As I watched the roof being put on a house being built down the street, I noticed that the roof trusses seemed really long. I know each truss is designed specifically for a home’s dimensions, but I was wondering how far can a roof truss span without support?
A common roof truss can span up to 80’ without support. Many factors affect this number, such as the size of lumber used in the truss, the slope of the roof, potential snow load, and the type of truss used.
In this article, we will take a look at all the factors that affect truss span. We’ll also discuss the differences between trusses versus other types of roof framing, as well as the pros and cons of roof trusses.
- How Far Can a Roof Truss Span Without Support?
- How to Calculate Maximum Roof Truss Span
- Do Roof Trusses Need Support in the Middle?
- Standard Roof Truss Sizes
- What is the Standard Spacing of Roof Trusses?
- Factors that Affect Truss Roof Span
- Wood Roof Truss Span Table
- Roof Truss Span Calculator
- What Is a Standard Scissor Truss Span?
- Benefits of Roof Trusses
How Far Can a Roof Truss Span Without Support?
A roof truss can span up to 80’ without support, however in any home that distance would be impractical and incredibly costly. Trusses are designed to span spaces without interior supports, and spans of up to 40’ are the most common in today’s homes.
The added interior space a truss provides in a home is highly desirable, as there are no walls or posts to impede the space. Also, trusses span much further than the other most common roof framing technique – rafters.
Trusses maximize structural engineering to provide massive strength in pieces of wood, often no larger than 2x4s. A truss is usually fabricated off-site by a third party and delivered to a worksite, where it is put in place by a crane. While trusses over 40’ are used, they are less common as truss fabricators often have to build them first in two pieces, then fasten them after the fact.
How to Calculate Maximum Roof Truss Span
Calculating the roof truss span of a truss is usually not a task a typical DIYer would do themselves. Since each truss is built to the specifications of a specific project, it is impossible to calculate the span yourself.
The best way to calculate a roof truss span is to use the roof truss span table below. A truss span table will show you spans of different types of trusses. As you’ll see, there are thousands, if not more, variations of trusses.
Remember, the primary benefit of a truss, besides being exceptionally strong, is the ability to bridge a space without needing additional supports. You may still encounter homes or spaces that utilize interior spaces for trusses. If that is the case, then the trusses used were likely undersized. A truss span chart assumes you are spanning an entire space.
Note that the overhang of a truss is not used to calculate a truss span. The span of a truss is only the length of the bottom chord. Any overhang by the top chord beyond the bottom is not considered part of a truss’ span.
Do Roof Trusses Need Support in the Middle?
No, roof trusses do not need support in the middle. Trusses are designed to bear only on outer walls. Whether your home has 2×4 or 2×6 framed walls, a roof truss does not need interior support provided the distance between bearing walls – the span – aligns with the span rating of the truss.
Some contractors will order undersized trusses to save money. For instance, a contractor may need a truss for a 30’ span, but order trusses that are 30’ but underbuilt with a smaller dimension of lumber or just less lumber. In these cases, an interior wall is needed to support the underbuilt truss.
Therefore if you have a home with trusses, you cannot automatically assume you can remove all your interior walls without detriment to your home. It is always critical to consult with a building engineer before removing interior walls, even if you have roof trusses.
Standard Roof Truss Sizes
There is no “standard” size of the truss. There are only standard roof sizes, and these are largely dependent on the area where you live. In new subdivisions, homes are larger. But asking what the standard roof truss size is like asking what is the standard sized lot for a house or what is the standard size of a kitchen – it depends.
Roof trusses come in all sizes, as they are used in the garage and even shed construction. According to The Atlantic, the average US home size is 1600 square feet. That means a house that is 32×50 is average. Going by those numbers, it stands to reason that a 32’ truss is the average size of a truss for a home in the US.
The most commonly used trusses in residential construction are fink – or standard – roof trusses. If you were to look at one, it would be triangular with webbing in the shape of “W”. The other most common truss is a gable truss, which fits either end of the room and provides vertical webbing for attaching wall sheathing.
What is the Standard Spacing of Roof Trusses?
The standard spacing for residential roof trusses is 2’. A quick look at any span table will show that the spans given are for 2’ truss spacing. Most span tables do not address spacing any smaller than 2’.
Some contractors may space their trusses closer together. Reasons may include the home being in an area prone to high winds or hurricanes or the area experiences extremely wet and significant snowfall. However, most trusses will be designed with snow and wind loads in mind – for 2’ spacing.
Factors that Affect Truss Roof Span
Many factors affect how far a truss can span. The pitch of a roof can drastically alter the length a truss can span, as well as the type of truss. Types of lumber used – such as 2x6s instead of 2x4s, can also alter truss span along with snow loads.
Steeper pitches allow a truss to span greater lengths. For instance, a standard truss for a 2/12 roof can span only around 24’ while the same truss spanning a 7/12 roof can reach nearly 48’. If you consider trusses with 2×6 lumber, a truss for a 7/12 roof can span much more than double the length of the same type of truss for a 2/12 roof.
There are hundreds of different types of trusses, but by far the most common is the standard Fink or Howe truss – also called a “common” truss. These trusses do not span any further than other types of trusses. All trusses can be constructed to span any length.
However, it is most economical to use common trusses as they are cheaper yet still strong enough for any residential application.
The greater the potential for a heavy snow load, the stronger your truss will need to be. For instance, if you live up north with a snow load of 55 pounds per square foot, then a 4/12 common truss using all 2×4 lumber can span up to 41’. That same truss can span 46’ when built for areas with a snow load of 40 psf.
Therefore, the greater the snow load, the less distance a truss can span. You can still have very long trusses in a northern climate, but they will need to use larger lumber and potentially a different truss design to work properly.
If you’ve ever bothered to inspect a bridge while out on the road, you’ve probably noticed that the use of steel trusses is quite common. However, in-home construction, only wood trusses are used in conjunction with steel truss plates to fasten the pieces of wood together.
Trusses are pre-fabricated in a shop, where the wood is measured and cut using computer-guided measurements. Then it is assembled on a jig where it is run under a presser roller that compacts the truss plate into the wood, firmly attaching each piece of the truss.
Size of Lumber Used
One of the easiest ways to make a truss stronger is to use 2×6 lumber instead of 2×4. Manufacturers can mix and match 2×4 and 2×6 pieces to create incrementally longer trusses of varying strength. Industrial trusses may use larger lumber, but in residential construction, you will only see 2×4 and 2×6 lumber.
Wood Roof Truss Span Table
A wood roof truss span table will assist you in determining what type of truss you’ll need for your desired span and at what pitch. No matter your desired pitch and span, there is a truss that will fit your needs.
A span table will have a column of roof pitches for each truss type. Rows will include the spans each truss can bridge with different types of lumber. For common trusses, you will be able to compare a truss built entirely with 2x4s vs. trusses that use only 2×6 lumber. You’ll also get to see the spans for common trusses that mix the use of 2x4s and 2x6s.
Roof Truss Span Calculator
A roof truss span calculator can help you in a few ways. A span table will tell you what type of gable you’ll need for what pitch using what type of lumber. It doesn’t tell you the length of one of the rakes – the length of one of the top sides of a common truss.
Truss calculators can help you find the rake distance, which helps estimate your roofing material, such as your sheathing and shingles or another type of finishing roof material.
It is important to note that no truss calculator will design a truss for you – you’ll have to consult a manufacturer or contractor for that. But it can help you estimate costs and find exact measurements.
What Is a Standard Scissor Truss Span?
A scissor truss has bottom chords that are pitched similarly to the top chords, allowing for a cathedral ceiling in the space beneath the truss. Many scissor trusses have top chords with a pitch of 6/12. The lower the pitch of the bottom chord, the further it can span. For instance, bottom chords with a 2/12 pitch and 6/12 top chords can span over 40’.
If you want a larger ceiling beneath, then the bottom chords will need a greater pitch. A scissor truss with 6/12 top chords and 4/12 bottom chords can only span 22’. If you add larger lumber, like 2x6s, then you can increase the span by roughly 9’.
Benefits of Roof Trusses
There are numerous benefits to using roof trusses. First, they use smaller-sized lumber than the alternative roof framing – rafters. That makes trusses cheaper. Secondly, they are manufactured offsite. That saves you time and money because manufacturers can mass-produce trusses cheaply. They are also incredibly strong and can span huge distances.
- Built off-site, saving builders time
- Huge variety
Let’s not forget that trusses are incredibly simple to install due to their lightweight. While many contractors will hire a crane for the day to lift trusses into place, a DIYer can easily install trusses on a shed or even a garage with a few friends.
Consider trusses for your next building project. Rafters cost more as they use larger lumber and are much more cumbersome to install compared to trusses. With so many different types of truss, there is one for any type of project you are completing.
Lastly, make sure to get a professional to review your plans for installing trusses before you go ahead with the project. Building trusses yourself isn’t a good idea, especially since you can buy them for nearly the cost it would take you to build them!