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This house has had the remedial work necessary to secure the crawlspace foundation by a licensed foundation specialist. The crawlspace foundation and 1st floors of this home are rock solid. My question is in regards to the 2nd floor joists. Throughout the house the ceilings of the 1st floor sag any where from 1-3 inches from the wall to the center of the room. This same sag is evident in the floors of the 2nd floor. The deflection is evident with a hammock like effect toward the center of the room which causes a pronounced bounce or trampoline like effect when walking across the room. Currently the home has (170 year old) true 2x8 joists on 2 foot centers spanning 19'x19' rooms. My question is this: will running a tripled 2x8 LVL girder perpendicular to the existing joists be adequate to stabilize and re-level the floors? The existing joists would be cut and hung on the new LVL girder and then sister-ed with new 2x8 lumber. My concern is the Tripled 2x8 LVL girder spanning 19 feet, will it be strong enough to maintain level with live weight? would it be advisable to use flitch plates between the LVL's to further strengthen them.

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  • Hello, and welcome to Stack Exchange. My guess is that we won't be able to answer this; you'll need an onsite structural engineer to get an authoritative response. Sep 25, 2018 at 15:07

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Excellent description as to existing condition and what is occurring on site.

You have several issues to deal with: 1) due to the age and amount of sag, you will not be able to jack it level, 2) cutting the sag out and sistering another joist will not solve the sag on top, 3) installing a beam will require a post and footing at each end, 4) solutions will be limited to the height of the ceiling.

Code requires a minimum of 40 lbs. per square foot live load (furniture and people) plus I’d guess about 15 lbs. per square foot dead load (joists, flooring, ceiling finish, etc.) for a total load of 55 lbs. per square foot.

When retrofitting older buildings, we like to “evenly distribute” loads rather than create point loads. That is to say, adding joists to span the room rather than adding a beam will probably create fewer problems. ...but here are two options:

Option 1: Add Joists: 2x8’s can’t span 19’, so I’d use 2x10’s and hold them down to the bottom of the sag. (It’s okay to notch them at the ends to sit on the plate.) However, 2x10’s spanning 19’ will only support about 60 lbs.per square foot, (depending on the species and grade,) so you’ll need to install them 12” o.c. (Be careful about creating squeaks between the new floor joists and existing subfloor. I’d recommend you nail or screw the subfloor to the new joists.)

Option 2: Add Beam: I’d recommend not trimming the bottom sag off the existing joists if possible...just sister a joist to make the bottom level. The existing 2x8 joists can support about 80 - 100 lbs. per square foot at 24” o.c. and spanning about 9’. So, I’d sister 2x4’s or 2x6’s to the bottom and make the bottom level. For a beam to span 19’ and support the floor, you’d need to support about 520 lbs. per linear foot. You’d need a 6x14 (depending on the species and grade) or a glu-lam beam. Option 2 does not seem plausible because you’ll reduce the height in the room by 14” or so and create about a 5,000 lbs. point load at each end.

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  • just wondering if you have any thoughts on the triple LVL recommendation. We could certainly triple a 2x10, run it perpendicular to the existing joists and then cut and re-hang existing joists on triple LVL beam. Planing to sister new 2x8 joists no matter what and to install them 12" o.c. Sep 25, 2018 at 17:09
  • would the flitch plates offer any benefits to the project? Sep 25, 2018 at 17:15
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    Sure...a flitch plate girder or any beam that reduces the span of your existing 2x8 joists will help. However, be aware that you’ll need to provide a post (and connection) at each end along with a giant footing. (The load needs to get transferred to the ground.) if you don’t install the new footing, you’re just moving the problem from sagging joists to cracking foundation.
    – Lee Sam
    Sep 25, 2018 at 17:24
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In nearly any US jurisdiction, you would need a building permit to complete this level of work. And they will almost certainly require a structural drawing drafted by a structural engineer. This is not DIY territory.

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