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On an upstairs floor I'm building, around the stair well I have (2) 4 ply 2 by 8 joists. They are laminated. The bottoms are even, but the top where the sub floor would screw onto the joist is roughly different for each joist. My thought was using an oscillating tool and (plane the top), cut down the higher joists, but that is a lot of time on the saw. I also dont want to cause a problem for the joists. How should I go about this?
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Remodel and New Build GC
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Your beams as laminated are not compromized....but you don't want subflooring squeaks. (Plane down any real high spot...but I'd just use some glue/adhesive caulk on the lower cut lams)
 

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Let's call each layer 1-2-3-4. 1 is nearest to me in photo. Joists fastened to 1 look like they are sticking up, so probably 1 &4 are narrower. Depends on each lumber but looks like 1/4" difference and that is too much. But check 2 things. With string, is the hump straight? With straight edge, 4' level, is the beam relatively on a flat plane coming from the joists? If yes on both, use the hump to nail to and bridge the gap. Meaning that you don't want one sheathing to end on lower joist then another sheathing to begin to higher joist.
If the hump is too high, use plane. Electric or hand. If hand, chisel may help. Oscillating saw probably will start a fire trying to cut that much.
 

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Is there any particualr reason the tops need to be even? I've seen numerous beams like this that function just fine without being 'fixed'. If the plies are connected together properly, they all share the load, regardless of which ones actually touch the subfloor, etc.
 

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Is there any particualr reason the tops need to be even?
Right. Typically you only have one floor joist every 16 or 24” to support the plywood subfloor. Here you have 4 joists ganged together, one is high, that’s the one that is going to carry the load. Just be sure there isn’t a plywood joint over that beam.
 

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Is there any particualr reason the tops need to be even? I've seen numerous beams like this that function just fine without being 'fixed'. If the plies are connected together properly, they all share the load, regardless of which ones actually touch the subfloor, etc.
Yes it is normal to just make sure you join sheets on the high one but this is a stair opening so the edge wants to be fully supported for railing or a wall.
 

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If the beam is part of the well, and even if finished railing on top, any finish fasteners probably will not depend on the rough lumber. Rail posts also don't sit on top of rough lumber without adjustments. You won't compromise the beam even if you cut out the hump. But it will lower the beam in relation to the joists there, and if sheathed, the subfloor can bend there or maybe even crack.
 

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One of the reasons I use Glulam beams. Don't have to deal with uneven lumber with them.
Nice thought. We use glue lam all of the time in residential construction…and while they are SUPPOSED to be all the same…but depending on their journey they might not be. For ex, l get three 24’ at 11 7/8” depth to make up a beam. One has seen some weather, two haven’t, meaning the one is going to be deeper than the other. So while standard residential glue lams don’t have built in camber (crown), but you very well,Ishtar have to work up from one end to the other flushing up the tops. we Would then just power plane any excess off of the bottom….only at the ends where the beam sits into the hanger. Differenxe is not huge, but have run into 3/8”
 
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