Concrete slab -- how flat is flat enough?

[Big_Ed]

Got my new slab in this weekend in preparation for install soon (Artesian Piper Glen).  Slab is generally level, but I noted today as I quadruple-checked the level that I have a slight depression towards the center of the pad.  It's no more than 1/4" lower than the highest points.

I'm a little paranoid given the cost of the unit sitting on it, so wanted to check with the experts.  Whaddaya think?  Am I good to go or should I be breaking out another bag of Quikrete for a level job?

[Jordy]

A quick easy fix would be to even it out with some sand in the low spots. Good luck and good tubbin' Big Ed.

[zacman]

You may also want to check out a product made expressly for uneven concrete floors.  It is a sort of self leveling concrete product you mix and pour on.  It levels itself.  I have never used it myself, but saw it demonstrated. You can check it out at the local home improvement store.

[Brewman]

I used the concrete leveling compound to even out an area of basement concrete to install a shower base.  
The stuff worked as promised.
Actually, I was quite impressed.  And that's not easy to do.
Brewman

[Big_Ed]

What I was hoping for was that you guys would say "get the %$@! out of here, 1/4 inch is no big deal!"

I guess instead I'll be making a trip to the store for this self-leveling stuff that you speak of.  

[ebirrane]

Get the (#*@ out of here.  1/4 inch is no big deal.  

Some slabs have a tilt, just not towards the center, for water drainage.  Assuming your slab is bigger than, say, 2 feet square (hopefully the tub is bigger than that!) 1/4" isn't horrid at all.

The major problem is that the depression in the center will cause water to pool there which, over time, could cause problems with a tub sitting there.  What is the base of your tub made of? Is there a chance standing water under there will attract bugs? Spiders?

Also, watch how thick and heavy the hot tub is.  Some big ones get to be over 6000 lbs.  A 1/4" layer of concrete on top might crack with that kind of weight and not look as nice?  Not a concrete expert so I don't know, but your local Home Depot/Lowes/Whatever guy should be able to answer those questions.

1/4" structurally should make no difference for the tub, even if it was a real tilt going all the way down the tub. Our dealer (HS) said that it would take a pretty good slope to structurally stress the tub and so long as the top jets had water over them the tub will go just fine.

-(little) Ed

[autoplay]

If was me,I probably wouldn't touch it. 1/4" depression doesn't sound bad. My opinion,the 4 corners of the tub will support the weight. Keep in mind,I don't install hot-tubs,so my opinion means squat

Check with some of the tub installers on this board,and,in your surrounding area.

Oh,and if you do decide to fill it,(the depression) make sure you bond to the existing slab with a bonderizer I.E. "link" etc. Concrete over hardened concrete will not bond.

GL

[autoplay]

Brewman......did you use the self leveling compound for the pre-float shower pan,then use waterproofing over that?  

Just trying to help,don't wanna see ya with an added expense down the road via water damage.

[Brewman]

I poured the stuff on existing concrete that needed to level to support a fiberglass shower pan.  
I used some type of etchant to treat the existing concrete before pouring on the leveling stuff.  Then I put the pan on top of that.  
When dry, it seemed very strong.  And that was about 10 years ago, far as I know, the shower is still holding up okay.  
We sold that house several years ago, though.  
Thanks for asking!
Bewman

[autoplay]

Ahhh now I understand  For a fiberglass or precast unit,that's fine  I was thinking possibly you were in the process of tiling the shower pan.

[Warkovision]

Slab is generally level, but I noted today as I quadruple-checked the level that I have a slight depression towards the center of the pad.  It's no more than 1/4" lower than the highest points.

OK. I think I have the same problem. My slab was poured yesterday and in sprinkling it down this morning, (for curing) I noticed water gathering in the center similar to Big_Ed's situation. This slab is in very close proximity to our swimming pool. It's an actual continuation of our pool deck. I'm concerned when the kids are splashing away in the pool, (and of course when it rains,) that water will collect under there. I'm curious to know if people put those spa pads on slabs as well. It would seem to me that this would provide  (how thick are they?) a ledge for the tub to sit on and make water underneath the spa less likely. How expensive an endeavor would that be? Would it work or be worth it? Or am I just obsessing about nothing?  :-/

[lobeknee]

Assuming the slab itself is thick enough, (4"min to 6" or more)and it has some rebar or wire mesh in it, the only concern is as others have said, water sitting in the low spot. Instead of adding anything you could  notch a drainage channel from the center out to a discreet side of the slab itself, let the water drain out.
Good luck

[Brewman]

A spa pad on top of the concrete should eliminate any water pooling issues.   If I recall, the ezpad is about  3" thick, and it had an edge bevel, so it looked nice enough.  Cost of the 8' X 8' ezpad was somewhere between $400 and $500- not exactly a cheap solution.  There may be less ezpensive pads out there. 
Brewman

[Warkovision]

I went down to my dealership to look at the HS Vista again,particularly around the bottom, and found about a 2 inch black edge on which the tub sits. It was on a carpeted floor so the seal with the floor may have been exagerated, but the dealer told me that the bottom of the Vista is filled with fairly rigid foam. If this is true, it makes feel more safe about my slab's mild central depression.
This information gave me another idea that I wonder if others here have done or considered. I was thinking of running a bead of silicone sealer along where the bottom of the tub meets the slab. Would this void the warranty. Is this a good idea?? Thanks guys.

[UnderTheStars]

I would not caulk the tub pedastal.  No caulk will form a permanant seal.  As it expands/contracts it will pull away in small areas.  That will let the water seep in but it will never have a chance of drying out.  It works for a while but over time it will trap the water, not keep it out.

Take a look at the bead of caulk around your bathtub or kitchen sink backsplash & you'll see what I mean - little random "pockets" that have let go from one of the surfaces.

I think I would set the tub down and not worry about it.  I'm trusting that my HS is built to be out in the elements - hopefully time will prove that to be true!