How many inches between roof rafters

I wondered if roof truss spacing was different and if so, I wanted to know how far apart trusses should be? 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. A truss is a prefabricated roof framing member for a structure. 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. Rafters are often more expensive because the lumber used is wider. 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. 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.

This will nominally increase the price of that truss per linear foot. Local building code may also address rafter dimension, although this usually is considered in addition to roof pitch and span.

As roof pitch is lowered, the weight burden on the rafters increases. For low-pitch roofs, building codes may require larger dimension rafters. For instance, if your roof has a steep pitch, you may be able to use 2-inch by 4-inch rafters, but if your roof has a low pitch, building code may require that you used 2-inch by 6-inch, or 2-inch by 8-inch rafters. Covenants are building codes that apply to a single development or subdivision, but covenants are just as enforceable as other local building codes.

When it comes to roofs, covenants may require a specific roof pitch so your home blends in aesthetically with the rest of the homes in the development. When they run in the opposite direction creating a scenario where they are not attached to the rafters, then rafter ties can be used to achieve the tie requirement.

This connection is important because it resists the horizontal thrust from gravity loads and rafter thrust caused by the uplift from wind loads. Using either a rafter tie or the ceiling joists themselves can vary slightly in the size of the roof rafter used which is why the code creates two types of span tables. One for when ceiling joists are used as the ties and one when they are not used, thus requiring the rafter ties.

Another thing to note is that the roof rafter span tables are limited to the ceiling joists or rafter ties being located at the top plate or within the lower third of the attic space.

If the ceiling joist is not located at the top plate but located within the lower third of the attic, a rafter space adjustment factor is applied to the rafter to account for the additional stress on the roof rafter due to the tie or ceiling joist being located heigher up in the attic. However it is important to note that if the ceiling joists or rafter ties are located higher up in the attic as to where they would exceed the one-third requirement, then the span tables can NOT be used and instead would require the roof rafters to be designed in accordance with accepted engineering practice.

So basically if the design loads are exceeded or the ceiling joists or rafter ties are located more than one-third into the attic, the design of the roof rafters are outside the prescriptive standards of the tables and at that point they must be designed in accordance with accepted engineering practice. As you can see, unlike ceiling joists or floor joists , roof rafters have many factors that must be taken into account when selecting the correct size. Now with all that out of the way, lets take a look at those tables to learn how to correctly size roof rafters using the prescriptive standards:.

For simplicity, only the table headers are shown above. The table header must be understood first before we can correctly select the right table for sizing the roof rafters.

Each table is labeled. As you can see Table R Remember what was said earlier. Rafters must be tied, so if they are not tied to the ceiling joists, then they would require rafters ties to be installed.

A 2x8 up to 12 feet; 2x10 to 15 feet and 2x12 to 18 feet. How far can a 2x6 beam span? When supporting joists that span 12 feet with no overhang beyond the beam, a double ply beam can span in feet a value equal to its depth in inches.

A double 2x12 beam can span 12 feet; a 2 2x10 can span 10 feet and so on. How much load can a 2x6 support? The type of load in question will also determine how much weight a 2x6 can hold on edge.

For example, a 2x4 lumber may hold a 4-cylinder engine comfortably but may not hold a bigger one. On the other end, a 2x6 can support a V8 engine of between — lbs. How much weight can a 2x6 ceiling joist hold? Fine Homebuilding notes that 2-inch by 6-inch garage joists will support a weight of up to 50 pounds per square foot. If you overload your garage's ceiling joists, the ceiling may appear to sag from the strain.

What size lumber can span 20 feet? How far can I span a double 2x8?