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Second floor beam size

KVSteve's picture

Recently purchased a cottage build in 1993 and am gutting it to repair many "issues" including what I think is an undersized beam supporting joists for a second floor sleeping area (2 bedrooms).

The interior dimensions are 15'W x 23'L.

Joists are simple span and butt up to a center beam just toe nailed (poorly) to the beam!

Outside ends of joists are also just toe-nailed to a ledger which in turn is nailed to the 2x6 wall studs with 3 common nails per stud.
The roof load is carried by the studs where the ledger is nailed so the beam is not carrying roof load.

I've mentioned numerous issues above which I am addressing. 1) adding joist hangars 2) adding Headlok structural screws to secure ledger to studs but the big unknown is the beam.

It's a 3.5" x 12" LVL beam. All the tables I find start at 20' supported joist span where my span is 15'.  
At 20' the LP span table wants an 18" x 5-1/4" beam using 40# live and 15# dead load. 

Problem... I just don't have the ceiling height to go to 18".

With the 12" beam bare I have 6'6" of headroom. Going to 18" and I'm going to be bashing my head on the beam (I'm 6'2).

Midspan supports are also a problem, this is the kitchen area and a post in the middle of the room is no good. 

I'm tempted to leave this beam and live with the 1" sag at midspan as it's been up there for 24 years now... BUT,... is this beam dangerous and should I really be replacing it?
If so, can I use a multi-ply beam and be able to stay at 12"?



Simple questions. (post #215787, reply #1 of 20)

What is the beam span? What is the span of the joists resting on it? From all your information I'd guess that the beam span is 15' and the joist span is 11.5' if the beam is in the center;  but none of this is clear. 

No it's the other way (post #215787, reply #5 of 20)

No it's the other way around...

Beam span is 23' clear

2x8 joists are hung off each side of the beam, they span 7.5' each side (or 15' total).

Kv (post #215787, reply #4 of 20)


A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


The forum stripped it first (post #215787, reply #6 of 20)

The forum stripped it first time.

trying a screenshot of the photo instead...

Capture.PNG1.43 MB

Steve (post #215787, reply #2 of 20)

I would draw up what you have and take it to a good lumberyard.  They will fax to a distributor for sizing.  Easiest way to go.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Bolt a 3" wide, 1/8" thick (post #215787, reply #3 of 20)

Bolt a 3" wide, 1/8" thick steel plate to the BOTTOM FACE of the existing beam, NOT to the side. 

5/16" 2" long bolts thru tight holes spaced 3" apart at the outer ends and then increasespacing by 1/2 inch each bolt till you get to the center.  Paint to match.

Is this what you're talking (post #215787, reply #7 of 20)

Is this what you're talking about regqarding the 1/8" plate?

Do I need 2 bolts or just 1 in the center on 3" increasing spacing?

Last question, should I jack the beam up in the center to eliminate the 1" sag then bolt the plate or just leave the sag and bolt to it to stiffen it?


Thanks, this might be an easier solution than replacing the beam, especitally since I'm fairly certain what comes back from a designer will require a taller beam or mid-span support.

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Yes, jack the beam up in the (post #215787, reply #8 of 20)

Yes, jack the beam up in the center with the stell against the bottom edge of the beam. , 1 bolt is OK, 2 is better, bolts get closer together as you go toward the outer ends of the beam span.  You can jack it up a bit higher even if you dont start cracking plaster in any walls above, then it will deflect bown to dead level when jack is remove. . 

Goocle 'built up beams' and lateral shear if you want to calculte what bolt spacing and exact bolt size if you re mthamatically inclined. ,

If it is a 20+ foot span, the typical steel stapy you would buy is 20 ft long.  The steel does NOT have to be out to the ends, a foot or so shy on each end works.  A couple of beads of constuction adhesive in addition to bolts is good also if you leave the; jack in place till adhesive cures.


PS: do NOT just try to nail th strap up, that is close to worthless as the nails deflect and the stell is not ut in tension. You need tight fitting bolt holes ITIGHT) of good adhesive. 

here is a link on how just naiing is close to worthless. (cannot past link)  try searching:   "reinforcing-wood-joists-with-steel-strapping-"

LVL? (post #215787, reply #9 of 20)

Do you foresee any problem with the bolts going into the edge of an lvl beam?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Thank you for your input. I (post #215787, reply #10 of 20)

Thank you for your input.

I did google the subject and found into a thread at houzz dot com about reinforcing sagging floor joists in the same manner.
The particular post was from a PEng. His bolt spacing in that application was 8" but more is better.

I'm going to try this approach, makes perfect sense. I'm basically adding the bottom flange of a steel beam to the LVL.

It was indicated that with large sag it will take time (weeks) for the beam to settle back when jacking. Given the 1" of sag developed  over 23 years I think I'm likely looking at a few weeks of jacking before I get it back to straight but that's ok, I have the time. No worries of damagining drywall etc, it's all being removed anyway so now is the time to try and get it straight.

My last question is you seggest 5/16 bolts, I assumed you meant lag screws?
Not sure if you're familiar with Fastenmaster line of "LOK" screws but something like Timberlok is made for laminating LVL beams, I was wondering if it would be superior or just stick with good old lags? Timberlok are geared more to axial (holding) power rather than shear though. Fastenmaster makes a ledgerlok product designed for shear loads too.

The houzz forum post warned about NOT using hardened bolts, that softer grade 5 bolts were better as the hardened variety will not give but break under heavy shear. 

Thanks again for your solution.

Yes, meant lag bolts.  Any (post #215787, reply #11 of 20)

Yes, meant lag bolts. 

Any grade bolt is fine, themajor give is in the wood.   A specialty bolt for tension where the thread diameter is greater than the shnk is no good, less effective than nails even .  Just plain steel lags are fine.  3/8 even better than 5/16 if you predrill a 3/16 pilot hole, any larger and Calvin's concerns come into play.  

You want to keep the steel from moving iwith respect to the wood so the steel develops tension.

Since you have the time to let the jacked beam settle, I'd for sure get some structural adhesive good for shear (any decent epoxy is good) and apply that to the steel  (the outer 8 ft on each end is good if the center under the jack) before you do the bolting. (e.g - yu will have the steel hanging from the center jack point till you apply the epoxy.  I've even used rebar setting epoxy, Wall board adhesive out of a tube is too flexible to use. 

A quart of polyesther boat resin will work just fine also. 

Perfect, thank you! (post #215787, reply #12 of 20)

Perfect, thank you!

BTW, the beam is 23' 1" (post #215787, reply #13 of 20)

BTW, the beam is 23' 1" unsupported and the longest flat bar is 20' so the steel will be 1.5' short of each end.

Only other way to go there would be extend the 20' by 2.5' and use a mending plate (overlapping the 2 pieces by say 12" each side).

From your previous post though it seems that is not necessary and having the strap 18-1/2" short on each end will yeild the same result?

Steve (post #215787, reply #14 of 20)

Dont forget to provide solid bearing down to foundation (floors/subfloors blocked, floor system etc.) so you don't end up pushing the floor down instead of the beam up.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Correct, 18-1/2" short on (post #215787, reply #15 of 20)

Correct, 18-1/2" short on each end of 23 ft beam will be almost the same improvement as full length steel  (93% as effective in reducing deflection to  be exact) .  Anything over 90% is 'almost' to me for this type case.

Local steel supplier tried to (post #215787, reply #17 of 20)

Local steel supplier tried to upsize me to 1/4" flat bar as the cost is peanuts ($28 vs $22).

Is there any reason to increase thickenss from 1/8 to 3/16" or 1/4"?



You have a good steel (post #215787, reply #19 of 20)

You have a good steel supplier, go for the 1/4", it will be stiffer.  That will be the best $6 you ever spent.

Hope yu have a drill press thugh <G>  It helps to drill a 1/8" pilot hole even with a drill press, then open it up to yur bolt size.

1/4" it is then. Definitely (post #215787, reply #20 of 20)

1/4" it is then.

Definitely will be setting up the drill press for this one!


Yes I will be jacking (post #215787, reply #18 of 20)

Yes I will be jacking directly over the crawl space center beam which has piers under it.

I'll also set blocking to distribute the jack load over a few joists.