Securing a safe to a post-tensioned slab - DIY

[John Browning]

If you have a post-tensioned slab, you cannot drill straight into it to bolt down your safe. I need to be clear that you should never, ever, ever, ever drill a PT slab on your own. You risk serious injury, death and structural compromise. No rational contractor will advise you to drill a PT slab without GPR or x-ray, and that costs much more than this project. However, you can still bolt it down using this simple do-it-yourself primer.

At each step, double check all your measurements, making sure that things are square, level and right. It is very hard to undo mistakes, so get it right the first time.

First, how can I tell if I have a PT slab?

LET ME BE CLEAR, If you are not sure that you have a PT slab, stop. Ask a professional before you drill. NEVER EVER DRILL A PT SLAB. I CANNOT STRESS THIS ENOUGH. Do not rely on your slab being stamped. Many are marked in the garage, and many are not. You can look at the outside of your slab if you are not sure. There will be grouted holes 3 to 4 inches in diameter every 3 feet or so. The best way to tell if you have a PT slab is to look for these holes, and look at a side where there is no concrete poured next to the foundation and the top 12in of the slab are visible. Do not rely on the slab being stamped as PT. If you think you might have a PT slab and don't want to ask a pro, then assume it is a PT slab.

I took these photos for two reasons. One is to help you guys. Two, if I ever have questions on what is underneath this slab, I have proof I never drilled it. If I were you, I would document this project as a CYA.

Step 1: Go to USPS and pick up a change of address packet. This has a coupon for 10% off your entire purchase at Lowe's. Home Depot will also take this coupon. You can save 10% on your safe and everything else, so buy everything you need in one swoop. The coupon used to be instant, but some now apparently require registration and take a few days to arrive via email.




Step 2: Plan your shopping list.

Here are the things I used for this project:

2x6 for forms
2x4 to screed
Sakrete
Portland cement
#4 rebar
Wedge anchors 5/8 - 6in
concrete edge and finishing tools
Skilsaw w/ wood and masonry blades
Sawzall w/ metal blade
Drill w/ Phillips driver, 3/4 Metal bit, 5/8 Masonry bit
Wheelbarrow
Sika LatexR Concrete Adhesive
Level
Wedges to level
Doobie blocks
3 in screws or concrete form nails
Respirator
Safety glasses
Extension cord

Then go to Home Depot (they have more of this stuff) or Lowe's and buy everything you need. Remember, you can buy your safe at the same time and get 10% off with the coupon.





Step 3: You'll want a diagram of the anchor hole dimensions of your safe if you are ordering it like me. I called AMSEC and the emailed me a diagram of their HS safes in a few minutes. If you buy it at HD, you can just measure it. You will want to sketch out the dimensions of your new housekeeping pad. I sketched mine out several times to get an idea of where I wanted it. My new safe is 40w by 26d. I made my slab 45x30 because those are great calibers, and because you want to be 4-6in larger than your safe. I"d go larger, but that would extend too far into my garage. Anyway, just sketch out what works for you. I went through a few different sketches before I figured out how large I could go and not be intruding on too much garage space. The bigger you can go, the better, but everything in life is a compromise.



Step 4: Once you have your area sketched out, you need to scuff the slab. This will allow the new concrete to better adhere to the old stuff.

Before you start, you NEED a respirator. This step will create a ton of concrete dust, so empty out the garage and cover what you can. Then, eyes, ears and respirator on.

I used a masonry blade on my Skilsaw and a FatMax Fubar to do the job. I gouged the slab with the Fubar and dragged the masonry blade at a 45 degree setting across the concrete. Ideally you want to scuff the entire area of the slab, but I wound up going a few inches less than that. Remember, you just need to scuff the surface, not go deep. You can also use an angle grinder to scuff up the slab.

Once the slab is scuffed up, make sure and get it cleaned off. Dust will severely weaken the bond, so get it as dust free as you can.



Step 5: Once you have the slab scuffed, you can set your forms. I won't go into how to do this as there is a ton of info out there on it, but just make sure that you have them level and secure. I secured mine to a workbench and then directly to the wall. A proper slab will not be perfectly level so that water drains out, so don't just set yours on the slab and go. You will need to level it out.



At each step, double check all your measurements. Just reminding you.

Step 6: Now it is time to put in your rebar. This is where knowing the anchor hole locations is important. Put in the rebar on top of the doobie blocks so that you are certain to avoid the anchor hole locations. THIS IS CRITICAL. Once you are certain that the rebar is nowhere near where you are going to set your anchors, check your forms and your measurements again.

[John Browning]

Step 7: Double check everything. Check that the forms are level and secure and that the rebar is not going to be in the way of the anchor holes.



Step 8: Now you are ready to mix and pour the concrete. The only special step you need to take here is to take the SikaDur LatexR Concrete Adhesive and make a slurry to put down. The directions are on the bottle.

Then take the rest of the Sika and mix it in with your first batch of concrete. I mixed mine in a wheelbarrow. adding a 1/4 a bag of Portland Cement to each bag of Sakrete High Strength concrete. You don't want it to be too wet, and it needs to be well mixed. Then just dump it into the form. If you're not sure how to work and finish concrete, there are a ton of DIY primers online.

Finish the slab, and once it is cured you can pull the forms. Now you've got a slab that you can bolt your safe down to. See the DIY sticky on how to bolt down a safe and go for it.



Step 9: Optional extra safe straps

Once your safe is bolted down, it is probably not going anywhere. The weakest link in the chain is most likely the bottom of the safe, which will probably be ripped off before the bolts shear or the slab moves. To reinforce that, here is what I do.

My last safe came with 10 gauge steel supports instead of a pallet. I take these, and drill two 3/4 in holes spaced correctly for my safe. Then I use these below the washers and nuts on the redheads to reinforce the bottom of the safe.

On the left is what it looks like before, and on the right is after. Then I take a sawzall and cut the side off and cut it to length if needed. On a side note, this is the same steel used in most of those low-end Winchester/Canon/Browning/Cabelas/Stack-on/AMSEC safes. You'd be amazed how quickly a Sawzall can chop through them.

On another DIY, I'll show you how to turn a $1000 safe into a much more secure container.



Anyway, I hope that this helps.

[John Browning]

Brilliant idea for a garage install. I had to put my safe inside, did not bolt down though because of the post tension slab.

Thanks. I am hoping this will get a sticky because safes really need to be secured. A lot of people think that because you have a PT slab you can't bolt it down and it has to be weakly secured. That is not the case. The slab I added will weigh about 600 pounds and with the adhesive you would need a winch and a F450 to pull it out. Making the slab larger than the safe also adds another lever arm that makes the safe harder to move. Best of all, this project only uses about $100 in materials if you already have the tools.

I am making sure my safes are all secured to where they can't come out. I have a friend in our development (also with a PT slab) who had a safe, and it was broken into with a crowbar. Now her safe was crap, but even then they made the effort to pull it out and get it on its back. It was lagged into the plywood subflooring and came out easy.

This housekeeping slab is the way to go if you want to take your safe with you if you move. I'm putting a very, very expensive safe here and don't want to leave it if I move. I'll be doing another DIY project with a much less expensive safe showing how to secure a safe to a PT slab if you really plan to keep it with the house.

[John Browning]

Should be a stamp.. ^^

But OP are you saying that you cannot drill into a post tension slab? Or you can't drill into a tension cable on a post tension slab? ..which are pretty hard to cut with a mason drill bit... they will bounce off and you will know you hit one... Just x-ray it and don't drill the cables.

But nice write up and not a bad Idea at all.. looking forward to your safe re-enforcement write up.

Drilling a PT slab should never be done unless you've had the slab x-rayed, and since a slab x-ray costs a pretty penny, I'd never drill one. That is after 20 years of construction experience and years working with concrete and PT.

Those cables are stressed to a degree that hitting them with a masonry bit won't cut them, but it will snap them. It isn't like hitting rebar. The tension in the cables makes them want to fly apart as soon as they are hit. More importantly, drilling a PT slab will void your home warranty, and if you hit a cable you're looking at a five figure repair bill.

The risks of drilling a PT slab are enormous, and the effort it takes to solve the problem is a full day project at best with minimal cost. Seems like a no-brainer to me.

[paul0660]

drilling a two inch deep hole in any concrete is going to snap a tendon as often as airliners having a head on. Use epoxy to secure the anchors, not redheads.

Too much rebar, too many dobies (they create a void). Mesh pulled up to the middle of the pour would have been ten times better.

But, a good training project for sure.

[John Browning]

drilling a two inch deep hole in any concrete is going to snap a tendon as often as airliners having a head on. Use epoxy to secure the anchors, not redheads.

Too much rebar, too many dobies (they create a void). Mesh pulled up to the middle of the pour would have been ten times better.

But, a good training project for sure.

Mesh is a terrible choice for such a small slab, but whatever. I could use Epcon or Hilti Epoxy, but the bottom of the safe will rip off long before the redheads give, so no need to epoxy a safe.

Only a moron drills a PT slab.

[paul0660]

Only a moron drills a PT slab.

Well ok, but I didn't spend 20 years overbuilding stuff because I couldn't figure out how it was made in the first place, as you apparently did, and didn't. Most of the people reading this probably have an appreciation for the facts, what their time is worth, etc.

[John Browning]

Well ok, but I didn't spend 20 years overbuilding stuff because I couldn't figure out how it was made in the first place, as you apparently did, and didn't. Most of the people reading this probably have an appreciation for the facts, what their time is worth, etc.

I now appreciate air conditioning in the summers and a five minute commute so construction is no longer my thing, but I still have a General Contractors license in both CA and NV. Your resume?

Yes, this slab is totally overbuilt if it were a driveway or a sidewalk or some other regular slab. However, the overbuilt part of it ran me about $4, and the extra strength in the slab makes that $4 well spent.

[John Browning]

Yeah you could park your pickup on it if it would fit.

My resume is a dozen building projects and getting generals and subs to not rip me off, no matter how smart they act, and 3 lawsuits that I won.

I mentioned epoxy because half the time noobs will rip the red head hole up by over tightening.

I offered an alternative because I know how post tensioned slabs are constructed. They aren't hand grenades, and the tendons are not within two inches of the surface, which is why I noted that dimension.

Seeya!

So, in other words, your only contribution here is to offer nothing. The only thing you did mention, to look for a stamp, is also wrong. My home is not marked anywhere, and many homes with PT slabs are not marked. We'll get to that later.

A 2in hole in a PT slab voids a home warranty for most people. That is very bad. Generally those tendons are 2in from the surface, but they don't always run parallel to the surface so you can absolutely hit one at less than 2in. That alone would be a reason to never drill a PT slab. Even if you were to drill a 2in hole and miss a tendon, a 2in epoxy anchor offers virtually no strength. A 2in hole with epoxy offers less than 1/8th the strength of a 6in hole, and the force requred to pull the anchors is less than the weight of a decent mid-level safe.

A real contractor will get an x-ray or use GPR before they ever drill a PT slab. If you want to go ghetto, mess up your life DIY like you're suggesting, here is some factual info to digest.

LET ME BE CLEAR, AS A LICENSED CONTRACTOR, ONLY A TOTAL MORON DRILLS THEIR OWN PT SLAB. NEVER EVER DRILL A PT SLAB. I CANNOT STRESS THIS ENOUGH. If you're still wanting to drill a PT slab, this is what a very stupid hypothetical contractor would do. I am not advising anyone to do this, because it is very stupid. Despite any reasonable professional advice to avoid it, he would look at the outside of your slab. There will be grouted holes 3 to 4 inches in diameter every 3 feet or so. The best way to tell if you have a PT slab is to look for these holes, and look at a side where there is no concrete poured next to the foundation. Do not rely on the slab being stamped as PT. He would guess where the cables run and drill, which is stupid. PT cables will be covered typically in a red plastic, so if he drilled it and hit plastic, he would stop what he is doing and then drive away, leaving you with a big problem. He probably had no idea that is the case, though, so when he hits the plastic, he keeps going until he snaps the cable. Then he realizes something is wrong, packs up his tools, and drives away hoping you don't have his information to track him down. That is why you don't do this yourself or hire very stupid contractors.

The tendon anchors are under 10,000-30,000 pounds of stress in most residential slabs. If you hit a tendon, they fly apart like something under 30,000 pounds of stress, which is violentlly. The anchors can (and do) fly a few hundred feet and people have been killed by drilling PT slabs. Smart people don't drill them ever on their own, and professional contractors will not drill them without x-ray or GPR.

If you don't mind ruining your foundation and killing yourself or someone else in pursuit of a 2in hole that offers no support, take paul0660's advice. Make it clear to your attorney that it was paul0660 offering that advice, and that I advised against it when you want to file your lawsuit. If you want to have a safe bolted to a PT slab, do what I said above which is to create your own housekeeping slab that you can drill and not worry about. Do not drill a PT slab. You can listen to a licensed contractor talking the talk and then walking it, or some internet certified keyboard expert. It is your house.

Here is a nice video showing what happens when just one PT cable fails.

[John Browning]

Yes, I also inquired about drilling in my PT slab. I was told it would void my home warranty, possibly my homeowners insurance as well. I already have a hard enough time finding homeowners insurance due to my high fire risk.

The only option was to get a licensed company to mark the tendons with X-ray. Not worth the risk or hassle in my opinion.

If you had literally nothing, you could honestly do this project with hand tools and have the whole thing finished for under $150 easy buying the essential tools. You could almost certainly borrow the essential tools and be done for less than $50 if you are in a pinch. Definitely not worth the risk or hassle.

[John Browning]

scobun - sidetracking a little bit. I have a newer built home (2004). I don't remember if it has a PTS, don't remember seeing any markings since I coated the whole garage floor. Would city hall be able to check what kind of foundation my home has? Foundations go through inspections right?

The foundation has been inspected many times, but I don't know that many cities keep records of the inspections. The markings on the garage are usually stamped into the concrete, but like I said before, mine is a PT slab and is not marked anywhere. If you're not sure, have a contractor buddy take a peek at it. If you're in a subdivision, see if any of your neighbors know what their slabs are. Most tracts that have PT slabs have them on most of the homes. You can also look for the grouted 3in holes every three feet on exposed slab.

[damngato]

Thank you very much for this DIY! I have the PT slab and thought I was SOL as far as bolting it down. This looks like my next project. When you talked about a proper slab not being completely level you were talking about the existing floor, correct? So we need to take that into account when we make our new slab is what I got out of that. Just wanted to double check.

On a side note, I'm not home but I know the garage has the stamp but we also have an outside patio area which was there when we moved in. Would that usually be the same PT slab? I'll look for the holes when I get home but just curious. Thanks again for the great info!

[John Browning]

Thank you very much for this DIY! I have the PT slab and thought I was SOL as far as bolting it down. This looks like my next project. When you talked about a proper slab not being completely level you were talking about the existing floor, correct? So we need to take that into account when we make our new slab is what I got out of that. Just wanted to double check.

On a side note, I'm not home but I know the garage has the stamp but we also have an outside patio area which was there when we moved in. Would that usually be the same PT slab? I'll look for the holes when I get home but just curious. Thanks again for the great info!

Correct, the existing garage slab floor should have a very slight grade to it.

All exterior residential concrete like driveways and patios that I've ever seen are just standard rebar. I would doubt that they are PT. I'm glad you found it helpful, that is what I was hoping for.

[odysseus]

I also want to note for post-tensioned slabs that if you drill into the slab - and avoid the tension cables, and yes void the warranty and such, if you later try to sell the house - you can get hit by prospective buyers on inspection with this. They can try to dock you on the asking price with the cost of fixing that. It is not going to help the sale, is what I mean to say. Of course, they might not be happy with the extra concrete work as noted here, but that is not a point against structural integrity.

[John Browning]

I also want to note for post-tensioned slabs that if you drill into the slab - and avoid the tension cables, and yes void the warranty and such, if you later try to sell the house - you can get hit by prospective buyers on inspection with this. They can try to dock you on the asking price with the cost of fixing that. It is not going to help the sale, is what I mean to say. Of course, they might not be happy with the extra concrete work as noted here, but that is not a point against structural integrity.

Good point, and another good reason to have photo evidence of what you did.

Also, when you drill for the anchors, make sure and drill a hole that is less than the depth of your new slab. Redheads sunk at 4in require something like 6,000-8,000 pounds of force to pull, so no need to drill the hole to China or into the PT slab.