Roof rafters form a vital part of the roofing frame. The type of rafter used in roofing matters, but the functionality comes down to the size.
The most common types of rafters are the 2×4 and 2×6 sizes. They are fit for all construction needs that are not industrial scale. A roof rafter serves to distribute weight on the roof.
If the size does not fit a weight limit, it will cave in to pressure after some time.
With that in mind, people may tend to make a run for a 2×6 rafter. Since it is bigger, it should be able to take more weight than the 2×4 rafter.
But there is more to choosing between a 2×4 and 2×6 rafter than weight distribution.
The choice between a 2×4 or 2×6 rafter depends on your roof slope and the overall weight. You also need to factor in the effects of snow and wind. Then consider how much span length you want to achieve. The 2×6 rafter works for longer span lengths than the 2×4 rafter. These factors put together will help you make a choice.
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Should Rafters Be 2×4 or 2×6?
The size of roof rafters you use depends on a couple of factors. First, you should know the roof span you would like to achieve. The 2×4 rafter goes for shorter spans, and the 2×6 goes for longer roof spans.
Sometimes, you can use the 2×4 to achieve longer spans if the spacing between the rafters is less.
Wind deflection and snow load have parts to play in choosing between 2×4 or 2×6. The 2×6 serves to support roofs in areas that get heavy snow loads.
But the 2×4 would not hold for long under heavy pressure. On the other hand, it fits in conditions that translate to light snow loads.
Another thing to consider is the size of the structures on which you intend to use the rafters.
The primary role of the rafter is to support the roof. So, the rafter size should match the roof size. It is a no-brainer; large structures have large roof sizes, and small ones have small roofs.
It is wise to keep 2×4 rafters for small setups and 2×6 rafters for large ones. The 2×6 offers better structural support compared to the 2×4 rafter.
You would not want your roof to cave in after it stands for some time.
Talking about caving in, the slope of the roof comes into play. The slope of the roof also determines the rafter size you should use.
The 2×4 rafter goes for 3:12 slopes and greater. For slopes that are less than 3:12, the 2×6 rafter can serve.
The 2×4’s build does not fit flat roofs, no matter the spacing between the rafters.
Should I Use a 2×4 or 2×6 Rafter for a Shed Roof?
For shed roofs, lumber, roof slope, span, snow load, and wind determine the verdict between 2×4 or 2×6. First, 2×4 should go nowhere near flat roof structures.
That is, roof structures with slopes that are less than 3:12. Dealing with such slopes is tricky. It is basically down to gravity.
Flat roofs tend to have more gravity weighing down on them. More gravity equals more load.
For snow loads, 2×4 fits for light snow loads and the 2×6 rafter for heavy snow loads. But the slope of the roof is still critical here.
Light snow loads on roofs with 2×4 rafters may still wreak havoc if the pitch is closer to 3:12. But generally, a 2×4 is a safe choice in an area that does not get much snow.
Both 2×4 and 2×6 rafters can get up to 22 feet spans for shed roofs. But again, the 2×6 is the best choice for heavy snow loads.
Also, the lumber type for the rafter determines the span you can get. Generally, 2nd-grade SPF (spruce, fir, pine) gives a longer span than SYP (Southern yellow pine).
So, 2×6 is the real deal if you want to go for spans longer than 22 feet. As per wind, 2×4 works for areas with light winds (winds with low velocity).
But the 2×6 is a good fit in hurricane-prone areas or locations that get strong winds. It is sturdy enough to help deflect winds and stay intact.
Do well to consult the wind load chart to know the wind speeds for your area.
Span Length for 2×4 & 2×6 Rafters
Typically, the maximum span length for 2×4 or 2×6 rafters depends on the lumber type and on-center (o.c.) spacing. Those factors must meet wind deflection and live/dead load limits.
Regardless of the lumber type and grade, the span length decreases with increased o.c.
The span length also drops with increased load readings. So, 20 pounds per square foot (PSF) tallies light loads. But then, 50 PSF means heavy loads.
Generally, 2×6 rafters span longer than 2×4 at any dimension and for any lumber type. But let us get into some specifics, shall we?
At light loads, 2nd-grade SYP lumber 2×4 rafters span 9 feet at 12 o.c spacing. They reach 7-feet-9-inches and 6-feet-4 inches at 16 and 24 o.c.
But 2nd-grade 2×6 SYP rafters span 13-feet-6-inches at 12 o.c, a massive 4-feet-6-inches gain over 2×4 rafters. At 16 and 24 O.C spacing, 2nd-grade SYP 2×6 rafters span 11-feet-8-inches and 9-feet-6-inches.
Let us peek at 2nd-grade SPF rafters at heavy loads (50 PSF). The 2nd-grade SPF 2×4 rafters span 9-feet-10-inches at 12 O.C spacing.
They reach 8-feet-6-inches and 6-feet-11-inches at 16 and 24 O.C spacing. Then, the 2nd-grade SPF 2×6 rafters span 14-feet-4-inches at 12 O.C spacing.
We notice a 4-feet-6-inches advantage over the 2×4 rafters at 12 O.C spacing.
At 16 and 24 O.C spacing, 2nd-grade SFP 2×6 rafters span 12-feet-5-inches and 10-feet-2-inches.
So, it is clear that 2×6 rafters have longer maximum spans than 2×4 sizes. So then, 2nd-grade SPF rafters have longer spans than SYP types for 2×4 and 2×6 rafters.
The maximum span lengths for other lumber grades will differ from 2nd-grade lumber. You can check the maximum span lengths of other 2nd-grade 2×4 and 2×6 rafters here.
Are 2×4 Rafters Sufficient?
Your decision to use a 2×4 should first stem from what span length you want to achieve. The overall load on the roof and roof slope also come into play.
But consider span length first before other things. The maximum span length for 2×4 rafters with 2nd-grade SPF lumber is 7-feet-1-inch at 12 O.C spacing.
That is under heavy loads. Note that 2nd-grade lumber is the standard for most construction work.
But you will get a wider span length (9-feet-10-inches) for 2×4 rafters under light loads. The span length also drops as the O.C spacing increases.
Then, consider the wind speeds and how much snow you get in your area. The 2×4 rafters are not fit for locations with raging winds and heavy snow loads.
So, if that is the case in your area, 2×4 rafters are NOT SUFFICIENT! But if you have it otherwise in your location, then it is safe to go with 2×4 rafters.
Lastly, construction experts warn against roof slopes less than 3:12 taking 2×4 rafters.
The reason is that the flat roof puts more pressure on the 2×4 rafter. But the 2×4 does not do well with weights like the 2×4 rafter.
So if you plan to have a flat roof, do not risk using 2×4 rafters on your structure.
Shed/Patio Build Roof Framing 2×4 or 2×6?
The most important thing to consider for sheds or patios is the weather in your area. There is no stamp on one slope you can use for both structures.
So, that favors both the 2×4 and 2×6 rafters. But a 2×4 rafter is a no-go for anything less than a 3:12 slope.
So, the 2×4 rafter goes for areas with fair weather, but the 2×6 goes for locations with ‘extreme’ weather. Overall, the weather translates to how much weight rafters will have to bear.
Sometimes, it is best to go with 2×6 rafters to be on the safe side. One great thing about them is that you can get wide span lengths even at 24 o.c. That would help you use a lesser number of 2×6 rafters.
Sheds and patios do not need long-span lengths. So both the 2×4 and 2×6 rafters can check the box for both structures.
You can choose between the 2×4 or 2×6 rafter after considering the weather in your area. The weather in your area is the defining factor.
First, you must consider the weather before choosing between 2×4 and 2×6 rafters. Factors like roof span, slope, and structure size also impact your choice.
The 2×6 rafter can handle more weight and reach wider span lengths than the 2×4 rafter. Weigh your needs against what the 2×4 and 2×6 rafters offer, then choose what suits you.