Types & Benefits of Roof Trusses

Reduced costs, superior construction and a greater range of roof designs are just a few of the benefits of roof truss systems.Conventional roof rafters and ceiling joists are less often used in new home construction these days. In fact, nearly 80% of homes built today use pre-manufactured roof trusses instead of traditional rafters to support the roof!

Roof trusses are pre-fabricated, triangulated wood structures which are built in a factory and carefully designed to carry the load of a home’s roof to the outside walls. They are then shipped to the construction site and installed using a crane after the home’s walls have been framed.

Ask most builders today and they will tell you that engineered roof trusses are the only way to go and are far better than the old roof frames. The primary benefits of using pre-fabricated roof trusses are cost savings and construction speed.

The flexibility in the roof design and complexity that roof trusses enable have also made them increasingly popular. With today’s home styles, more complex roof designs, angles, cross gables and other features have added cost, which can be at least partially offset by using pre-manufactured roof trusses rather than building a roof frame on site.

The Advantages of Pre-manufactured Roof Trusses

While the basic advantages have been mentioned above, a more detailed list of the advantages of roof trusses in building a home include:

  • Professional design and fabrication techniques; in conventional roof building, the carpenters build a roof frame, leaving much to chance and the builder’s opinion in terms of the roof’s strength. By comparison, a trussed roof system is designed by engineers to accommodate the specific roof design and meet building codes and enabling a more uniform size and roof pitch.
  • Higher quality materials and strength; roof trusses are fabricated inside a shop, where the materials are not exposed to inclement weather or moisture conditions.
  • Trusses can typically be installed in a single day, speeding the home construction process and getting the structure closed up sooner, which helps prevent moisture and other weather elements from getting inside.
  • Trusses are cheaper than conventional roof framing due to labor savings and since they are built from shorter lengths of 2×4 lumber rather than the large size lumber required in building rafters and ceiling joists.
  • Roof truss systems allow for almost any custom roof design and shape imaginable, allowing features such as cross gables and cathedral ceilings without being prohibitively expensive.
  • Roof trusses can span much longer distances without the need for load bearing interior walls

The only real disadvantage of roof trusses is that the homeowner ends up with less usable space in the attic area; a very small price to pay!

Diagram of a basic roof truss configuration.

Types of Roof Trusses Used in Building Homes

Two basic types of roof truss designs are used in home building; the pitched or common truss and the parallel chord or flat truss.

A common truss is recognizable by its triangular shape and is most often used in roof construction. Most often, variations of the common truss are named for their web configuration, such as the King Post, Fan, Fink and Howe trusses, with the chord size and web configuration typically being determined by the load, span and spacing.

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, Bowstring, Dropped Chord, Gambrel, Hip Step-down, Howe, Parallel Chord, Pratt Bottom Chord, Pratt Top Chord, Raised Heel, Scissor, Semi Howe, Slope Frame and Warren.

Each type of roof truss offers pros and cons and is designed to suit specific structural and aesthetic purposes. It is important to understand the different types of roof trusses when designing a new home, doing major remodeling or a room addition. This will help match the look and function of your home’s roof to the correct roof truss type and design.

Both aesthetic and functional considerations are required in choosing the best type of truss to use for a given roof shape, size and design complexity. You will want to discuss all of these factors with your architect and roof truss engineer before ordering roof trusses for any new home or major remodeling project.

Climate is another consideration, since each type of roof truss has different characteristics that make it more or less desirable in terms of the insulation and air-vapor barrier.

We have expanded on the advantages and disadvantages of several of the major types of roof trusses:

A Raised Heel Truss is designed to span an area and provide adequate space for full depth attic insulation. While enabling greater energy efficiency, Raised Heel Trusses also enable an air tight vapor barrier to reduce problems arising due to condensation, dry rot and mold. It is also more expensive than other types of roof trusses due to the need for soffit siding, higher manufacturing cost and the additional insulation required.

A Dropped Chord Truss has two segments; a convention truss, with a secondary chord truss suspended below to help reduce truss uplift, which is when an interior ceiling may “lift upward” and result in ceiling and wall damage. A Dropped Chord truss design enables a vapor barrier and full depth attic insulation, as with a Raised Heel design. Taller studs and additional blocking and siding are required where walls and ceilings intersect in order to accommodate the air-vapor barrier, which adds to the construction cost.

A Scissor Truss also uses lower chords, but instead of being horizontal, the lower chords are sloped inward to form a shape that looks somewhat like a hang glider. Cathedral ceilings many times require Scissor Trusses and eliminate the need to use a bearing beam and wall. Insulating the attic area is more difficult with this type of truss and manufacturing costs are typically higher than other roof truss types.

A Parallel Chord Truss may also be used with cathedral ceilings, but allows for fuller and easier installation of insulation of the attic area. Because it requires steel braces and several wood products to manufacture, the cost is higher and thermal bridging caused by the steel braces can decrease energy efficiency.

Diagram of various types of roof trusses typically used in home construction.

Ordering Roof Trusses For Home Construction

It is very important that you specify the correct type, measurements and other factors when ordering roof trusses. Common mistakes include inaccurate specifications and failing to inspect the roof trusses when they are delivered to the construction site.

A list of the basic specification required when ordering roof trusses includes:

  • Truss Span – specifies how long the bottom chord needs to be.
  • Overhang Length – specifies the horizontal distance between the end of the bottom chord and the bottom edge of the top chord.
  • Number of Trusses – specifies how many trusses are required. The uniform spacing of roof trusses (usually either 24 inches or 48 inches on center) makes this relatively easy to calculate.
  • Design Load – specifies both the live and dead loads of the top and bottom chords, as well as wind and any other loads to which the trusses will be subjected.
  • End Cut – specifies the plumb or square cut and any custom specifications
  • Heel Height – specifies the vertical distance between the bottom of the bottom chord and the top of the top chord.
  • Roof Slope – specifies the vertical rise in inches per each 12 inch horizontal run.
  • Bearing Width – specifies the requirements for the truss bearing.
  • Type of Truss – specifies the type of truss required.
  • Overall Height – specifies the total vertical distance from highest point of top chord or peak to the bottom edge of the lowest bottom chord, not including the overhang.

18 Responses to Types & Benefits of Roof Trusses

  1. Rickie Bell says:

    Well written article, straight and to the point with no hype. Anyone considering building a new home should read this web page before deciding on a roof system to use.

  2. Rafter Tales says:

    Thanks Rickie. Let us know what other types of tools you think we should give away in future months’ contests. Being a framer, maybe a big ol’ framing nailer would be nice?

  3. Ricky sturz says:

    Hi, could you tell me which roof truss design would be the most efficient? And by that I mean, which one can hold the most weight and still be as light as possible. Thanks!

  4. Gobi Gopinath says:

    Thank you!

  5. Napoleon Wak says:

      Hi I am A architure student at  University of Technology Papua New Guinea. We are doing a Studio project On a Mausolum And I was Using a  Slab roof somting which is more monumental type. Could tell me any thing about the weight of the roof. It, a concrete structure by the way. Give anything on conrete or slab roofs.

  6. yo says:

    hi i want to ask you about a widespan wooden truss….can you tell me more about the vaulted parallel chord type? tq

  7. Bedford says:

    I’m considering changing the roof line so that it has a nice prominence by increasing the rise.

    The present roof has a low rise but I would like to change it to a higher rise, nearly double.

    Is the old truss system removed or can you build onto it?

    The roof size is 24′ x 45′.

  8. Norm says:

    I have an older home built in the late 60’s where the roof was constructed of 2×6’s alone. The ceiling joists are over lapped in the center of the house on a load bearing wall. The roof rafters are nailed into the outer edges of the ceiling joists over the outside walls and merely beveled and leaning into each other at the peak of the roof. I have concerns with the heavy snow load we are getting this year (winter 2010/2011) in the northeast. I’ve noticed over the last ten years cracks in the ceiling sheet rock that lead me to believe the stresses are pushing the outer walls apart in my kitchen/dining room area which doesn’t have a load bearing wall to support them. I’d like to see articles on how to convert/add supports in the attic to create an improvised truss of sorts to improve the structural strength of the roof rafters in situations like mine. I’m looking at a couple hundred dollars of materials and my labor to convert these rafters and ceiling joists into trusses of sorts. I know they’d be less than desirable torsionally speaking, but my situation leaves me with less than ideal ways to correct the situation before the snow load gets the last say in the situation. My preemptive strike is my best option of saving my structure from a possible failure.

  9. jg says:

    Norm,
    You”re up early with these questions. I’ll assume the span in the kit/dn is 16 -20′. Since you have no ridge beam the snow load is indeed pushing the exterior walls out. If you can get in your attic space above the rooms you can install 3/4″ plywood gussets on each side of the rafters to help with the movment. The longer the better up to 8′ long. Giant triangles glued and screwed are the best but lots of ringed shank nails will do also. You can also use collar ties [same principle using 2×6 or 2×8] about 1/3rd of the lenght from the top of the rafter spanning parallel to the ceiling joists. If you can, take the snow load off before you do the work.
    Good luck.

  10. Bob McQ says:

    We need to do an addition on our home and now is a good time since construction costs (labor) are down considerable in our area. We have a 750 SF home that was a 2 bdrm 1 bath that we’ve converted to a 1 bdrm 1 ba so we have some living space.

    We want to go straight off the back of our (built in 1945) home about 16 feet. We will enlarge the living space, enlarge the master bedroom and add a mstr bath, and add two bedrooms upstairs in the attic space that will be left.

    We were thinking of scissor trusses with pony walls about 4 feet in from sides (approx 3-4 feet high) for the two bedrooms.

    While looking at your site I noted two other types of trusses that might work better upstairs than scissor trusses. They are Double Inverted and Attic Room.

    My house is 32 feet wide and I will replace the entire roof and just run trusses all the way front to back. The back half (addition) will have the two bedrooms (and a bath) on top. I’m thinking a 10/12 pitch will give me the room needed to get needed sq ft upstairs. Can you tell me how far in I need to come with both Inverted and Attic trusses to do what I need?

    Thanks,

    Bob

  11. Brian Jordan says:

    I was trained as a young boy to use 2x 12 solid rafters plate to risen plate for slope. This was 40 years ago. currently i am working on a retirement cottage in Hawaii. I noticed the mono and scissors rafters will they allow me to use smaller wood 2×6 or 2x 4?
    If so on a 36 ft long building would a spread of 32″ or 24″ be preferable?. 1 808 938 3911

  12. ncwala dlamini says:

    thats tha best can i have more

  13. kennie says:

    The county is trying to fine me for redoing a roof that was torn off in a storm. I replaced the roof with 2×4 trusses and they’re telling me that because the room is 11’3″ I had to use 2×6’s. My problem is they have pics of me starting this project and waited until I was finished before starting the complaint.

  14. Pingback: ROOF TRUSSES | Materials Find

  15. Paula wetzel says:

    We would like a quote for your typical roof truss shown on page 3 of your website. We are building a new space and need 11 trusses with a span of 16 feet. You can reach me at
    (610) 529-4627.

    Please respond ASAP

    Thank you,

  16. Pingback: TRUSSES CONSTRUCTION | Materials Find

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