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I'm wanting to remodel our attic to add a few bedrooms. We currently have 2x6 floor joist that span 16' on one side and 18' on another. All meeting up on the load wall. We were going to sister 2x8 to the existing 2x6 but are worried about the extra weight and the huge amount of pain it will be.

All our current 2x6 joist are spaced 16" on center.

I'm being told that we could just put in braces in every 16" to form a 16" x 16" cube structure. Is this true?

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    The 16"X16" cube idea will not work, as Michael Karas puts it, the joists need to be 2X10's to carry the span you have. I looked it up in the 2012 IRC code book, wood needs to be at least Hem-fir, S-P-F will not cut it – Jack Jun 27 '17 at 6:48
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    Modern building practices (codes notwithstanding) don't allow for 16' spans even with 2x10. That requires engineered I-joists or trusses, or 2x12 lumber in the case of an exterior deck. That said, 2x10s might reduce bounce to acceptable levels. – isherwood Jun 27 '17 at 15:25
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2x6 joists at the span that you are talking about (16' and 18') are not strong enough to support a floor as a living space. With a span like that the timber size that you sister in would have to be at least 2x10's. Look this up in a joist span table (available in numerous places on the web) if you do not believe me.

Trying to install cross bracing on the existing 2x6's can stiffen the overall construction if it is installed correctly. It will not however make the overall floor any stronger. Instead it just spreads localized loads across multiple joists to lessen flexure due to live loads moving around. Depending on the method used it will add to the dead weight of the existing structure. If you tried to install cross braces using 1x3 or 1x4 material in an X format you would have to remove the ceiling from below in order to properly nail in the bottom ends of the cross braces. If instead the bracing would be done by installing 2x6 blocking between all the exiting joists these could be nailed from above using the appropriate tools. Although nailing 16d spikes in between 16" on center joists with a conventional hammer is no picnic. It is also likely that nailing to the existing joists like this will lead to some flexure and corresponding ceiling damage below.

It would be typical that a remodel like that would have, at a minimum, the roofing and roof sheathing removed on one slope of the roof so that the long members can be poked into the space from the outside. If the whole attic was basically clear it may be possible to make one hole in a gable end to push in the timbers and then position them in place in the attic space but it would be a total pain.

Then with that all said you would have to decide if the loss of an additional four inches of floor to ceiling height under a sloped roof area ends up making a usable bedroom living space anyway.

Rooms built up under a sloped roof often face a huge compromise of insulation in the rafter area where the sloped part of the ceiling comes into play. Done blindly with the idea of just thinking of attaching sheet rock to the bottom if the rafters you lose even more insulation due to the need to provide air vent paths in this space from the rafter tail area up to the peak area. Be prepared for a lot of heat loss in winter and the rooms potentially getting very hot in the summer.

A better choice remodel may be to consider an addition to extend the existing floor levels out the amount needed to provide for the added rooms. Or if there is an existing attached garage consider a garage conversion.

  • A shed dormer may help him get some valuable square footage.... A boatload of work compared to his original concept – Jack Jun 27 '17 at 6:44
  • The shed dormer could offer more full height square footage but the joists would still have to be dealt with. Part of the boatload of additional work is in redesigning the upper end rafter support back into the load carrying structure of the building. Shed dormers also have two attributes that detract from their usage. First they can change the appearance of a nice looking structure to one that is ugly. Secondly they lead to a roof that has very little pitch and this can be a problem with regard to roof quality in rain and snow conditions, – Michael Karas Jun 27 '17 at 11:25
  • I had a cape cod in Maryland with a shed dormer facing the rear, well designed. I was also in agreement with the 2X10s I had added a comment to the question as such. I spelled your name right this time:) The boatload of work I was referring to was raising the roof line to keep a decent pitch, since I was only adding a comment, I felt it wasn't necessary to elaborate too much. – Jack Jun 27 '17 at 15:29
  • Maybe I'm looking at the wrong span tables, but I don't see where a 2x10 can span 18'. Looks like doug fir and southern pine SS can make the span, but only if this is a sleeping or attic area (30 live, 20 dead). If this is a living area (40/20), looks to me like you'd need 2x12 SS lumber for an 18' span. – Tester101 Jun 28 '17 at 12:08
  • @Tester101 - I had checked a couple of tables and you are indeed right that the best grades of 2x10's can just make it for that span with the load analysis on the light to medium scale. In my book the existing 18' span of just the existing 2x6 ceiling joists is barely good enough, They would be more than enough if they were built into a rafter truss as the lower chord and tied into the triangulation structure of a properly engineered rafter, – Michael Karas Jun 28 '17 at 13:19
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Sistering a 2x8 joist to your existing 2x6 joists at 16" oc will work. However, I'd use a No. 1 grade rather than No. 2 & better.

The Code requires a minimum of 40 lbs. per square foot live load and I'd assume about 15lbs. Per square foot dead load for a total design load of 55 lbs. per square foot. The 2x8 plus the 2x6 with No. 1 grade joists will give you about 68 lbs. per square foot (depending on species of wood where you live) for a 16' span and about 56 lbs. per square foot for an 18' span.

I'm assuming there is an existing ceiling you don't want to disturb, so be sure to install the new joists "crown" up. There will be wires, ducts, etc. you'll need to work around too. (By the way, no waterbeds.)

There are a hundred other considerations too: minimum head height required, minimum roof insulation, minimum tread depth, maximum riser height, minimum egress window size, added electrical load, etc., which you didn't ask about, but it's do-able.

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    To get advantage of the 2x6's to help carry the load from above the sistering would require the 2x6's and 2x8's to be firmly fastened together all along their lengths. Without that the 2x8's would be carrying the load from above on their own and thus the reason it would be better to go right to 2x10's and not worry too much about fastening the members together. – Michael Karas Jun 27 '17 at 11:31
  • In my experience, "sistering" implies load sharing. Obviously this can't happen unless the existing 2x6 lumber is structurally involved. I think the downvote was inappropriate. – isherwood Jun 27 '17 at 15:28
  • @isherwood Downvote for "sistering" ??? – Lee Sam Jun 28 '17 at 0:28
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    I think in a case where you're sistering joist to increase the strength of the floor, you're going to need engineer documentation proving that it's adequate. You'll also have to make sure the fastening is done as per the engineer's instructions. – Tester101 Jun 28 '17 at 12:11

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