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How many rafters do I need for a shed?

What is this? If you have a simple gable shed roof, then using 2×4 rafters makes more sense unless you have a very large shed. If you have a single sloped shed roof, then using 2×6 rafters is a better bet because the price differential won't be huge and your shed roof will have a much greater load capacity.

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Rafters are a simple, straightforward way to frame the roof of your shed. They require less lumber than trusses and once one is cut, all the others follow the same cut. However, one of the main issues you will encounter when cutting rafters is, should you use 2×4 or 2×6 rafters for your shed roof?

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 won’t experience heavy snow loads, a gable roof can span up to 22’ with 2×4 rafters. For roofs that will experience heavy snow or wind and will span the same distance, use 2×6 rafters. If you have a simple gable shed roof, then using 2×4 rafters makes more sense unless you have a very large shed. If you have a single sloped shed roof, then using 2×6 rafters is a better bet because the price differential won’t be huge and your shed roof will have a much greater load capacity. Below we’ll take a closer look at when to use 2×4 and 2×6 lumber for your shed roof, and hopefully, you’ll be able to make a more informed decision on what type of lumber you should use when building your shed roof rafters.

Roof Rafters and Shed Roof Pitch

When talking about framing a shed roof with rafters, you’ll first need to know how steep your roof is – in other words, what is the pitch or slope of your roof? Most rafter span charts are organized into 2 sections: roofs with a slope less than 3:12 and roofs greater than 3:12. What does 3:12 mean? It means a roof that rises 3 inches for every 12 inches of horizontal run. If you look at the chart linked above, you’ll notice that for roofs that are less than 3:12, the chart does not include 2x4s as an option. 2×4 rafters are not strong enough for low pitched roofs under any circumstance, so avoid if you are building a shed with a flat or very shallow roof. Many roof material manufacturers have minimum pitch ratings for their products. Many shingle manufacturers recommend no less than 2:12 when installing. The reason for this is that flat or shallow roofs withstand much more abuse and loads than steeper roofs, making wider shed roof rafters a necessity. Lastly, remember that we are not talking about trusses in this article – just rafters. Trusses only use 2×4 lumber and are constructed using a “web” of 2x4s for strength. Rafters simply rely on a center ridge beam and the outside walls for support. While there are benefits to both, only rafters vary in the size of lumber that is used. For more information about roof framing and how understanding the differences between types of shed roof construction, visit my guide to shed roof framing.

Type of Lumber Matters

We will be looking at roofs that have a pitch of 3:12 or greater. Many rafter span charts delineate between rafters using SYP (southern yellow pine) versus SPF (spruce, fir, pine) lumber. SYP is one of the densest softwoods, so it is strong but more difficult to cut and nail into. SPF is light and easier to use when framing. There is a slight drop off in strength when using SPF for shed roof rafters, but in most areas, you only have one option when you go to your local home reno store, so at least know what you are buying. If you forget to look at the store, don’t worry – the wood itself is usually marked SPF or SYP.

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, you’ll have two sets of rafters, with a ridge beam in the center uniting the rafters on each side.

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

Understanding Roof Loads

You may live in an area where your shed roof will experience very little disturbance – no snow, regular rainfall amounts, and occasional wind storms. In this case, you’ll be looking at rafter span charts that estimate your roof load at 20 pounds per square foot. If you have substantial snowfall that stays all winter or frequent severe weather that includes high winds, then you’ll be looking at the rafter charts for 40 pounds per square foot. If you look at a wind load chart, you’ll see that various parts of the country experience winds of greater velocity, on average, than other areas. If you live in “tornado alley” or in a hurricane-prone area, then you’ll likely have special building codes outlining what type of rafters to use. If your area experiences high winds, then you should use 2×6 shed rafters. 40 pounds per square foot is a substantial amount of pressure and these types of loads are often used only in home construction, where pressure is much greater due to foot traffic, furniture, etc. If you aren’t sure if you live in an area with these extreme environmental factors, then you are probably safe to use 2×4 rafters.

Span Length for 2×4 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 ⅔’. 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 ⅔’ 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 ⅔’. 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.

Span Length for 2×6 Rafters

2×6 shed roof rafters are enough for nearly any type of shed a homeowner wants to build. While a 2×6 might be nearly double the cost of a 2×4, you can likely space the 2x6s further apart, negating the added expense of increasing your lumber width. An SYP 2×6 rafter at 24 O.C. can span up to 13’4”. That means it can span a total length of over 26’ for a gable roof. An SPF roof at the same spacing and lumber size will span 11 ½’. These numbers are for roofs supporting light loads of 20 pounds for square foot. If you place the 2x6s at 16” O.C., then SYP can span over 16’ and SPF can span 14’.

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For heavy loads at 24” O.C., 2×6 SYP can span up to 9 ½’. SPF 2×6 rafters can span 8’. If you move the rafters closer together at 16” O.C., then your SYP rafters will span 11 ½’ and SPF up to 10’. Therefore, if you want a 10×12 single sloped roof and you live in a heavy snow area, then you can space your rafters 16” apart and use SPF 2×6 lumber.

Should I Use 2×4 or 2×6 Rafters for Shed Roof?

You should use 2×4 rafters if you live in an area without heavy snow or wind loads. Remember, you can span a gable-roofed shed up to 17’ with SYP and 15’ with SPF 2×4 rafters at 24” O.C. For most shed owners, this is more than enough span for our backyard sheds. You can always put your rafters closer together to achieve an even greater span. Use 2×6 rafters if your roof will experience heavy snow or wind loads. SYP 2×6 rafters at 24” O.C. can span over 18’ for gable-roofed shed. An SPF rafter can span nearly 15’ at the same dimensions. Again, put them closer together if you want to achieve a greater span. Remember, this advice applies only to shed roofs that have a pitch greater than 3:12. If you have a roof with a pitch that is less than 3:12, you should not use 2x4s. If you do opt to use 2×4 shed rafters for your roof construction, you will always use a 2×6 ridge beam even though the rafters will be 2×4. For more guidance building and installing your rafters, see my article on how to build rafters for a shed.

Conclusion

Determining whether to use 2×4 or 2×6 rafters depends entirely on your environment. Heavy snow and wind mean that your roof will experience significant loads. One cubic foot of snow weighs about 20 pounds, so if you have a 100 square foot roof area, you have a ton of snow literally on your roof. 2×4 rafters may not be able to handle that sort of weight. Secondly, the style of roof you have also plays a major role. You can span much greater lengths with rafters supporting a gable – two-sided – roof rather than a single sloped roof. Consider your shed design and materials cost before constructing as you may be able to alter your roof design and use 2x4s instead of 2x6s, saving you the cost of larger lumber. Whichever size of lumber you choose for your shed rafters, it never hurts to build a shed stronger than it needs to be. There are other ways to save costs on a shed instead of skimping on the shed roof framing, and you don’t want to wake up one morning with a ton of snow that has collapsed into your shed. As always, thanks for taking the time to read this article and I hope it has helped you figure out which size of rafter will work for your next shed project. Please feel free to comment below with any advice or input you have about working with shed rafters.

plasticinehouse.com - Should I Use 2x4 or 2x6 Rafters for Shed Roof? - Plasticine House