Strength of 2x4 and 2x6

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Forum topic by D_Allen posted 07-05-2011 03:41 AM 37773 views 0 times favorited 33 replies Add to Favorites Watch
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495 posts in 2984 days

07-05-2011 03:41 AM

Got a question for you builders.
I am desiging an awning off of my garage and am wondering about the difference of a double 2×4 compared to a 2×6. There isn’t much room for a lot of pitch so it will have to be a 2/12. I need to use 2×4 rafters but am wondering about possible sag. If I double every other one with 2×4s, would it give me a strength that is close to a single 2×6? They will be 16” on center and the longest ones are 10 foot. The blue lines below would be the double 2×4s

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33 replies so far

View lew's profile


12428 posts in 3955 days

#1 posted 07-05-2011 05:13 AM

Our single car carport has virtually a flat roof- just enough pitch to get water to run. The house is almost 60 years old. The “rafters” are 2×4’s 16” on center covered with tongue and groove 3/4” boards and rolled roofing (the roofing has been replaced several times. It has held up- with only minor sagging we live in south central Pennsylvania so there have been winters when the snow has been quite heavy.

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View Dark_Lightning's profile


3342 posts in 3309 days

#2 posted 07-05-2011 05:40 AM

In this picture, it appears to me that the studs are doubled. You are better off with one 2X6 versus two 2X4 sistered. Rafter size is dictated by your local code. In the past, I have used 2X4 rafters where now 2X6es are required due to the 1994 earthquake here in Southern California. I am not qualified by law to tell you what to do, but for that span, you should be OK with 2X6es. Depends also on what you use for a ridge beam.

-- Random Orbital Nailer

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495 posts in 2984 days

#3 posted 07-05-2011 12:54 PM

AtomJack, I think you are seeing this differently than intended. All of the area shown is the roof, viewed from above. There is no ridge beam as it attaches to the gable end of a garage. The odd shape is to take in as much of the patio area as possible.
Thanks lew, that leads me to think that the doubled rafters will work. And I’d suspect that a 60 year old 2×4 was much heavier and stronger than the stuff sold today!

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View HorizontalMike's profile


7770 posts in 3114 days

#4 posted 07-05-2011 02:05 PM

The strength of rafters comes from their width. You will gain very little doubling up a 2×4 when compared to using a single 2×6, especially when trying to span 10-feet. And besides, you will use less lumber using 2×6s. Just saying…

-- HorizontalMike -- "Woodpeckers understand..."

View helluvawreck's profile


32087 posts in 3066 days

#5 posted 07-05-2011 02:24 PM

I’m not a carpenter but I agree with Mike. From a common sense standpoint the 2×6’s would cost less and take less work and the roof would also be lighter.

-- helluvawreck aka Charles,

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Don W

19009 posts in 2767 days

#6 posted 07-05-2011 02:32 PM

I’m not sure how much snow you get where you are, but its not going to take to much with snow load to over stress 2×6’s in a 10 ft unsupported span.

-- - Collecting is an investment in the past, and the future.

View nailbanger2's profile


1041 posts in 3343 days

#7 posted 07-05-2011 02:54 PM

I would tend to agree with Don W. Hope this helps.

-- Wish I were Norm's Nephew

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4541 posts in 3274 days

#8 posted 07-05-2011 04:59 PM

A slightly different perspective for consideration – -

Usually, a 2×6 is preferable to 2 2×4s. There may be an exception if you are dealing with inferior material. A board is only as strong as its weakest spot along its length. If you are dealing with wood that has weak spots, sistering 2 boards together can be a good idea.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View HorizontalMike's profile


7770 posts in 3114 days

#9 posted 07-05-2011 05:03 PM

D_Allen didn’t say whether or not this awning will be supported or not. If NOT supported then even wider boards should be used and Nailbanger’s link will come in handy.

-- HorizontalMike -- "Woodpeckers understand..."

View Sawkerf's profile


1730 posts in 3268 days

#10 posted 07-05-2011 05:16 PM

Since you’re in Ohio, you’ll be dealing with snow loads. You don’t say why you need to use 2×4 rafters, but if this were my project, I would find a way to use 2×8 rafters. I would also have a rafter at the apex of the roof line (where your 10’ dimension is located) so the facia boards aren’t hanging in space.

-- Adversity doesn't build reveals it.

View derosa's profile


1577 posts in 3036 days

#11 posted 07-05-2011 05:47 PM

As long as you have support posts you should be fine with 2×6s at that spacing. My house’s roof is fairly steep which helps a lot but is made of 24’ 2×4s set 16” on center with only one support each in the center and they have held for 93 years without any noticeable sagging despite being in upstate NY. The load could get heavy being flatter but if they’re properly braced to the wall and supported at the open side a 2×6 roof can hold a lot. Just make sure your support posts are buried deeply enough for a proper anchor and the roof is properly attached to them or I’d be more worried about the roof blowing away in a good storm.

-- --Rev. Russ in NY-- A posse ad esse

View D_Allen's profile


495 posts in 2984 days

#12 posted 07-05-2011 07:47 PM

OK, this needs more splainin I guess.
This is NOT a clear span and not cantalievered into the existing structure.
There will be post supports at the 3 points opposite the garage wall and there will be headers between the posts, probably double 2×4s.
There will be a 2×6x12 at the top where the rafters tie in and I will use hangers.
There will be at least 5 horizontal ceiling ties that I may also use to help support the rafters. These will tie the garage wall to the headers.
My effort here is to keep the dead load to a minimum as I know there will be some snow load.
I calculated this to be about 5 pounds per square foot so with a snow load of the same I am within tolerance on some charts.
The reason for the 2×4 rafters is because going to a 2×6 would increase the tie in height and I would give up some of the pitch to do that. I cannot go higher as this will be attached to the gable end of a 24 foot wide garage. I plan to use vinyl screen doors around the perimiter so that is my height at the low end of the roof.
This is a poor attempt to see what it would look like. The reason for the weird angles is because the garage and house are not in line and the gable ends are in a V configuration about 12 feet apart.

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View nailbanger2's profile


1041 posts in 3343 days

#13 posted 07-05-2011 08:09 PM

You could always make your own trusses. This would increase the strength without losing any height. Top member 2×4, bottom member 2×4, with webbing of 2×4 or plywood. It’s important to have very strong joints, which is why I mention plywood. You could cut a groove and glue it in between the members. Of course this would more than double your lumber cost. Just a thought.

-- Wish I were Norm's Nephew

View GBS's profile


30 posts in 3907 days

#14 posted 07-05-2011 08:17 PM

A doubled-2×4 is only half as strong in flexure as a single-2×6. The bending moment of inertia “I” of a rectangular section is proportional to the cube of the section’s height. I = 1/12 [b*h^3] where b is the width and h is the height. Every little bit of additional section height counts WAY more than adding (a whole bunch of) width.

To put it differently:
Under the same weight per sq ft with identical spans, a roof made of ALL doubled 2×4s at any spacing will deflect TWICE as much as a roof built of single 2×6s at the same spacing. This is due to the section’s bending moment of inertia described above.

Building codes aside, (!!) what deflection will you find acceptable?

View D_Allen's profile


495 posts in 2984 days

#15 posted 07-05-2011 10:00 PM

What if I braced the rafters like this?


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