adding up to the construction cost.

Why sometimes a roof truss makes sense

Sometimes a roof truss can make sense as it will be able to meet certain design weather considerations that can trigger special building code requirements. These are the following advantages that can be obtained when using roof trusses:

  • Workers with less experience can install roof trusses reducing labor costs and facilitating the installation process.
  • Some designs can require having fewer load bearing walls as these trusses can be used for achieving longer spans, creating open living spaces.
  • There can be some savings in building materials due to using some lighter and smaller materials depending on the type of truss being used.
  • Installation of trusses can be achieved in 1 day, as they can be pre-fab near the site and erected using a lightweight crane or construction equipment.

    How much can this cost and how can you save money?

    There are certain factors that will have an important impact on how much the truss will cost. Recent studies have shown that for an average American house the roof truss can cost somewhere between $12,000 to $15,200. As an average the roof truss labor costs can be near the $2,300, this is about 112 man-hours times $20.50 per hour, the crane rental which can be around $700, the amount of lumber used which will be around $10,000 and the scrap disposal cost which can be something between $200 up to $500.

    Other studies have shown that conventional framing can take up to 66% more time to build than a pre-fab truss, conventional framing uses 33% more lumber and wood than pre-fab, and even the waste disposal and scrap can increase when the traditional framing method is used.

    In general terms, and if you want to save money while being practical, a 4/12 pitch roof is the most economical, as it is strong but shallow, maximizing the use of lumber using available and ordinary lumber sizes.  A 4/12 pitch means that the roof rises 4 inches for every 12 inches of run.