Reinforcing Floor Joists in Bump Out.
I’m renovating a kitchen that has a 2 ft bumpout (cantilever). New cabinets with granite countertops will be installed in the bumpout and tile will be installed on the floor so to be on the safe side I’m considering doubling up the floor joists. The 2×8 floor joists are approximately 14 ft in total length including the 2 ft that cantilevers over the foundation.
My initial plan was to sister 12ft 2x8s to one side of the existing floor joists under the kitchen and sister 6 ft 2x8s to the opposite side of the existing joists which would extend 2ft out to the cantilever. But the thought occurred to me that it might make better sense to open the exterior wall at the cantilever (new siding is also part of the plan) and install full 14 foot 2x8s. One benefit of this method would be that I wouldn’t have to temporarily remove some water pipes that run on the bottom of the existing joists.
Any thoughts, pro or con?
Replies
The longer your new joists the better on an exterior canteliever, although 2' isn't bad. Using a structural adhesive like a polyurethane construction adhesive and lots of deck screws to pull it all together tight and even the short 2x8's will go a long way towards beefing everything up. (normal construction adhesive isn't rated as a structural adhesive since it creeps over time)
On any older canteliever there is normally quite a bit of droop - how far has the floor sagged there?
There's not going to be much strength difference between a 12 foot sister and a 14 foot one.
jersey
14'span and tile on 2x8 joists is not a good idea. Too much spring. Will doubling them up help?
Yes, how much I have no idea.
JohnBridge forum might be able to help with their span calculator.
As for the granite-part of the weight will be right over your foundation wall-don't know that full length sisters will be real necessary.
It's only a 12 foot span.
dan
originally, the 2x8x12 is low on the list for tile.
Adding scabs to the sides?
Using I-joists in addition?
Beats me, but something to think about.
Thank you for using the John Bridge Forums Deflect-O-Lator :-)
For joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 12 feet long between supports, the deflection calculated is 0.478 inches.
This translates to a deflection of L / 302.
Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.
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The longer your new
*duplicate*
Sorry I wasn't more clear with my description. The entire length of the 2x8 floor joists is 14 feet. 2 ft of that makes up the cantilever portion. My intent was to double up all the joists but of course a 14 footer won't go in from the basement so I was going to do it in 2 stages. Use 12 footers on one side of the existing joists inside the basement and slide 6 footers into the bumpout section attaching them to the now doubled existing joists. This would leave 2/3 or 4 feet inside the basement. I just thought it might make better sense to remove the outside sheathing and sister the full length of the run with full length 14 footers.
But in light of some of the comments, maybe beefing up the bumpout would be overkill which I've been frequently known to be guilty of. I'm at least going to do some more research and maybe find out what the load bearing capacity of that 2 foot bumpout is. It does bear the weight of a 10ft section of the roof and ceiling.
Thanks for the input and the link to the Deflectometer (Dan). At least I know I need to double up the joists under the tile floor area.
If you have 16" OC 2x8s (&
If you have 16" OC 2x8s (& no. 2 or better syp or df), and if the roof above is 12 ft wide to the ridge (or less and you are in a 20# sf snow load or less), you do not need to do anything to the overhang if your added granite and counters are < about 500 # ft. with one side of the counter (eg. front) over the wall under the 2x8s. E.g. a 1000 pound fridge 2 ft wide is OK.
see all the (if)? That is the type info you did not provide that is critical.
The only way ylu can beef up the overhang is to remove the blocking an put in 14 ft sisters, a 2 ft piece outboard DOES NOTHING
They wouldn't have to be 14-foot sisters to be effective. They just need to go back 6-8 feet, though longer is better.
It's not unusual to see a noticeable sag in a 2 foot overhang. Structurally it's fine but it looks like sith.
one more look not good
Seeing the cracks in tile or grout telegraphing the 4x8 subfloor edges. Seen too many times in high end homes. Looks good from a distance.
junkhound wrote:
If you have 16" OC 2x8s (& no. 2 or better syp or df), and if the roof above is 12 ft wide to the ridge (or less and you are in a 20# sf snow load or less), you do not need to do anything to the overhang if your added granite and counters are < about 500 # ft. with one side of the counter (eg. front) over the wall under the 2x8s. E.g. a 1000 pound fridge 2 ft wide is OK.
see all the (if)? That is the type info you did not provide that is critical.
The only way ylu can beef up the overhang is to remove the blocking an put in 14 ft sisters, a 2 ft piece outboard DOES NOTHING
I didn't put all those (ifs) in my post because I didn't think anyone would be inclined or interested in doing load ratings for me. I was mostly interested in opinions on whether it made good sense to remove the outside box and sister the entire joist run as a unit rather than in two stages.
But the questions you and others raised made me question whether I needed to reinforce the bumpout at all.
I did some digging to get those roof run measurements you mentioned so I could add up the loads and found that there is a full (foundation supported) girder over the kitchen that supports both the roof and ceiling up to the bumpout section. So the bumpout is basically only supporting itself. I'm satisfied that it will easily support the new cabinets and granite so I'm only going to double the joists under the floor.
Thanks for the help guys.
If you're going to sister them, I'd say sistering full length is the way to go. If you don't do the cantilever I doubt it would be a big deal. But it won't hurt either.
Your span/depth ratio isn't great. So whatever you put in should be solid stuff. I'd suggest either the highest grade lumber you could get, or maybe some 7 1/4" LVLs.
Or maybe buy 14" LVLs and rip them in half. If they're a little shorter than 7 1/4" they'd be easier to slide in place.
Maybe Doing Something Different
While doing some more research I came across some threads and posts on gluing and screwing a 2x4 to the bottom of a joist to strengthen it. I had read about this before but what I had missed in my previous readings was that this is as good if not better than sistering the joist. More than 2x less deflection. Basically would turn the 2x8 joist into a 2x10. I have no problems sistering them but it would require a bit more work since I have to pull some of the wiring running thru the joists. But if the 2x4 method is even better and it's less work, seems like a no brainer.