Do I Need To Reinforce My Floor If???

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Old 02-16-10, 06:29 PM
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Do I Need To Reinforce My Floor If???

I wasn't sure where to start this thread, so mods, if there's a better spot for it please feel free to move.

My home is about 85 years old and has, to the best of my knowledge, the original hardwood flooring. I just purchased a large (600lbs) safe and am curious if I will need to reinforce the floor underneath. There is crawl space underneath the house and was wondering if I need to build a brace between the ground and the floor or if the floor can hold that much weight in a concentrated area permanently.

Appreciate your help.
 
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Old 02-16-10, 07:13 PM
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I'm always surprised what kinda loads framing can handle.
But to start off everyones gonna want to know the dimensions and spacing of your floor joists.
And what do you see supporting each end of those joists? Specifically check under the room you'll be placing it. An 85 year old house might have an old addition that don't follow the same construction method as the rest of the house. Sometimes the joists of old room additions got a ledger board instead of being let in to the foundation!
 
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Old 02-16-10, 07:23 PM
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Good point mickblock. The room, or rather the closet it's going in is definitely part of the original build though. I've got to go under the house for something else, so while I'm there I will check out the joists. I'm a total noob at this, can you link me to what some of the different setups might look like so I can verify which one it is? I'll try to take a camera down with me, but don't know if there'll be enough light, even with the flash.
 
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Old 03-08-10, 08:24 AM
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I'm not sure why you're fretting or hesitating. Go under the house, find the spot where the safe goes, and "sister" the framing members. Simply screw and glue lengths of plywood onto the framing members flush with the floor above and a ways past the load point (if the safe is 2 feet square, sister 4 feet square). That will dramatically strengthen the framing members (like really dramatically), and spread the load out as well. It's easy to do, costs little, and is totally invisible. Plywood has incredible sheer strength, and the glue/nail attachment will cause the laminated sisters to become "one with the joists", making them incredibly strong.
 
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Old 03-08-10, 10:19 AM
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I went under the house on Saturday to bolt the safe to the floor. It was raining outside so I didn't want to risk damaging my camera, but I will explain what it looked like.

The crawl space is approximately 2 feet high. The floors are wood and supported by what looked like 2x8s spread approximately 16" apart. The safe is next to the wall, so going 2 feet out is not possible on one of the sides. But I could mount something like a 4'x4' sheet of plywood to the 2x8s, pour a 2-3" thick concrete slab directly under the 4'x4' plywood so as not to lay wood directly into earth and then work in some 4x8s on the four corners of the plywood.

Would this be overkill?

Not sure what "sistering" is. Can you please explain?
 
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Old 03-08-10, 10:31 AM
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Sistering

Your floor joists don't need additional support to the ground. You just need to ensure they don't deform ("sag"). So, cut a length of plywood the same width as the floor joist (7 1/2 inches or so for a 2 by 8), and maybe two feet long. Lay that plywood along the joist so it is up against the flat side of the joist with the edge of the plywood against the floor (just like the actual joist....) Put some glue (any glue, wood glue, liquid nails, whatever, and be generous) on the plywood so you can "stick" it to the joist (so now the joist will be thicker....) Nail it to the joist in several places so it stays while the glue dries.

Do this on one or both sides of joists that are supporting the safe. It's not important that your "sister joists" be supported or that they be very long, they are meant to keep the joist from sagging from the weight. The actual 600 lb safe is well within the joists' weight limit (think about a 300 lb man standing on two small footprints versus your safe standing on a fairly large footprint.....) What you want to avoid is sagging.

It seems counterintuitive, but plywood is VERY strong along that dimension. It is used in "I-beam" floor joists to span incredible lengths. As a "sister", it stiffens the joist so it won't sag. The glue holds very tight. The nails are there to hold the "sister" while the glue dries.
 
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Old 03-08-10, 11:17 AM
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I understand what you saying and will do this for sure. I'm amazed though that the sister joists will hold with enough strength without adding some support under the joists like a sheet of plywood. What I mean is picture an upside down T, where the bottom part (perpendicular to the joists) is the additional sheet of plywood.

I'm just worried that one day I'll come home to find my safe fallen through the 85+ year old floor.
 
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Old 03-08-10, 11:31 AM
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I don't think you need to do it, but if it makes you feel better, after you sister the joist, find a couple of cement blocks or a flat rock and use the rock/block as a "footer" to support a simple two-by-four or four-by-four post holding up the joist. Just use a cheap car jack on a couple cement blocks to VERY SLIGHTLY raise the joist so you can fit your post under the joist sitting on the "footer". Don't raise the joist much (maybe 1/4"), or you risk deforming and damaging the floor above. It's 85 years old and pretty brittle.

To improve the stability of this (unnecessary...) post, you could make a "sandwich" of three two-by-fours nailed and glued together forming a "saddle joint". The middle 2x4 in the sandwich is recessed, so you slip the assembly over the joist and nail it (using the outside 2x4's as nailing flanges). To keep the rock/cement block from moving, just bury it a bit.

Or, just stack up cement blocks to the joist level and ship with wooden wedges to make a tight fit.

Either way, that area will be supported just fine. I'm assuming the house's bones are good (It's lasted 85 years, after all...) and you don't have existing problems with springy floors. In any case, the sistered joists will be very strong in resisting sagging. If you add the support under them, it'll be overkill, but you may sleep better. And it costs very little.
 
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Old 03-08-10, 11:35 AM
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You can also box in that area for a bit more stiffness. Basically that involves cutting 2 x 8s to fit between the existing joists and then screwing or nailing them to form a box slightly beyond the area to be reinforced. This helps distribute the weight to the adjacent unloaded joists. Of course it is more for out in an open area (like for an attic access scuttle for example)..not right up against a wall..but if you tie into the rim joist with the blocking...then that will help support the next floor joist in.

I would suggest that if you go with the sistering of the plywood that you cut it as long as can be easily done. As Jim said..if the safe weight is on 2' of joist..then a 4' length of ply would be great.

Also...don't use drywall screws, use appropriate length deck screws or galv nails (though really after the glue dries I guess it wouldn't matter too much).
 
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Old 03-08-10, 01:15 PM
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Ok, you have convinced me that the sister joists will be sufficient. No need for overkill. I am going to have to head back to the lumber store for some more plywood as I literally just used up my last 4'x8' for some closet shelving. Still got some sawdust in my hair, but the garage is clean.

Thanks for the help fellas!
 
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