In a nutshell, my cabin has 2x6 floor joists on 16" centers over a very tight crawspace, spanning 7'-4". I was planning on installing ceramic, if not slate tile overtop. I have a substancial subfloor, doubled 3/4" plywood for a total of 1-1/2".

My concern was the joists themselves.. I know 2x6's arent exactly massive, but on the other hand, the span is rather short at less than 8 feet.

The floor is basically level except for one spot: The five joists under my hearth are sagging down about 3/8" at the lowest spot. The reason is probably simple: the hearth is built upon 2x10s running on top and perpedicular to the 2x6 joists [to get the stove up to waist level] and in addition to the plywood and hardibacker [for fire resistance] it was boxed in with, there is 2" flagstone [and mortar] across the top.. 3'x5' hearth, so that is alot of weight spanning 15 square feet on top of those particular 5 joists, even with such a short span.

I have been doing some research, and HAD decided to full length sister [screw and glue] with another 2x6 on every joist under the floor [I would definatly jack up the sagging joists under the hearth before sistering.] A buddy of mine, one of those, 'jack of all trades' people, suggested sistering, well I guess a more accurate term would be sandwiching, a 5.5" sheet of 3/4" plywood, again, full length, on each side of every joist instead??

Basically, he is saying to rip some 5.5" sheets of 3/4" plywood and cut them to 7'-4", glue them up with contruction adhesive on each side, and put three [vertical] 2" deck screws every 12".

I cant see how that would add any strength, though he ensured me that it would definatly add strength and stiffness for the tile, as well as help the joists under my hearth to resist the dead load overtop.

Thoughts?

If I'm understanding correctly, you would be making the opposite of the way a door/window header is constructed?

In the orientation indicated, that plywood has it's greatest resistance to flex.

I'm sure others here will give good advice. There may be online programs to calculate the max weight bearing possible for that joist/spacing/span system.

Can you calculate a weight for the tile/slate?

Exactly. I was hoping my explanation would make some sense; sometimes, it's hard to describe this sort of thing without pictures!

At any rate, you are correct.. instead of a over-window or door header, where two 2x's are sandwiching a piece of plywood, it would be the exact opposite, a full length sheet of equal-width plywood sandwiching a floor joist on each side.

His reasoning was the arraingment would provide similar strength as with sistering an identical joist to the existing one, but would be much easier to work with, as I had mentioned, the crawspace, over dirt [with SPIDERS I may add.. ugh] ranges from about three feet high to about a foot high to the bottom of the joists.

TJIs have ply as the center of a wood "I" beam and are damn rigid in that orientation. Lay them flat and they flex more than a dive board!

It sounds workable to me, but there are some engine-ears on here that could run some calcs for you.

I think you're on the right track. Only thing I would change would be to use SDS screws.

Your buddy has the right idea, but you need to define 'tight crawlspace'. If this is a get away cabin I'd suggest some roll stock vinyl, or place and press and avoid 'tight crawlspace'.

A tight crawlspace for me is not being able to roll over, and lots of rats and black widows..

Perfect spot to send the apprentice in.

When you get to putting your underlayment in look at ditra as opposed to hardi backer or wonderboard. Ditra is really easy to work with and in your case you may not have to do much to reinforce the subfloor and framing. Also by design it allows for some movement in the framing/subfloor.


http://www.schluter.com/6_1_ditra.aspx

2x6's aren't conventional for floor structure. I would never use 2x6 in a floor close to the ground.

However, 2x6's can actually go a little over 9 feet span (from the book, Building Construction Illustrated). Your deck thickness is very strong as is.

I would level the sagging areas with a sistered piece, 2x6 or plywood. I may even do every other one glued and screwed. You want a stronger than normal floor for large tile.

Here is a handy site I have used in the past.

http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

Seems to me it would be much, much easier to add bridging between the joists or maybe an additional beam under them halfway between that span than it would be to sister or laminate them...
Heck, even screwing a perpendicular 2X4 to the bottoms of the joists would help them "share" the load and stiffen things up. You can't need much more with that sub-floor.
Good luck.

Can you build a pony wall @ mid span? That is usually the cheapest and most effective fix.

do not sandwich the ply wood to the joist = waste of time and adds 0 to the spanabilty! your 2x6 construction in weak...u need to run a header board and split that span in 1/2.. ..not enough space for a wood beam then move to steel. add- any flex in the floor is going to make cracks in the tile joints... tgi/bci's only add stiffness > as there height increases.... u can sandwich all day and do yourself no good.

Seems like this would be the best option. You've gotta jack it all up anyway, fix the problem instead of putting a bandaid on it.

I did two jobs like you are talking, except I didn't have a real tight crawl space. I built floor trusses in place.

ps, your cabin floor is a quite-a-bit less than what i'm doing on my current project -notice the use of a 16"/90 series bci to use as a plank to walk while spreading trusses. was it safe? yes....imo... and it served me well but sucked bad removing it.

Goodness gracious...have a little sway in that?

Engineer checking in.
Let's talk stiffness for a moment. The stiffness of a beam increases proportionally with its width. Double the thickness (by adding a sister beam), and you effectively double the stiffness of that beam. However, the stiffness increases to the 3rd power for an increase in the depth of the beam. Do, double the depth of your beam and get a beam 8x as stiff. Sistering then, while providing some benefit, has significantly less increase in stiffness than providing a deeper beam.

If you want to sister it, I would suggest the sister beam being something deeper than a 6x. This will increase the stiffness more than just slapping another 2x6 on the side would. If need be, you can notch the end of the new sister beam at the ends so that it sits on the support simiarly to the existing 2x6 - the benefit of a deeper sister beam is derived at the midspan of them beam, and notching the ends will have no appreciable impact on the performance of the new beam).

If you want to consider other options, providing a new support at the midspan of the joists would help as well. A beam can support 4x the amount of uniform load as a beam just twice its length. Like others have mentioned, a pony wall or midspan header would work. If you use a header, that header must span to a support as well - simply adding bridging between the existing joists will not do anything beyond potentially dragging a couple other of the existing joists into the party - certainly not enough considering the level of weight you have for these joists to support.

19' off the floor ...no sway all 2x8' construction..a bit much of a spot to hang out at till the boom trk got the truss up

I was thinking along the same lines-if space permits, go with a deeper joist like a 2x10 or even 2x12. I will differ with you on the notching, though. I'd recommend either not notching the deeper joist, or, using hangers or possibly a ledger board to support the bottom of the new joist to eliminate the possibility of a longitudinal crack occurring at the mouth of the notch in the new, deeper joist.

If plywood is used to sister the joists, be sure and use something that is moisture resistant, such as marine or maybe just treated plywood to resist de-lamination due to moisture.

This is your solution. A sandwiched 2X6 will help in some spots but not in others due to the nature of imperfectly straight lumber. A perpendicular beam mid span will give more support overall even if you have to dig on either end to install and dig an access hole also.

You can buy a device designed for this. It is a steel post with a screw jack on the end like on top of a hydraulic jack. For the beam I would use a 6" thick lam beam or steel I beam for it's straightness.

If you are less than 18" off the ground, I wouldn't spend any big money on anything because rot will eventually destroy it anyway. Just put enough into it to enjoy it while you can(best option) or go big and have a professional jack up the whole structure then reinforce the floor.

Here's what I read:

2 related problems:

1) The 2x6s on 16" centers used as the floor joints for support are/were too small from the get go.

2) Some are now sagging which probably has negatively effected the plywood flooring contact.

Both reduce the floor loading which now you want to increase with the addition of slate tiles.

The best solution, as I suspect you know, is to redo the entire foor before adding the new slate tile. Cause if you don't, you run the risk of more sagging and having too fix it ($$,$$$) again.

If you could add both the center supports with the sistering to the joints you could get away OK. IMO, I don't think just center supports or Just sistering either alone is going to work.

Agreed on the notching. Just throwing the option out there depending on the support condition.

I'd dig a couple of footings, and beam the sob at the half point.

Easy peasy, no hassleasy.

the deflection is too great for slate using 2x6's simple as that. 2 layers of 3/4" ply aren't helping you any either.


ML

atv has it figured out. Split the span.

I'm with the "split the span" crowd, a treated 4X6 with the tall dimension vertical, even just blocked with patio blocks every couple of feet will do a bunch more than scabbing on the plywood. Does your crawl space get exposed to freezing? If so you might want to dig a trench, back fill with gravel to build your supports on top of.

They make a concrete "footing" block for under decks, that might fit the bill.

This way you can string line your timber and jack where needed blocking as you go. You can also go back underneath and add blocking later if settlement occurs.

I'm afraid the scabbing will create the opportunity for moisture in between the pieces to help rot the 2X6's over time.

All the form loading books, I've got a shelf full of them, are for "short term use", something long term like a floor joist is different.

Shortening the span is the answer.

My 2 Cents,
Mike

Let me try to explain what I'm working with here and perhaps that will help to clarify.

What started off as a simple Shack for hunting, ended up growing and growing to what it is now, more or less my home.

It started as 6, 6x6 treated posts, sunk 48" into the ground, on 8' centers. 6x6x16 beams laid across the posts and 7'-4" treated 2x6 joists were spanned between the 6x6 beams and hung with joist hangers. The outer spans between the beams are spanned with 2x12s. The rest is pretty standard, 2x4 walls, etc.

Eventually, two more identical spans were added with the 6x6 posts and beams with the 2x6 joists running in between. Total size is now 24'x16'.

The cabin is built on a slight slope, downhill at an angle from one corner to the opposite corner. The low corner has the joists approximately 16-18" off the ground, the high corner is about 36".

Everything under the subfloor is treated and the posts are soaked each spring with a petrolium/tar solution. The joists, beams, posts, and ground beneith are bone dry, no water runs underneith, even during a downpour, so whoever mentioned rot, i dont think its gonna happen, at least not in my lifetime.

As far as the suggestion to split the span, i agree, and wish it were that easy, but its not. There would be no way to dig deep enough to get under the frost line while under there to install posts to support a cross beam.

Sistering with deeper 2x's (say, 2x10s) is certainly do-able, as someone suggested, but other than screwing and gluing to the existing 2x6, i have no way to attach the ends of the 2x10s to the 6x6 crossbeams, aside from toenailing them in on the upper half of the 2x10?

I hope that clears things up a little. Maybe itll help you to guide me to a fix to this dilemma. If need be i could snap.some photos or a drawing to help to explain the existing structure.

u still have to split it ... u could put in two concrete pads for footings and place the "screw jack post" on top they do make Simpson strong tie face hangers that would work ...u have to order them in...as they are not joe homeowner items.

A complete waste of time and materials.

Since it stays dry, don't worry about the frost line. Just get something underneath to split the span.