View Full Version : Monolithic Slab or Seperate Footers?


burger
07-02-2007, 02:39 PM
Having battled the zoning board, I am now designing my 23x24 detached garage. Before I get too far, I need to decide if I am using a monolithic slab or a seperate slab & footer with a block wall.

As I understand it:

The monolithic slab is a lot cheaper and quicker to install.

The seperate slab and footer offer a superior design, but requires much more time and money, especially if several rows of block are to be laid on it.

Question is, exactly how much better is a seperate slab and footer?


Thanks,
Ed

chaingang
07-02-2007, 04:36 PM
Not sure of the cost differences or time savings for that matter. It probably has a lot to do with what part of the country your in. I think monolithic is more common in the south having built two detached garages in GA. I still had to have 18 x 18 footers. Then I put 3 rows of block plus a cap on top of the slab. I did this to get the ceiling height over twelve foot and I can also wash down the inside without worry since I have floor drains.:thumbup:

ron in sc
07-03-2007, 04:41 AM
My detached garage, 24' x 26', under construction, has a stem wall. It is more expensive. For one thing in my case it requires three separate concrete pours; one for footers, one to fill blocks with cement and one for slab.

Link is to good article of description of stem wall and advantages.
http://www.bobvila.com/HowTo_Library/Storm_Ready_Home_Stem_Wall_Foundations-Foundation-A2020.html

JCByrd24
07-07-2007, 06:31 PM
Ed, here in Maine there are basically two schools of thought as far as I can see it. Attached to house requires a 4 ft frost wall (to the depth of frost line=4 ft below grade), where detached they will do a floating slab, basically anything goes. Mine appears to be 6" (maybe only 4") all the way around, no enlarged perimeter or anything. When I say 4 ft frost wall it is always poured here in ME, never blocked. In NJ, I'd say a truely detached could definitely get away with a monolithic slab with good foundation/drainage, i.e. compact gravel base not in a hole where water will collect. If I knew more at the time I would have had them enlarge the perimeter to 12" or 18", but relying on professionals got me a 6" slab all the way around. It's been there 2 years and I'm just starting to build my garage on it. It doesn't appear to have moved much at all but I'll keep all posted how it goes.

wayoff
07-08-2007, 07:42 AM
It is mostly to do with the frost line. Where I am you have to go down 4' to get under the frost line. Any building over 24'x24' has to have a four foot frost wall. With a slab, if the floor heaves, the walls could be compromised.

Stuart in MN
07-08-2007, 08:00 AM
I have a 24x40 garage sitting on a floating slab. I built it about 15 years ago and it's still holding up fine. From what I've seen here in Minnesota it's a pretty common method.

burger
07-09-2007, 09:04 AM
Thanks a lot for your replies!

I'm waiting to hear back from two local concrete guys. I'd like to hear thier opinions since they're used to the weather factors for this area.


Thanks,
Ed

wayoff
07-09-2007, 01:11 PM
I have a 24x40 garage sitting on a floating slab. I built it about 15 years ago and it's still holding up fine. From what I've seen here in Minnesota it's a pretty common method.

Sorry, I meant to say "In my area". 24'x24' is the standard here, but it also varies from town to town.

Splinter
07-13-2007, 11:48 PM
what exactly is a floating slab?

Junkman
07-14-2007, 10:01 AM
When I lived in NJ, I had a garage on a floating slab, and in the winter, the slab would raise, and come back down in the summer. It never cracked, but you could see the movement because the water melting off the cars would run to one side of the building, which didn't happen in the summer. I would spend the extra money and build it with a footing and poured or blocked walls. The only time that I think that I might go the less expensive route, is if I were planning on selling in the near future, where the additional cost might not be recouped. Build it right the first time, and you will never have any regrets...

Tman
07-14-2007, 11:17 AM
Monolithic slab and floating slab are two different beasts. Floating is just that, a slab on top of compacted base. Monilithic combines the footer AND the slab in one pour. My shop/house has monolithic. The floor is 6" thick with the thickened edges12"x18". We are in very unstable soil and have noticed no chages in 2 years cycles. Your concrete guys will know what is most common in your local.

GarageGuy09
03-31-2009, 05:13 PM
Hello all, I have a few generalized questions for a garage that I am about to build. Firstly, I am going to be extending my current garage, 18x24, with a new 18x24 garage and am wanting to know if I have to creat a footing where the 2 cement pads will be meeting, and what the best way is to connect the 2 slabs together. I am thinking about drilling holes, adding liquid concrete in the holes and then driving in rebar bars to join the pads. Is this ok, is there another way to do it. Also, is a monolithic slab the way to go. I feel that it is the best way but when will I need to add the block for the perimeter wall, when the concrete is wet, or when it dries, and if so what adhesive is to be used for the blocks. Many thanks for any thoughts that you might have, Cheers.

IHI
03-31-2009, 07:40 PM
Locally footers are dictated by sqft of the building. The two two we do most of our work have a 850sqft limit for a floating slab, in the town i live in it's 900sqft and we're going to break ground on my shop and i discussed another option with our inspector that utilizes 2" ridgid polystyrene 2' down and 2' away from foundation work plus i'm doing radiant heat in the slab so this way i can build the building without a true trench footing because i'm going to extend the roof 10' beyond the front of the garage and they will be supported on pier footings that will not move in frost heave, so i explained the insulating method and how that will prevent frost from being an issue with my basically "slab" garage even though i'll be pouring rat footings around the perimeter.

ANYWHO. Personally i absolutely hate monolithic pours and i refuse to bid them in our packages, all of the 1 day garage builders use this method, and just like their product, it's garbage. I'm yet to see a monolithic pour stand up to our frost cycles, and once they crack, it migrates into the slab itself and the garage starts to seperate and eventually gets completely out of control. The worst case i seen 2 miles away was a slab in 20 pieces, one area had dropped 4" from the rest, another area split right at the poured curb/slab junction and had a gap wide enough to stick my entire hand down into.

We spec out block for the simple fact it's easily replacable should things get crazy. If HO's are super anal we'll pin each block and solid fill them if it makes them sleep better at night, but in the big picture it's not going to stop a crack when a crack decided to show it's ugly face.

You want the utmost stable wall system, a true footing that extends down below the frost line is the only way to prevent ground heave from compromising/heaving walls/slabs but adds considerable costs to the build. Locally we charge $55-60 per foot just for the trench footings to be dug/rebarred/poured. Slab and block are additional above and beyond that.

Circumstances where the garage is into a hill and serving as not only garage wall base but also as a retaining wall, we'll spec out a poured wall with lots of vertical/horizontal rebar tied together to keep it under tension and prevent wall it's holding back from buckling it. i just dont trust block in this situation. Same scenario for garage build on a backyard that slopes away and we have to build up a wall to fill to meet grade requirements, these "retaining walls" will also get rebarred and poured concrete since there is ALOT of pressure working inside once you bring in all the fill material and then pour a slab and get vehicles running in and out.

Hope some of this helps.

As for tieing to the two garage together, talk to your building department. We run into this a few times every few years and whenever it goes over the 850sqft rule for combined under roof we have to do footing, that's just the way it is here, no other way around it. and once HO's see what it costs to under pin old structure, then do footing for new struture to tie into old, it X's that idea right away. BUT if your under the sqft requirement by adding and addition structure, with a 4" slab (typical) we'll drill a 3/4" hole 12" deep into the old slab, squirt expoxy designed for cement use, insert 3/4"x24" treated steel dowel pins and hammer them in 3' o.c. to help prevent them from teeter tottering.

Just remember folks, there are only 2 garuntee's with concrete:
it will get hard
it will crack
we try to do everything possible to prevent cracking, and try to pick lines to cut reliefs in so when it does crack, it will crack where we suggest it should crack...but sometimes it will still do what it wants:mad: Even when everything is done by the book concrete is still it's own monster.

toadjammer
03-31-2009, 08:59 PM
No matter what everyone has said, prep and materials go a long way in avoiding cracking. Things can be done cheap and they can be done right. Make sure to use plenty of rerod and some 5" wire square don't cheapen out the PSI on the crete and make sure it is all compact. The cheap garage builders get bad results because they don't follow through and do it the right way. I have a monolithic slab and don't have any cracks that have opened up at all, I have a few hairline cracks. It has withstood the last 10 winters just fine and I don't have any stress cuts.

larry4406
04-01-2009, 04:43 AM
With a monolithic pour - it is done all in one shot, footers & slabs. This means weather must be perfect. If done in pieces, you can make better luck with the weather - footings, stem walls, then slab. So, take this in to account.

With a monolithic pour - your stem walls or framed walls are on top of the slab meaning if not flashed right (siding ovelapping slab edge), water can come in at the slab to stem/frame wall. Also, you better have a flat piece of property where the garage is going since you can't back fill against it.

If it were me (and this is what I did), I poured footings, then poured stem walls using aluminum form panels (I hate block). Then framed entire garage including roof. Last came the slab which was then protected from sun, rain, and the abuse of carpenters.

ddawg16
04-01-2009, 01:54 PM
As you can see from above, weather is what really dictates how you build it.

I live in an area where it never gets below freezing.....or even close....nor does it get that hot.....my garage is a single pour....18" footers...5" slab...8" stem walls....20'x25'....they used 20 yards......it's not going anywhere.....that works here but I doubt it would in your area.

I would be inclined to think that a proper footing going below the frost line would be best....then pour the slab.

BTW.....since you are doing this much work....have you considered a basement? Great way to increase the area......

GrageGuy....you basically have the right idea.....the way I see most people joining two slabs of concrete is to drill large holes into the existing slab....install some large rods with grease on them (allows expansion without cracking) and then pour the other slab with those rods protruding into it.

StingRay
04-01-2009, 03:37 PM
It gets 40 below here and the frost goes down 8 feet. My shop is 28 x 44 and is on a floating slab with a contoured edge. This is standard practice for garage type structures. It's nearly 10 years old with only a few very minor hairline cracks. If it were attached that would be a different story. It would need footings and the walls would be built on top of a short grade beam. The slab would get poured later.

rdg1276
05-06-2009, 09:06 AM
I am building a 24x24 garage in a sloped back yard. Most people who are building in the area are using a monolithic slab but because of the amount of backfill I am holding back on the high end of the yard (20" above the slab) I have chosen to go with three pours. A footer, a stem wall and then a slab after I build the garage.

When I pour the slab should it sit on top of the footer inside the stem wall, or inside the footer so that it may move independantly. Also should there be any poly between the stem wall and the slab to allow it to move?

1SlowFormula
05-06-2009, 10:14 AM
I am debating on a monolithic slab or the standard footer/stemwall/pad install myself. I worked in concrete and block work when working my way through school years ago. and back then I had never worked on a monolithic poiur job, and honestly we didn't have many anyway so I have no ereal experiance with them. I will probably just do the seperate footer/stemwall/pad method because I had done hundreds of those int he past and could probably be able to do it bline folded even though it has been many years since I touched concrete. The only reason I was debating on the monolithic slad was for cost savings and my build will be on a budget. But either way I do it I need to go down 3' according to the zoning inspector... Has anyone done a monolithic slab with the footer portion of the slab that deep?? if so was it really a cost savings at that point?

Tman
05-06-2009, 02:55 PM
I am building a 24x24 garage in a sloped back yard. Most people who are building in the area are using a monolithic slab but because of the amount of backfill I am holding back on the high end of the yard (20" above the slab) I have chosen to go with three pours. A footer, a stem wall and then a slab after I build the garage.

When I pour the slab should it sit on top of the footer inside the stem wall, or inside the footer so that it may move independantly. Also should there be any poly between the stem wall and the slab to allow it to move?

When I was a concrete guy we would pour them ON the footer. I dont like any separate element to move independantly.

I am debating on a monolithic slab or the standard footer/stemwall/pad install myself. I worked in concrete and block work when working my way through school years ago. and back then I had never worked on a monolithic poiur job, and honestly we didn't have many anyway so I have no ereal experiance with them. I will probably just do the seperate footer/stemwall/pad method because I had done hundreds of those int he past and could probably be able to do it bline folded even though it has been many years since I touched concrete. The only reason I was debating on the monolithic slad was for cost savings and my build will be on a budget. But either way I do it I need to go down 3' according to the zoning inspector... Has anyone done a monolithic slab with the footer portion of the slab that deep?? if so was it really a cost savings at that point?

I have done them that deep. On my current place they are 2' in one area. You need steel down in there for it to do any good and you WILL use more mud than if you poured a stemwall. But the plus side is one quick pour. we had 40 yards off the truck on my last place in less than an hour and were floating before breakfast.