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How far apart do trusses need to be?

plasticinehouse.com - Roof Truss Spacing: How Far Apart Should Trusses Be?

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I was doing some work in my attic last year and crawling around between the trusses. As I moved between them, I noticed that they seemed further apart than other framing members in my house. I wondered if roof truss spacing was different and if so, I wanted to know how far apart trusses should be?

Roof trusses should be 24” apart, on center. Trusses are allowed to be closer together, at either 12” or 16” on center, but building codes allow for 24” on center spacing without using heavier duty fasteners for truss to wall connections. If you live in a hurricane-prone area, you might consider moving your trusses to 16” on center. Trusses are engineered to the specifications of each specific house. That means the style and size of wood on a truss varies from house to house. In this article, we’ll go over the reasons for spacing your roof trusses 24”.

What is a Truss?

A truss is a prefabricated roof framing member for a structure. They are usually a series of 2×4 or 2×3 pieces of lumber connected via “webs” to support the roof sheathing and support any type of roof load, such as snow. There are many different styles of a truss. Some can accommodate raised ceilings below, such as a scissor truss, while others, such as a parallel truss design, are used for flat roofs. For every type of roof, there is a truss designed to support it. The important thing to remember is that no matter what type of truss design you use, they are always pre-fabricated before the job. Roof trusses differ from rafters in that rafters use dimensional lumber – usually 2x6s or 2x8s – for their main framing members – the rafters. They attach to a ridge beam at the top and the wall framing below. That’s it. Rafters are often more expensive because the lumber used is wider. Trusses never use more than 2×4 lumber. One reason builders prefer trusses is that they come to the job site preassembled, so all you have to do is attach them to the walls – much quicker. They are also pre-designed by an engineer, whereas rafters require the builder to consult span charts and decide upon the proper spacing and lumber dimensions.

Roof Truss Variations and Spacing

There are hundreds of different types of roof trusses. All have different load and span requirements, and therefore have different requirements for spacing them. Luckily, all trusses can be designed to fit the same spacing, making it much simpler for builders when it comes time to build a structure. How can trusses be fabricated to fit the same spacing? Trusses can be made to use more wood, a larger dimension of wood, or a higher grade of wood to provide a greater truss spacing. Take a standard gable truss design, for instance. A gable truss that needs reinforcement to adapt to a greater spacing, such as 24”, may have the bottom chord – a 2×4 – replaced with a 2×6. This will nominally increase the price of that truss per linear foot. Alternatively, reinforcing a truss might also mean adding a couple of extra truss webs between the openings of a truss. The added lumber will also increase the cost of that truss. Finally, truss manufacturers will make roof trusses with a higher grade of wood for trusses spaced further apart or need to bear heavier loads. As there are up to 10 grades of wood for some common dimensional lumber species, a truss needing reinforcement may get premium wood versus another that gets a #2 grade.

Standard Residential Roof Truss Spacing

If you were to take the roof off of most of the homes in your neighborhood with trusses, you’d more than likely find that nearly all of them are spaced at 24” on center. The reason for this is solely to do with cost – it is cheaper to space at 24 inches as you need fewer trusses. Will a roof be subject to failure with trusses spaced at 24” rather than, say, 16” on center? Not at all, and it all has to do with how trusses are made. When you build a house or a contractor is building a roof, you’ll have to order your roof trusses from a building materials supplier. A question they’ll eventually ask is for spacing – how far apart do you plan on spacing your trusses? Once you’ve answered that question, then the trusses will be designed to fit the span and spacing. Naturally, a truss designed to sit at 16 inches spacing will have less wood than 24” on center. How much less? It depends on the slope of the roof and the span of the truss. A truss designed for 16” centers will probably use a slightly lower grade of wood, resulting in a slightly lower cost per lineal foot. However, you’ll need several more trusses at that spacing, negating your savings. For these reasons, most builders opt to place trusses at 24”. This allows for easy placement of sheathing above and drywall below while still providing the adequate strength necessary to support all live loads a roof is meant to handle.

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Roof Truss Spacing Requirements

When it comes time to plan your roof truss spacing, there are a few things you need to keep in mind. The first is to understand if you are in a zone that experiences extremely high winds. Typically, this would be anywhere than experiences hurricanes. If so, you’ll have to check a wind uplift forces chart. This may alter how you space your roof trusses. The further apart you space your trusses, the greater the need to anchor them more securely to the walls becomes. Therefore, if you live in a zone that experiences 140 mph wind uplift and you space your trusses at 24 inches, you’ll have to reinforce the truss-to-wall connections with something more than nails. You’ll need steel strapping and brackets, too. On the other hand, spacing your trusses at 16 inches in the above scenario might result in allowing you to forgo any special fastening requirements, as the decreased truss spacing may negate the need for reinforcing truss wall connections. However, most of us do not live in hurricane zones and don’t have to worry about wind uplift. Many of us do, however, live where there is snow. The more snow an area gets, the greater the load on a roof. Luckily, any competent truss manufacturer will consider local snow conditions when constructed a truss. Your snow load could push 40 pounds per square foot for those who live in snow-heavy areas out west. In that case, you’ll be getting trusses that have more reinforcement – wood – to bear that potential weight. They will cost more, and regardless of truss spacing, they will still need to be larger due to the potential live load. You won’t save money by narrowing your spacing.

Other Truss Spacing Considerations

When spacing your roof trusses, you’ll want to consider a few other factors that might be important to you when your structure is complete such as:

Roofing material and sheathing

Insulation

Attic Space

Roofing Material and Sheathing with Truss Spacing

Remember that roof sheathing is 4’ x 8’. That means that you’ll want your trusses in some multiple of 12”. Suppliers will not be able to design a truss for you unless it is based on one of these multiples of 12. Therefore, your only options for spacing trusses are 12”, 16”, 24”, or some multiple of one of those numbers. Otherwise, you’ll have an unsupported edge of sheathing when it comes time to finish your roof. Also, the type of roof material you’ll use may affect the design of your trusses. When you order your trusses, be sure to mention what type of roof you have. If it is metal, the trusses may be designed to withstand a lighter load, making them slightly cheaper. On the other hand, if you install something heavy like slate or clay tile, then be sure to mention this when buying your trusses, as they’ll significantly increase your dead roof load.

Insulation

When you insulate your roof, you’ll have fewer thermal breaks the further apart you space your trusses. A thermal break can be a framing member that you put insulation between. Wood is not an ideal insulator and can reduce your energy efficiency in a structure. Fewer trusses potentially mean you can fit more batts or foam snugly together, which means greater efficiency in terms of heat or cooling loss.

Attic Space

One of the downsides of trusses in a house is that the webbing – the inside part of the truss that is often “v-shaped” – provides little room for a person to move around comfortably. If you want a living space in your attic, you’ll need a different type of truss that will probably be more expensive. Also, if you opt for a different type of truss to allow for more attic space, your spacing requirements may have to change, too. An attic truss may use more wood and require closer spacing than a standard gable truss. On the other hand, you can always ask your supplier for trusses of different kinds to all have the same spacing, which is commonly done. If so, be prepared for different truss types to have different costs as some will contain more lumber or a higher grade of lumber than others.

How to Calculate Truss Spacing and Cost

Calculating your truss spacing is an exercise in economics. When it comes time to plan your building, contact your building supply store to get quotes for your truss: slope, span, and spacing. Ask for a quote for trusses at 16” and 24” on center. Once you get that, you can do some math. Before we do some math, keep in mind that you’ll always need one extra truss than you’ve calculated to account for the very first truss that will sit on your sidewall and serve as the first truss. Let’s say the supplier will sell you trusses spaced at 24” on center for $4 a linear foot. If your trusses are 30’ long, you will simply multiply 30×4, which is $120 per unit. If you have a 40’ long house, then you’ll need 20 trusses plus one more for 21 total. 21×120 is $2520 for your trusses.

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Now, let’s say the supplier will sell you trusses for 16 inches spacing at 3.75 per linear foot. First, we’ll multiply 3.50×30, which is $105 per truss. Now we’ll need more trusses for this spacing, so 40’x12” is 480”. Divide that by 16” and you get 30 trusses, plus one more. Now, we’ll multiply our price per unit, $105×31 which is $3255.

Truss Spacing Calculator: How Many Trusses Do I Need?

If you are not a math person, then a roof truss calculator can save you some time and ensure you don’t make any mistakes. You’ll need to know 2 things: the length of your roof (not the width), and your desired truss spacing. For instance, if you want to space your trusses at 24 inches and your house is 50’ long, then the calculator will tell you that you’ll need 26 trusses, as we divide 50’ by 2’ (24”). Of course, 50/2 is actually 25, but you’ll need one extra for your first truss that sits on the first edge wall of your home.

Roof Truss Spacing for Metal Roofing

Roof truss spacing for a metal roof will not differ from the spacing you would have for asphalt roofing. Metal roofing is typically much lighter than asphalt shingles, and trusses designed to support a standard asphalt roof will be more than enough to support a metal roof. Remember that trusses are designed with snow load, uplift forces, and spacing in mind. Therefore if you think you can space your trusses even further apart with a metal roof for trusses designed to fit 24” spacing, then you are asking for roof failure. Just because your metal roof is a lighter dead load, the live loads and wind uplift play a much greater factor in terms of forces acting on your roof.

Pole Barn Truss Spacing

Pole barns are a bit different than other structures in that the spacing of roof framing members such as trusses can spread out quite a bit more. Why?

Pole barns are supported by posts within the wall framing. Therefore the forces a truss puts on the walls are absorbed by the posts instead of standard stud wall framing as you’d find in a house. For this reason, you can have fewer trusses that will often align with the placement of posts in the pole barn walls. Pole barn truss spacing will be either 4’ or 8’ apart to connect with the columns in the pole barn wall. Trusses will be engineered to fit either spacing, and you’ll have to determine what is best for your pole barn. The further the trusses are spaced, the more expensive they are. However, you’ll need double the number as you would for 8’ o.c.

Are All Trusses Spaced the Same?

No, trusses vary widely and there are hundreds of designs. Granted, your average home will be made up of one or two of a handful of designs. Depending on the type of truss you have, you may have to space them differently. There are many different types of trusses, and the manufacturer/engineer will tell you how to space them. For instance, gable trusses are designed to fit at either end of a roof, with extra lumber to allow for sheathing to be attached. Scissor trusses allow for more headroom beneath or raised ceilings. These trusses, as well as many others, may have different requirements for installation and may be spaced differently than others.

Conclusion

When it comes time to plan your next roof, remember that you should get estimates from your local truss supplier for all different spacings. They will determine the load and uplift requirements for you based on various truss spacings, as well as quotes for each. Also, keep in mind that more trusses aren’t always better. More trusses mean a greater dead lead on the rest of your house framing. And while that isn’t necessarily a bad thing, it also shows that the more framing you pile at the top of your house it doesn’t mean it makes your house sturdier. Consider the end product. How will you affix roof sheathing and what type of roofing material do you want? How will you insulate your ceiling? All these factors should be considered before you space your trusses. And finally, never start building until your truss spacing has been determined and your trusses ordered.

plasticinehouse.com - Roof Truss Spacing: How Far Apart Should Trusses Be?

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