Checking Slab Thickness

[scootermcrad]

Hey everyone!

I read through the previous threads that came up in a search about checking slab thickness. Drilling, core samples, and ground penetrating radar seem to be the most common responses. However, nobody ever updates their threads on the methods they used and the results.

With the economy in a strange place, I'm pushing off the new shop build. (again!) So this means that I'm going to have to get clever with my existing 21'x24' 1937 garage and install a 4-post lift for car storage and flexible work space. I need to check the thickness of the slab. I'm pretty confident that it will be 4" or better, but I want to confirm before taking that step. I'm not opposed to drilling a small hole (or two) to see what I can learn. If I do this, will a 1/2" hole likely give me the information I need, or has anyone gone smaller? What would be the correct method of filling this hole? A concrete adhesive of some sort? Mortar mix? Something else?

BEFORE I do that, I'm going to look into having someone out to check it with radar or scanner, or whatever gizmo they use. Has anyone had this done recently? I feel like this could be pretty expensive, but a non-invasive check would be preferred.

I would like to hear from those that have successfully checked their existing slab thickness.

THANKS!

 

[gizardlizard]

I drilled my garage slab in four places to check thickness before a two post lift install. I drilled 1/2” holes. Bottom of hole will blow out though and give you a false reading. Doubtful you’ll have 4” of concrete as standard framing lumber will be 3 1/2” or less. I had as little as 2” and as thick as 3 1/4”. I ran a flexible tip bore scope into each hole. To be honest though, if you’re doing a four post you’re wasting your time and money. You have nothing to worry about. Two post however would be a different story. I ended up digging two 4 foot squares 12” deep with rebar and keyed them both under the existing slab. Filling in the holes is super quick and easy. Choose your weapon: quickcrete, post cement, etc. Don’t overthink it.

 

[scootermcrad]

I drilled my garage slab in four places to check thickness before a two post lift install. I drilled 1/2” holes. Bottom of hole will blow out though and give you a false reading. Doubtful you’ll have 4” of concrete as standard framing lumber will be 3 1/2” or less. I had as little as 2” and as thick as 3 1/4”. I ran a flexible tip bore scope into each hole. To be honest though, if you’re doing a four post you’re wasting your time and money. You have nothing to worry about. Two post however would be a different story. I ended up digging two 4 foot squares 12” deep with rebar and keyed them both under the existing slab. Filling in the holes is super quick and easy. Choose your weapon: quickcrete, post cement, etc. Don’t overthink it.

That's actually a really good point about nominal lumber forms. Nominal 2x4's used in our 1937 house were 1-3/4" x 3-3/4".

But, you raise another good point... even though it's recommended by the 4-post manufacturer, 4" is probably overkill. And the car(s) that would be living on it are pretty small/light. Still makes me nervous, though. I would prefer it be 4".

 

[gizardlizard]

I’m definitely not a four post guy but if I was, I wouldn’t think twice about placing a lift on your floor. You’ll love having a lift. It’s a game changer.

 

[scootermcrad]

I’m definitely not a four post guy but if I was, I wouldn’t think twice about placing a lift on your floor. You’ll love having a lift. It’s a game changer.

Yeah, I have a dual purpose need. I need car storage, but also, I have a more immediate need for flexible, convertible and creative work space, so the lift is going to serve as a temporary platform to potentially store a chassis fixture and/or work bench. My garage is bursting at the seams and building the new shop is off the table for a bit. So I'm going to have to get pretty creative. And, it will be nice to have something for oil changes and basic stuff.

 

[mikedodge]

Right or not the easiest way I've done it is drill a hole, blow it out and then use a dental pick to scrape down the sides. You can feel how far down the sides of the hole go. A bent wire should work too. Do that in a couple spots if you don't know if the floor is all a consistent thickness or not.

 

[scootermcrad]

Right or not the easiest way I've done it is drill a hole, blow it out and then use a dental pick to scrape down the sides. You can feel how far down the sides of the hole go. A bent wire should work too. Do that in a couple spots if you don't know if the floor is all a consistent thickness or not.

What did you back fill it with? Thanks for the comments!

 

[Old tool guy]

What did you back fill it with? Thanks for the comments!

Get a bag of hydraulic cement from home depot. Or concrete patch, self leveling cement, whatever. Don’t try to use regular sakrete, the stones won’t go down the hole.

 

[scootermcrad]

Get a bag of hydraulic cement from home depot. Or concrete patch, self leveling cement, whatever. Don’t try to use regular sakrete, the stones won’t go down the hole.

Perfect! Thanks!

 

[mikedodge]

What did you back fill it with? Thanks for the comments!

The last ones I did I haven't filled with anything yet but I'll probably use some sort of sealer in a caulking tube since there's a better chance of filling more of the hole. I might experiment use cement mix of some sort in one and that in another out of curiosity to see what the long term durability is and what works the best.

 

[MikeC55]

I've used the small hole and bent wire method too. I guess you could get fancy with GPR, etc., but a few holes will provide a sanity check.

 

[racecougar]

Assuming small hole(s), I'd consider stuffing something in as a backer and filling with Sikaflex.

 

[Dig Doug]

Get a concrete drill bit measure from the tip up 4 inch wrap tape around it so you have 4 inch sticking out

drill slowly - Don’t apply very much pressure if any. Let the drill bit do the work

once down about half way pay close attention to the concrete surface and the tape line, stop and vac or blow the concrete dust away if you can’t tell (eyeball ) 1/2, 3/4 or 1 inch stop and measure as you go

you will blow out the bottom about 1/2 or 3/4 inch from the bottom of the slab if you reach 3 1/2 deep with out blowing out the bottom, I’d say your good !

I’d use a 1/4, 3/8 inch roto hammer bit

you can always get a 1/2 inch thick metal plate and place between the top of concrete and the lift to spread out the load of the lift and car on top

as for patching I’d just get some concrete crack filler, mix it so it flows, and make 2 applications to make sure it’s topped off

 

[Dig Doug]

I filled this crack w/ a sika floor leveler - 2 applications and then ground it flat to eliminate a trip hazard

the floor leveler had more glue to stick

 

[LS1-IROC]

I drilled 1/4" holes in my slab to check thickness. Filled with epoxy when I was done. 20+ years later you can't even see the filled holes anymore.

 

[scootermcrad]

Get a concrete drill bit measure from the tip up 4 inch wrap tape around it so you have 4 inch sticking out

drill slowly - Don’t apply very much pressure if any. Let the drill bit do the work

once down about half way pay close attention to the concrete surface and the tape line, stop and vac or blow the concrete dust away if you can’t tell (eyeball ) 1/2, 3/4 or 1 inch stop and measure as you go

you will blow out the bottom about 1/2 or 3/4 inch from the bottom of the slab if you reach 3 1/2 deep with out blowing out the bottom, I’d say your good !

I’d use a 1/4, 3/8 inch roto hammer bit

you can always get a 1/2 inch thick metal plate and place between the top of concrete and the lift to spread out the load of the lift and car on top

as for patching I’d just get some concrete crack filler, mix it so it flows, and make 2 applications to make sure it’s topped off

That sounds like an excellent path forward! Thanks for the quick write-up!

 

[scootermcrad]

I appreciate people sharing their techniques and the results! This gives me some good ideas on how to proceed. Not sure I'll get to it this weekend, but when I do get to it, I will post a follow up here. I have my eye on a 4-post and would like to get something rolling by summer.

 

[Briandel]

Hey everyone!

I read through the previous threads that came up in a search about checking slab thickness. Drilling, core samples, and ground penetrating radar seem to be the most common responses. However, nobody ever updates their threads on the methods they used and the results.

With the economy in a strange place, I'm pushing off the new shop build. (again!) So this means that I'm going to have to get clever with my existing 21'x24' 1937 garage and install a 4-post lift for car storage and flexible work space. I need to check the thickness of the slab. I'm pretty confident that it will be 4" or better, but I want to confirm before taking that step. I'm not opposed to drilling a small hole (or two) to see what I can learn. If I do this, will a 1/2" hole likely give me the information I need, or has anyone gone smaller? What would be the correct method of filling this hole? A concrete adhesive of some sort? Mortar mix? Something else?

BEFORE I do that, I'm going to look into having someone out to check it with radar or scanner, or whatever gizmo they use. Has anyone had this done recently? I feel like this could be pretty expensive, but a non-invasive check would be preferred.

I would like to hear from those that have successfully checked their existing slab thickness.

THANKS!

Most people just drill a small test hole. A 3/8"–1/2" masonry bit works fine — once you break through the slab you’ll feel the drop and can measure the depth on the bit or with a wire. It’s quick and cheap.

Afterward you can fill the hole with non-shrink grout, hydraulic cement, or even concrete patch and it’ll disappear.

Radar scanning works, but for a small garage slab it’s usually overkill and more expensive than it’s worth. For what you’re trying to confirm, a small drill hole is the method most people end up using.

 

[scootermcrad]

Most people just drill a small test hole. A 3/8"–1/2" masonry bit works fine — once you break through the slab you’ll feel the drop and can measure the depth on the bit or with a wire. It’s quick and cheap.

Afterward you can fill the hole with non-shrink grout, hydraulic cement, or even concrete patch and it’ll disappear.

Radar scanning works, but for a small garage slab it’s usually overkill and more expensive than it’s worth. For what you’re trying to confirm, a small drill hole is the method most people end up using.

Thank you!!

 

[Old tool guy]

I filled this crack w/ a sika floor leveler - 2 applications and then ground it flat to eliminate a trip hazard

the floor leveler had more glue to stick

I hate to break the news to you, but the patch failed.

 

[wssix99]

Check your instructions for the lift again. Typically, 4 post lifts don't need to be bolted and the concrete thickness shouldn't be critical. (Its counterintuitive, but the depth is required to develop pull-out resistance for the bolts.)

If your slab is shallow, you can just get shorter bolts. (if you want to bolt the 4-poster down) If you do something that causes the bolts to rip out of the slab, you have done something that will have caused you bigger problems than the bolts ripping out of the slab...

 

[Dig Doug]

I hate to break the news to you, but the patch failed.

That was before I patched

This was after
no separation or additional cracking …..


so far!

lol

these pics were taken a year ago march of 25

 

[Skooterj]

Just so you know, I did the math in a post sometime last year. A 4 post lift, with a car on it, exerts less downward force (PSI) on your concrete than just parking the car on the concrete does, since the contact patch of the 4 post lift is MUCH larger than the contact patch of your vehicle. So if your concrete will support the weight of your car, it will more than be able to support the weight of your 4 post lift.

 

[scootermcrad]

Just so you know, I did the math in a post sometime last year. A 4 post lift, with a car on it, exerts less downward force (PSI) on your concrete than just parking the car on the concrete does, since the contact patch of the 4 post lift is MUCH larger than the contact patch of your vehicle. So if your concrete will support the weight of your car, it will more than be able to support the weight of your 4 post lift.

Yeah, I mean that makes sense, if you're comparing contact patch of the tires relative to the area of the lift's feet, which should be approximately 5 times (?) the area of the tire contact patch.

I'm not particularly worried about it. I'm sure the lift companies recommend 4" min. slab thickness to cover their buts with a significant safety factor to avoid lawsuits in the event of an accident. I think that's even increased, over the years. I remember seeing a manufacturer that was 3-1/2", at one point. I also haven't heard of a slab giving out, anytime recently, but I suppose there's always "that one installation" that had issues and failed and nothing could point to why.

All of that being said, it's nice to know what I have, since I don't know the history of the construction. My Bridgeport hasn't cracked the slab, so I have to assume it's a reasonable thickness.

 

[scootermcrad]

Check your instructions for the lift again. Typically, 4 post lifts don't need to be bolted and the concrete thickness shouldn't be critical. (Its counterintuitive, but the depth is required to develop pull-out resistance for the bolts.)

If your slab is shallow, you can just get shorter bolts. (if you want to bolt the 4-poster down) If you do something that causes the bolts to rip out of the slab, you have done something that will have caused you bigger problems than the bolts ripping out of the slab...

This definitely won't be bolted down.

I would also assume that the 4-inch "requirement" would be based on a lift loaded to full capacity (8000 pounds in this case bringing total load with lift to around 9600 pounds), and then probably a safety factor of 3 on all of that, or something. Heaviest vehicle I would ever put on this thing is maybe 3500 pounds, and the vehicle to be stored on there is maybe 2500. So even by logical deduction, I'm probably well within the safety factors even at a 2-1/2 to 3" slab.

No real concerns, here. Mostly just a fact finding mission to know what I'm dealing with.

 

[racecougar]

This definitely won't be bolted down.

There is reasoning behind installing a couple bolts in each pad. It keeps the lift from moving around on you and takes the sway out of the lift when wrenching on a car up in the air. If you use recessed threaded inserts in your floor, the lift is still easily relocatable and no trip hazard is left behind.

 

[scootermcrad]

There is reasoning behind installing a couple bolts in each pad. It keeps the lift from moving around on you and takes the sway out of the lift when wrenching on a car up in the air. If you use recessed threaded inserts in your floor, the lift is still easily relocatable and no trip hazard is left behind.

OH! Okay! I was not aware of this! Thank you for that bit of knowledge. I've not owned one before, so I will keep that in my mind.

 

[Dodgeboy2020]

I honestly didn't read every word in this thread but something that just came to mind now reading this thread (not sure why it didn't occur to me in any of the other 4 post lift discussions) is what type of 4 post lift? If its the type most guys here have that is not 'open' on the front (the front 2 posts are connected via a crossbar just like the back posts) then I would think there's not a whole lot to worry about. At work I have an 'open front' 4 post with no bar between the front posts so in many ways they act similar to a 2 post in that there is considerable cantilever forces on them. The front posts are much larger, foot plates bigger and use larger diameter anchors than the rear legs do as they are not under the same type of forces. Again not saying its applicable in this particular case but something to at least keep in mind as all 4 post lifts are not the same geometry.

 

[scootermcrad]

I honestly didn't read every word in this thread but something that just came to mind now reading this thread (not sure why it didn't occur to me in any of the other 4 post lift discussions) is what type of 4 post lift? If its the type most guys here have that is not 'open' on the front (the front 2 posts are connected via a crossbar just like the back posts) then I would think there's not a whole lot to worry about. At work I have an 'open front' 4 post with no bar between the front posts so in many ways they act similar to a 2 post in that there is considerable cantilever forces on them. The front posts are much larger, foot plates bigger and use larger diameter anchors than the rear legs do as they are not under the same type of forces. Again not saying its applicable in this particular case but something to at least keep in mind as all 4 post lifts are not the same geometry.

Since I didn't include that information, I think it's absolutely good to mention. The thread's worth is only as good as the content it covers, within.

In my case, it's just a hobby-grade storage lift. Atlas or AMGO (basically the same) are the two I'm considering.
One of the things I look at over the weekend, was the placement in the garage. It got me thinking about location of the foot pads relative to the edge of the slab. Looks like I'm okay, (15"+ from the edge), but it got me worried for a brief second. Didn't get to the drilling, yet.

 

[BombShelter]

I drill through a lot of garage floors, I'd be really surprised to see a 1937 garage with a 4" thick floor, up here in the Midwest they seem to be around 2"-3" with a ton of heaving which causes big cracks like the photo above. I rarely see something from before 1965 without cracking. Another concern is floors will develop voids underneath them, especially if they have a floor drain which carries away the sand/soil when the old metal pipe rusts through, 4"-6" is pretty common in some areas of town. If your lucky, a local mud-jacker can fill the void but I've seen some with 1'-3' of space and I definately wouldn't put a jack or car on that.

Use hydraulic cement to patch and if you have big cracks, Fortress.com makes a product called Crack Stitch that can help.

 

[scootermcrad]

I drill through a lot of garage floors, I'd be really surprised to see a 1937 garage with a 4" thick floor, up here in the Midwest they seem to be around 2"-3" with a ton of heaving which causes big cracks like the photo above. I rarely see something from before 1965 without cracking. Another concern is floors will develop voids underneath them, especially if they have a floor drain which carries away the sand/soil when the old metal pipe rusts through, 4"-6" is pretty common in some areas of town. If your lucky, a local mud-jacker can fill the void but I've seen some with 1'-3' of space and I definately wouldn't put a jack or car on that.

Use hydraulic cement to patch and if you have big cracks, Fortress.com makes a product called Crack Stitch that can help.

Well, that's terrifying. No floor drains on this one, and the drainage around it is pretty good, as far as I can tell. Noted, but a terrifying thought.