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What Size Lumber For Roof Rafters

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How to calculate the length of roof rafters (captioned)

You can use either 2×6 or 2×4 rafters for roofs with a 3/12 pitch or greater, but it depends on the overall size of your shed. For roofs that wont experience heavy snow loads, a gable roof can span up to 22 with 2×4 rafters.

Thereof, How far can a 2×4 Rafter span without support?

One may also ask, how far can a 2×4 span without support? In general terms, joists spaced 16 inches on center can span 1.5 times in feet their depth in inches. A 2×8 up to 12 feet 2×10 to 15 feet and 2×12 to 18 feet.

Also to know is, How far can a 2×6 Rafter span without support? How far can a 2×6 Rafter span without support? The table states that 2 X 6 rafters spaced 16 inches on center can span a maximum distance of 13 feet 5 inches.

Subsequently, question is, Can I use 2×4 for roof rafters? You can use either 2×6 or 2×4 rafters for roofs with a 3/12 pitch or greater, but it depends on the overall size of your shed. For roofs that wont experience heavy snow loads, a gable roof can span up to 22 with 2×4 rafters.

Also, Can I use pressure treated wood for roof rafters?

As long as the rafters are not directly exposed to the elements, no need to be treated. Treated is generally for ground contact or exposed to the elements or high humidity areas.

How Far Apart Should 2×6 Rafters Be


Max. Live Load 20 lbs/ft2

Maximum Span
Rafter Spacing, Center to Center Lumber Grade
24 13′

Subsequently, question is, how far should roof joists be apart? A roof joist is always spaced no more than 16-inches apart from another roof joist. If the roof is not completely square, you can decrease the space between joists but never increase the space. In other words, you would space a roof joist 15-inches apart from another roof joist but never 17-inches apart.

Furthermore, what is the standard spacing for roof trusses?

These standards are as follows: typical roof truss spacing means that they are usually about 24 inches from centre which means at the centre of one truss, there will be a two-foot gap, or twenty-four-inch gap, until the centre of the next truss.

How far can a 2×6 Rafter span without support?

The table states that 2 X 6 rafters spaced 16 inches on center can span a maximum distance of 13 feet 5 inches. Another option are 2 X 8 rafters spaced 24 inches o.c.

Should I Use 24 Or 26 For Rafters

For heavy loads, 2x4s can span up to 5 ½. Most roof framing is 24 on center, and you can still build sheds up to 14 with 2×4 rafters at 24 on center. But again, if you are going to experience heavy loads, it just makes sense to err on the side of caution and use 2×6 rafters.

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References and Further Readings :

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Roof Pitch Between 30 And 45 Degrees Strength Class C24

Roof pitch between 30 and 45 degrees strength class C24

Size of purlins in mm Maximum

This diagram illustrates the typical arrangement of a rafter and purlin roof and shows the dimensions referred to in the tables above.

Gable walls need to be strapped to the roof at the maximum 2m centres using 30 x 5m galvanised mild steel straps.

Span Length For 24 Rafters


As mentioned above, a 2×4 SPF shed roof rafter can span 11. An SYP rafter can span 11 ½. Both of these are for light roof loads.

If you live in an area that requires a shed roof rafter to support a heavy load, then an SPF rafter can only span 7 . An SYP rafter can span 8.

While the heavy load span specs for a 2×4 rafter are much less than a light load, they would allow you to have a shed width of over 14 with a gable roof. It will make less sense to use 2×4 rafters on a single slope shed roof if you live an area with heavy snowfall.

Keep in mind the above measurements are for 12 on center. If you had a 12×12 shed with a gable roof, that would mean 26 rafters, which is a lot. Moving to 16 on center, you could span your SPF 2x4s up to 6 for heavy loads and 9 ½ for light loads, which would still work with a 12×12 gable shed.

Spacing your 2×4 shed rafters at 24 on center limits the width of your roof. For light load roofs, 24 o.c. 2×4 rafters can span up to 7 . For heavy loads, 2x4s can span up to 5 ½.

Most roof framing is 24 on center, and you can still build sheds up to 14 with 2×4 rafters at 24 on center. But again, if you are going to experience heavy loads, it just makes sense to err on the side of caution and use 2×6 rafters.

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Vertical Strapping At Eaves Pitched Roofs

At eaves level, vertical straps 1m long should be fixed as shown in the diagram opposite at a maximum of 2m centres.

This is the right way to fix truss clips connecting the rafter and wall plate. Note that these are normally only required to trussed rafter roofs and not for traditional purlin and rafter roof construction.

Roof Framing Is Not As Hard As It Looks

Image source: The Spruce

The whole process is rather complicated, but when you break down designing, measuring, cutting, and hammering everything together into small, digestible parts, framing a roof can be DIY.

Of course, this guide merely scratches the surface of roof framing. We don’t get into the nitty-gritty of building codes, gable studs, and roof sheathing. However, these fundamentals ideally provide a solid foundation for a gung ho roof framer.

Keep in mind that you can skip most of these steps by opting for pre-fabricated trusses. For those who would rather not spend on delivery, recruit additional labor for installation, and seek to hone the waning craft of roof framing, rafters are the way to go.

With enough experience, framing even a complex hip roof with hip rafters, jack rafters, and the like should come somewhat intuitively.

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What Size Timber Do I Need For A Flat Roof


Roof pitch between 15 and 22.5 degrees with rafter spacing strength class C16

Size of rafters in mm 400mm
3.00 2.76

Secondly, how long can a 2×6 Rafter span? The table states that 2 X 6 rafters spaced 16 inches on center can span a maximum distance of 13 feet 5 inches. Another option are 2 X 8 rafters spaced 24 inches o.c.

Furthermore, what’s the best covering for a flat roof?

Ethylene Polypropylene Diene Monomer is one of the most versatile rubber roofing options on the market. Considered to be one of the best materials for flat roofs, rubber is a popular choice for industrial and commercial applications thanks to its ability to withstand extreme weather conditions.

How far apart do roof trusses need to be?

The trusses should be a little longer than the exact width to allow for overhang, and you will need sufficient trusses to mount across the length. Most are spaced between 18-26 inches apart. Once you’ve calculated the number of trusses you will need, you should purchase the lumber.

Basic Roof Framing Instructions

Build your own wood trusses

The first step in conventional roof framing is to layout and cut a common rafter pattern. The rise and run of the common rafter will determine the height of the ridge board as well as the length of any hip / valley rafters that may be involved with a conventionally framed roof. Conventional framing of roofs falls under the heading of rough framing carpentry. Anyone who has ever tried framing a roof can tell you it is a fine tuned skill that can only be acquired with experience.

Many a carpenter has been frustrated by this seemingly dying art. Even though conventional framing has been around for centuries, with the advent of trussed roof systems, the demand for pro roof cutter’s has decreased. The modern day truss manufacturer can build them to match nearly any conventional roof. But not always.

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How To Build Rafters For A Shed: Last Guide Youll Ever Need

Over the years Ive had the opportunity to build quite a few shed roofs, and no two are ever the same. Ive done simple gables, slant roofs and a few lean-tos and probably a few others I cant quite recall. And of course, every time I build a new roof, Im always learning how to build different types of shed rafters.

I figured it was time to put that shed rafter knowledge into an article to share with you. I want to share with you how to easily build rafters for your shed. Well take a look at a gable roof, but youll be able to apply the rafter method outlined below to most types of shed roof styles.

Below well go through what it takes to construct rafters for a shed, step by step. Well also take a look at the materials and tools youll need to do the job.

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Sizing Roof Rafters Example 2

Now lets try another example, but this time a span will be given and you must select the correct size lumber to be used given the following scenario:

Question: When the ceiling is not attached to the rafters and where Hem-fir #3 roof rafters are to be spaced 12 inches on center supporting a snow load of 30 psf and a dead load of 20 psf, what is the minimum allowable rafter size when required to span 14-0 in length?

First step, lets collect all the given information within the question:

  • Ceiling not attached to rafters and a snow load of 30 psf: Use Table R802.4.1
  • Roof Rafter Spacing: 12 inches
  • Species and Grade: Hem-fir #3
  • Roof Rafter Span: 14-0

Now lets find the minimum allowable rafter size given the above scenario:

Therefore given the above example, the minimum allowable rafter size for Hem-fir #3 lumber spanning 14-0 is a 2×10 since it can span 14-5. A 2×8 member for example cannot be used since it can only span 11-9.

To Learn more about the design and construction of wood roof framing, be sure to check out Section R802 of the 2018 International Residential Code .

Fun Tip: To learn how to easily size ceiling joists, be sure to check out this POST.

Hopefully given the above examples you can find it easier to size wood roof rafters per the Tables within the International Residential Code.

In conclusion, lets take a quick second to answer some common frequently asked questions related to roof rafters.

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Figuring Out The Total Center Height Of Rafters

by Endureed | Aug 29, 2018 | Blog, HDPE, Installation, Synthetic Thatch, Thatch Roofing, Why Endureed?

Curious about how to figure out the total center height of a roofs rafters if you dont have design software? Maybe youre working on a do-it-yourself cabana or carriage house. No problem! You can do this!

The pitch of the roof will determine the height of the ridge board that runs the entire length of the structure. Determining the height of a ridge board lets you know how to cut angles on the rafters and how to set them at the right height for the pitch of the roof.

So, first, divide the width of the exterior structure frame in inches by the number two. This lets you know the run for one side of the roof. Then, divide the run by 12 to find the run factor. The 12 is actually for 12 inches. The outside of the width of the wall framing should be measured on the actual structure frame. Dont add in the siding of the structure or sheathing if there is any.

Then, multiply the the roof pitch by run factor you just calculated. Your answer will equal the total distance of the roof rises. That will be what you use to set the height of the bottom of the ridge board. Of course, thats specifically if the roof rafters and the ridge board use the same size lumber. Thats the most common way to do things, but if you are using a more creative design, that may not be the case.

What Prevents Rafters From Spreading

How to Size Roof Rafters


Collar ties and tie beams are both construction “fixes” that can be installed in a roof frame to help stop or solve roof spread issues. The tie beams are fixed to the rafters horizontally to stop them from spreading. The collar ties are used in a similar way.

Furthermore, can Rafter ties be spliced? If rafter ties need to be spliced, such as when they span a roof wider than the available stock, the same nailing is required for the splice as for the connection at the rafter heel. Also, when a rafter tie is acting as a ceiling joist, it must be properly sized and nailed for the span.

Correspondingly, are Rafter ties necessary?

Rafter ties resist the outward thrust that rafters exert on the exterior walls. They help keep walls from spreading due to the weight of the roof. Rafter ties are always required unless the roof has a structural ridge, or is built using engineered trusses.

What causes roof spread?

Roof spread occurs when the downward pressure of the weight of the roof starts pushing against the rafters enough that the structure itself begins to move: either the roof starts moving downwards, or the walls start pushing outwards. This is a prolonged process but can cause severe damage to the integrity of a home.

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Patio Roof Maximum Beam & Rafter Spans

An essential part of planning a patio roof or gazebo project is determining the number, size, and spacing of rafters, beams, and posts, according to the loads they will carry.

In areas with mild climates, patio roofs are generally designed for loads of 30 psf . For heavy roofs or in areas with substantial snowfall or winds, call your local building department for code requirements.

The tables below give maximum spans for rafters and beams. The figures are based on quality materials. For lesser grades, spans should be shorter. Keep in mind that these are maximumsin other words, shortening spans slightly will result in a more solid structure.

Find out the loads your overhead must bear, and then determine the rafter sizes. Next, calculate beam placements and how best to coordinate them with the rafters.

Keep in mind that these are minimums for the spacing indicatedyou can select larger sizes to handle excessive loads or for appearance. Of course, beefier sizes also increase your lumber bill.

What Is Span Length

The span length is the measure taken from the inside of the shed framing to the interior apex of the rafter. Another way to look at it is from the beginning of the birdsmouth cut to the opposite, interior facing edge.

Understanding this is key to knowing how far your rafters can span. For instance, 2×4 SPF shed rafters can span up to 11 when placed at 12 O.C. If you have a gable roof on your shed, youll have two sets of rafters, with a ridge beam in the center uniting the rafters on each side.

Therefore, for gable roofed sheds, the rafter span table is indicating how far a rafter can span half of your sheds width.

If you have a single sloped roof, then the 11 span distance for 2×4 rafters O.C. means that your shed should be no wider or longer depending on how you orientate your single sloped roof than 11.

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Roof Framing: Key Terms And Definitions


The rafters are the diagonal members that directly support the roof deck.

Ridge board

The ridge board serves as a nailing surface for the opposing rafters and provides no structural support.

Rafter tie

The rafter tie resists the outward forces imposed on the load-bearing walls. As the rafters sit on top of the wall plates at an angle, they exert horizontal forces on the exterior walls. Rafter ties, which often double as ceiling joists, prevent these horizontal forces from causing the walls to “pancake.”

Ridge beam

Some framing systems use a ridge beam combined with gable-end posts instead of a ridge board, which provides structural support to the rafters. For roofs with less than 3:12 pitch, codes require ridge beams because they reduce the extreme outward forces on the walls. A ridge beam can eliminate the need for rafter ties, which is often paramount for the cathedral ceiling aesthetic.

Cut The First Rafter Tail And Ridge Cuts

Framing a Big Roof with I-Joist Rafters

The handiest tool in your toolbox for this job will be the speed square. Youll notice that your speed square has regular measurements on one side and degrees on the other. Youll also see that the end with degrees has a set of numbers, 1 24, just above it.

Those single numbers correspond to the roof pitch. So since we are using a roof pitch of 4:12, well use the number 4 in that row to help us make our miter cuts.

To start, I already know that my rafter length is going to be 5 3 ½. Ill use my speed square to measure my first cut on one end of my 2×4 rafter. Ill use the corner of the speed square on one edge of the 2×4, where it says pivot. Ill then rotate the square on the pivot until the 4 is lined up with the outside edge of the rafter.

Ill mark a line along the edge of the speed square. This gives me an exact 4:12 cut. Now, I could do a few calculations and know that 4:12 is roughly 18.5 degrees, but a speed square is so quick that I dont need to do that.

If you want to use a miter saw, you can simply rotate your miter saw to 18.5 degrees and cut.

When cutting the other end, be sure both cuts are parallel! If the length of the cut is 5 3 ½, then youll be measuring only on one side from end to end. Using the miter saw, simply slide the wood through after you make your first cut. This ensures you keep the cuts parallel.

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