Assume that I have an attic which currently has 2x6 joists on 16-inch centers. If I sister a 2x8 to each 2x6, meaning that the bottoms of the joists line up but the top of the 2x8 is approximately 2 inches above the top of the 2x6…is there a span table for such a situation, or any rule of thumb for estimating strength as per code?
Not an engineer, just a construction guy. Sistering adds virtually nothing unless you are running from support to support. Otherwise it sags under weight and you gain nothing.
That said, factor the strength of the 2x8 alone, as the 2x6 contributes little. I base all of this on nothing more than 30 years of ripping crap apart due to failure. I am not a framer, I’m a tileguy. Lots of sistered joists in bathrooms from water damage and plumbers cutting right through structural members to run pipes.
Take that for what it’s worth.
depends on the wood used to sister and its grade.
southern pine 2x8 on 16 could span 13 to 19 feet depending on its grade.
It would add resistance to sagging away from the supports say in the center of the attic at least - no?
I can see how it might not add to the total load (and probably actually detracts due to the added weight) if the 2x8s are not attached to the supports, but I can’t imagine why anyone would sister the joists and leave them floating (not sure if that’s the proper term).
OK, let me clarify some things:
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I am a licensed Engineer, but not a structural one.
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I plan on having the 2x8 run the entire length of the 2x6.
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Assume the same wood and grade.
What I’m trying to find, after fruitlessly scouring span tables, is how much stronger the 2x6 + 2x8 are compared to just a 2x8.
If the smaller is good to start with then doubling the smaller figure should give a decent safety margin if properly attached. The attachment is the key.
Glue and nail a 3/8 to 1/2 inch plywood piece the same depth and the full length and that will really up the load carrying ability of that rafter.
If you are looking at just hanging a porch swing, simple sister-ing should do fine.
If you are going for a lot of weight like an second story and are not staying over the load bearing walls, then not only will you have to really check that the load bearing walls are strong enough and that the rafters will really need major strengthening…
Not seem many 30 year old or older houses that the ceilings did not sag a little with 2X6’s and longish spans. Just not enough for long term rigidity.
No schooling, lousy at this kind of math, not a real builder, just old & have seen a lot.
Just stuff to add to your thinking list… 
The issue is this: currently the upstairs has 2x6 joists which are mainly there to support the ceilings below. If I want to turn the upstairs into living space (to take on boarders, traveling sharecroppers, etc. as we get deeper into this Depression) then the code says I need 2x10s. However, I can’t practically take the 2x6’s out without destroying all the ceilings in every single room below. A 2x8 comes close to code, so I’m wondering if I sister the 2x8s to the existing 2x6s with liberal use of Timber-Lok screws and glue, then does the net strength push it up to the level of a 2x10. There are lots and lots of “rules of thumb” out there, such as “2 2x8’s = 1 2x10” but from looking at the span tables, that doesn’t seem to be true.
Una,
Sistering will help. You also need bridging between the joists. It’s important that the bridging is in a straight line.
Some information here.
I’m not an architecht or engineer, but given enough glue and screws to bond the two together, I’d expect the combo you propose to act similarly to a 4x6.
Is the consideration price or not being able to fit 2x10s and still have a code-compliant ceiling height in the new living space?
You really need to contact your local building department and “pick their brains” so to speak since they will have the final say.
How about engineered lumber? A 2x8 LVL should be more than equivalent to a 2x10 pine/doug fir.
Worst case scenario - is it practical to just call it a day and sister the existing joists with 2x10s?
The only reason I’m opposed to sistering with 2x10s is I’m trying to see how to minimize weight on the walls below, since a 2x10 weighs about 25% more than a 2x8. If I must go with 2x10s I must, but I was wondering if sistering to the 2x6s would be equivalent.
What is LVL? Laminated something lumber?
Here’s my 5-minute analysis, FWIW.
There are two issues: strength and deflection.
For strength, the section modulus (S) of the combined 2x6 and 2x8 member must be greater or equal to the section modulus of the 2x10. You must also make sure that the fasteners connecting the 2x6 to the 2x8 are of sufficient capacity and spacing to transfer the forces between them so they act as one member.
For deflection, the moment of Inertia (I) of the combined member must be greater than or equal to the 2x10.
And you can’t just add up the Ss and Is of the 2x6 and 2x8 - you would need to calculate the new centroid, then calc the S and I.
I’m a structural engineer but timber is not my thing, so there may some tables or aids out there that I don’t know about.