How long of concrete screws/anchors for safe to concrete?

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Oct 6, 2006
I want to bolt my safe to my basement floor. I will be using a piece of 3/4 treated plywood for the safe to sit on, sized 1/2" over the safe's footprint. Counting the thickness of the bottom of the safe and 3/4" plywood, how long of 3/8" concrete lag screws will be sufficient to keep the safe from easily getting moved?

I know most basement slabs are 4" deep, and I don't want to risk going going through the slab, but screw sizes are limited in length, and I don't want them mounted too shallow either.
I am wondering if 3" screws will work (like these). I am looking for a screw not wedge for ease of installation and removal (as I am in a rental house). A 3" screw will be mounted about 2 1/8 - 2 1/4" in the concrete (considering the 3/4" plywood the safe is sitting on) and I am wondering that is enough. They sell 4" model screws, but then I am over 3" into the concrete and am not sure how deep the slab is. I have heard slabs are typically 4", but don't want to go through it either, as this is an old house. Thoughts?
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I would put in threaded concrete anchors and use appropriate length bolts....I used threaded anchors in my basement floor to mount a manual tire changer 20 years ago. They have held up fine to the leverage of bead breaking. The anchors are between 2 and 3 inches in length.
Structural engineer answer is bottom 1/3 of the slab thickness. And, in practical terms, hitting the "halfway" of that bottom third is a practical ideal.

I know most basement slabs are 4" deep,
Depends upon the basement. A "Rat Slab" basement floor might only be 2-3" thick (if you set it on the edge of the footers, it's an economical solution for the floor for most basements.

Only real way to know is to actually drill a hole clean through the slab, sadly. For thin slabs push-through butterfly anchors can be an elegant answer.

Your mileage may vary.
Counting the thickness of the bottom of the safe and 3/4" plywood, how long of 3/8" concrete lag screws will be sufficient to keep the safe from easily getting moved?

By bumping into it or if someone placed a port a power ram between it and the wall?
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No answer on your screw length other than I'd try to get 1-1/2"-2" into the concrete. But FYI treated lumber is very corrosive and resting your safe directly on top might not be the best idea. Yeah, most are powder coated but it tends to get chipped off the bottom as it gets handled.
I wouldn't place anything that thick under a safe and create the possible area for a ...prybar to be inserted. If a vapor barrier is needed get plastic sheeting at the hardware. I advised a buddy on this same issue, he found (Home Depot?) some thin rollup 4' X 8' sheeting that could be glued up to make a shower wall. Super thin stuff less than 1/4'' as I recall.

Just my $.02 YMMV the main thing U R correct is BOLT IT DOWN
I would use the 4” screws, if it punches all the way trough the concrete it’s not going to hurt anything.

And it’s very unlike a 4” screw would punch through anyway, assuming the 3/4 plywood. Like you said most slabs are 3.5-4” thick.
Well, I bought 3/8" x 4" tapcon screws. With my hammer drill and masonry bit I drilled 3.5" into the concrete and didn't poke through. I wasn't thinking clearly and didn't realized the 3/8" holes in the bottom of my safe were not big enough for the 3/8" Tapcons because of the width of the threads. I went and bought a 15/32 steel bit and ended up burning it up after two holes. Had to go back a third time and get a 1/2" cobalt bit and used oil when drilling. Those last two holes went like butter. As I don't own an impact wrench, I used my 20v Porter and Cable drill to get the screws about half way and then used a 1/2" socket wrench to wrestle them in the remaining distance. Gun safe doesn't move.

I just saw the end of this thread where Bassjam mentions treated wood is corrosive to steel! Ugh!!! To late now. I hope it will not be a major deal. Having to undo all 4 screws, replace the plywood, and get the new piece lined up with the existing holes in the concrete and safe will be very, very difficult. I guess I am hoping the corrosion will be somewhat minor.

As for putting a horse matt or plywood under the safe, that was recommended by a1abdj here on The Highroad. He is a safe dealer and installer and contributed a lot of knowledge and clarified many misperceptions on the strength of gun safes. He seemed to stop posting in 2016 but a search will pull up a lot of good info from him around 2008 - 2010. He said the key is to have very little of the plywood (1/2" or so) sticking out of the sides of the safe.
I don't care to use screws but do like drop in anchors.
51396088471_5bca75af96_z.jpg 2021-08-22_08-43-44 by poofy27, on Flickr

The middle ones can be either set flush with concrete or embedded down into the bottom 3rd as previously mentioned.

They are easy to unscrew the bolt or all thread rod and infill when they are removed.
The stud anchors work but harder to remove. The other drop in and the right drop in have lips that prevent them from going in deep. I prefer deeper and good bolts.

All in all, it will also depoend on quality of concrete and holding power into that concrete.

The lower left anchor is referred to as a 4way and used more for hollow wall applications but could also be used if the concrete isnt good quality.
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