Home Forums Hunting & Shooting Hunter's Campfire Any engineers, contractors or overall 'smart' people got a moment?

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

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?

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.

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.