Contractor recommending 3" slab?

[Trav]

I'm in the beginning stages of planning a woodshop build on high clay content soil in Saskatchewan that has a reputation for shifting. The contractor I planned on using for the work recommended a thick, well prepared gravel base with a heavily insulated 3" thick slab and radiant heat. After researching a bit on this forum and elsewhere i'm definitely thinking radiant heat is the way to go and am pleased with his insulation plans but i'm concerned about such a thin slab. He seems to think adding thickness to the slab won't increase its stability on our soil. I won't be parking vehicles in here really, maybe overnight once in a blue moon, so there won't be alot of weight on the floor. I'm also concerned about the shifting soil out here possibly making radiant heat a bad choice.

I'm a complete construction noob, and this is my first post on the forum here. I read the stickied slab guide but didn't see anything about the real pros and cons of thin slabs.

 

[The Cobbler]

3" seems a bit light in my opinion for a floor . are the edges going to be thickened? what are the dimensions of the thickening & the slab? rebar, wire mesh?

 

[theoldwizard1]

One thing I would add is wire mesh. Holds the hot water tubing while pouring the cement and IF the slab does crack, it won't shift.

 

[billconner]

How much gravel base? A slab on a good base is like a finished floor on a sub floor. If the base doesn't heave, the slab won't. And if the base does heave, the slab will regardless of any reasonable thickness.

I'm guessing he's planning on pinning radiant floor tubing to foam - either foam designed for that or using those little U shaped barbed pins to anchor to the foam.

Can you get a reference from the contractor for similar projects? See where he's done a 3" slab?

 

[Trav]

3" seems a bit light in my opinion for a floor . are the edges going to be thickened? what are the dimensions of the thickening & the slab? rebar, wire mesh?

'24 x '40 or maybe '44 is what i'm going with. I remember him saying the edges would be thicker but I don't have dimensions yet. He said there would be standard rebar and nothing was said about wire mesh that I remember but he mentioned some sort of foam board with channels for containing the radiant piping.

This has only really gotten to the initial conversation stage so far as I just acquired the lot beside me to build the shop on last friday, so I have no real firm plans or anything. The contractor has done great non-concrete work on our house in the past but the 3" slab sounded so wrong to me that I wanted the opinion of others.

 

[Trav]

One thing I would add is wire mesh. Holds the hot water tubing while pouring the cement and IF the slab does crack, it won't shift.

Interesting, thank you. I'll look into that and speak with him about it

 

[rlme36]

3" is sidewalk depth in the states. 4" in min for most residential basement floors/house garages. If doing radiant heat you want the mass of a 6" slab. my 2 cents. I'd want 6" all day with mesh and fiber.

 

[jblnut]

If doing radiant heat you want the mass of a 6" slab. my 2 cents.

I did radiant in my shop and did a 6” slab partly for this reason and partly because I occasionally bring really heaving farm/construction equipment in the shop.

I'd want 6" all day with mesh and fiber.

Hard pass on the mesh. Rebar on chairs is the way to go. I’ve cut out a few slabs that had wire mesh and it was almost always laying on the bottom of the concrete and sometimes even pushed down in the dirt. “Oh we’ll just pull it up as we go” is usually said but it never works out. Even when rebar was placed and “pulled up as we went” it doesn’t usually work out.

I stapled the pex tubing to the floor and set the rebar on chairs. I happily spent the money to have a pump truck come onsite and place the concrete where it was needed so the tubing and rebar weren’t disturbed.

OP if you’re going to have a gas boiler make sure to have a plan to get rid of the condensate. I really wish I’d have put a pex line from the mechanical room to the floor drain so I didn’t have to have the condensate either fill up my holding tank or drop into a bucket that I have to empty twice a day. One of the few regrets I have in my build.

 

[Trav]

How much gravel base? A slab on a good base is like a finished floor on a sub floor. If the base doesn't heave, the slab won't. And if the base does heave, the slab will regardless of any reasonable thickness.

I'm guessing he's planning on pinning radiant floor tubing to foam - either foam designed for that or using those little U shaped barbed pins to anchor to the foam.

Can you get a reference from the contractor for similar projects? See where he's done a 3" slab?

The reference is an excellent idea, definitely will ask that. He stated this plan out so confidently that I found it awkward to question it so there should be an example somewhere nearby.

He did mention something about foam boards designed for containing the radiant piping. He didn't mention a base thickness. What would be appropriate? Seemed like it would be at least 14-16 inches from the conversation, but it was not explicitly defined and I know little about this myself. Are you saying it's possible a "3 slab could be enough?

Starting to realize I know alot less than I should and should really educate myself further before I proceed in planning this if I want an excellent result.

 

[mike93lx]

Starting to realize I know alot less than I should and should really educate myself further before I proceed in planning this if I want an excellent result.

Start here

The Concrete Underground's Guide to Floor Slabs

There have been many GJ discussions on garage slabs but I'm hoping a simple outline will be helpful to those who haven't done a lot of concrete work and are getting ready to place a slab. So let’s lay out the basics that will give the average guy a good shot at a great slab. Base - What’s...

www.garagejournal.com

 

[PCustoms]

I did radiant in my shop and did a 6” slab partly for this reason and partly because I occasionally bring really heaving farm/construction equipment in the shop.


Hard pass on the mesh. Rebar on chairs is the way to go. I’ve cut out a few slabs that had wire mesh and it was almost always laying on the bottom of the concrete and sometimes even pushed down in the dirt. “Oh we’ll just pull it up as we go” is usually said but it never works out. Even when rebar was placed and “pulled up as we went” it doesn’t usually work out.

Really seems the issue is that mesh/rebar wasn't on chairs, not that mesh was used...

OP if you’re going to have a gas boiler make sure to have a plan to get rid of the condensate. I really wish I’d have put a pex line from the mechanical room to the floor drain so I didn’t have to have the condensate either fill up my holding tank or drop into a bucket that I have to empty twice a day. One of the few regrets I have in my build.

Good idea. To fix you issue after the fact they sell condensate pumps...

 

[48windsor]

I'm in the beginning stages of planning a woodshop build on high clay content soil in Saskatchewan that has a reputation for shifting. The contractor I planned on using for the work recommended a thick, well prepared gravel base with a heavily insulated 3" thick slab and radiant heat. After researching a bit on this forum and elsewhere i'm definitely thinking radiant heat is the way to go and am pleased with his insulation plans but i'm concerned about such a thin slab. He seems to think adding thickness to the slab won't increase its stability on our soil. I won't be parking vehicles in here really, maybe overnight once in a blue moon, so there won't be alot of weight on the floor. I'm also concerned about the shifting soil out here possibly making radiant heat a bad choice.

I'm a complete construction noob, and this is my first post on the forum here. I read the stickied slab guide but didn't see anything about the real pros and cons of thin slabs.

NEVER pour any slab under 4 in. thick!
The compacted base is a good idea.
I might suggest adding fibermesh to the mix . works great

 

[f121]

Suggest checking local code and talking to a structural engineer, you may need a soil sample. 3” sounds very light, I don’t think you could put a lift on that.

Off the top of my head, for the heavy clay soil here, we have an 8” reinforced slab with edge thickening to 18” to support the walls. I had a hard requirement to be able to bolt a 2 post lift onto it

 

[rlme36]

Hard pass on the mesh. Rebar on chairs is the way to go. I’ve cut out a few slabs that had wire mesh and it was almost always laying on the bottom of the concrete and sometimes even pushed down in the dirt. “Oh we’ll just pull it up as we go” is usually said but it never works out. Even when rebar was placed and “pulled up as we went” it doesn’t usually work out.

just poured my 6" slab and had mesh on chairs along with rebar in strategic places and added fiber. I hear you on the pull it up ****, hence why I asked for chairs. without the mesh real ***** to get the pex mid to lower slab. Definitely didn't want all metal on top of the pex, even when pressurized, with the crete guys doing their thing as best they can.

 

[jblnut]

Really seems the issue is that mesh/rebar wasn't on chairs, not that mesh was used...

Good call. You are absolutely correct. I see chairs under rebar way more often than under mesh so my brain associated mesh with improper more than rebar. Both are a fail if they’re not properly placed in the slab.

Good idea. To fix you issue after the fact they sell condensate pumps...

Naw. Instead of the couple hundred for a decent condensate pump I put in an outdoor wood stove and all it’s needed things to get heat into the house and shop

 

[billconner]

The reference is an excellent idea, definitely will ask that. He stated this plan out so confidently that I found it awkward to question it so there should be an example somewhere nearby.

He did mention something about foam boards designed for containing the radiant piping. He didn't mention a base thickness. What would be appropriate? Seemed like it would be at least 14-16 inches from the conversation, but it was not explicitly defined and I know little about this myself. Are you saying it's possible a "3 slab could be enough?

Starting to realize I know alot less than I should and should really educate myself further before I proceed in planning this if I want an excellent result.

I'm not a more and bigger is better fan. I think it would be OK. People talking about lifts and vehicles but I thought you said wood working, no vehicles? I suspect most people recommending 4"+ don't have the 14-16" compacted base. Seems like a smart plan to deal with the clay. Did he mention drain tile?
ps: as article above says "4" is more than adequate" .

vapor barrier? you want one I believe between foam and base.

 

[ConCretin]

A 4" slab is generally considered the minimum thickness but since the base carries most of the load, 3" of concrete might be theoretically fine but it doesn't leave much room for error.

It's normal to have variations in base and top of slab elevations and if a high spot in the base aligns with a low spot in the slab, you could have some pretty thin areas. Also, if we're talking about a free standing monolithic slab on questionable soils, steel reinforcing is a necessity. Maintaining adequate cover above and below the reinforcing especially with radiant tubes will be marginal.

Finally, the cost difference for an additional inch of concrete is pretty minimal compared to the consequences of cutting it too close. In wouldn't take the chance if it were me.

 

[dcg9381]

Interesting, thank you. I'll look into that and speak with him about it

I'd ask around before I poured 3". And I've seen a lot of use of "fiber" slabs up north, which won't need wire mesh or rebar.

I'm on pretty much solid limestone, which sits on top of a layer of flint.. So nothing is moving around. We still pour 4" with rebar.

The "right" way to answer this question - especially if it's a big investment is a soils test - and you send that to a foundation engineer who can spec the right slab for your area.

I've done one non-engineered foundation and it turned out fine, but 4" depth, 24" beams all the way around, and rebar.

 

[Trav]

A 4" slab is generally considered the minimum thickness but since the base carries most of the load, 3" of concrete might be theoretically fine but it doesn't leave much room for error.

It's normal to have variations in base and top of slab elevations and if a high spot in the base aligns with a low spot in the slab, you could have some pretty thin areas. Also, if we're talking about a free standing monolithic slab on questionable soils, steel reinforcing is a necessity. Maintaining adequate cover above and below the reinforcing especially with radiant tubes will be marginal.

Finally, the cost difference for an additional inch of concrete is pretty minimal compared to the consequences of cutting it too close. In wouldn't take the chance if it were me.

I did not consider the alternate high and low spots in the base and slab that you're mentioning here and absolutely don't want those problems. This and many other replies have convinced me, I believe i'll be going for a 4" slab.

Much rather pay for it now rather than down the road.

 

[Trav]

I'm not a more and bigger is better fan. I think it would be OK. People talking about lifts and vehicles but I thought you said wood working, no vehicles? I suspect most people recommending 4"+ don't have the 14-16" compacted base. Seems like a smart plan to deal with the clay. Did he mention drain tile?
ps: as article above says "4" is more than adequate" .

vapor barrier? you want one I believe between foam and base.

Yes this will be 99% a woodworking shop, so not alot of weight going on it (might have 2000ish pounds of wood permanently in a corner would be the heaviest thing.) I might park my half ton in it for a night but not often and not for extended periods.

I think you're correct and the 3" would probably be fine but the other comments have convinced me to go for a 4" slab since it isn't an extreme factor as to the price, more thermal mass for the radiant, and more wiggle room for thick/thin spots in the base and slab without causing problems. I'm likely only going to get to build a dedicated wood shop once in my life so i'd rather overdo it than underdo it.

When I made this post, I thought there may be pros for thin slabs on dynamic soils due to limiting the weight of the slab, but that doesn't seem to be a thing.

 

[dcg9381]

When I made this post, I thought there may be pros for thin slabs on dynamic soils due to limiting the weight of the slab, but that doesn't seem to be a thing.

I don't think there is a case for "thin" slabs, but you absolutely can do monolithic slabs without steel these days (fiber cement). They're just uncommon where I am. They are common elsewhere. I have never done one.

As you're "new" to this, make sure you specify slope or "cut outs" near all the "garage" door entrances so water can't get inside. Driveways get sloped. Anything "exterior" to the building sides (sidewalks, driveways, whatever) - they need to slope AWAY from the building. Make sure this is specified in a drawing in your contract.

Fixing low spots that collect water is a real pain in the ***. And if the concrete is sloped wrong, you get water intrusion into the building.

 

[billconner]

I'm fine with 4", which is often 3 1/2 - based on 2x4 size. I agree with the pro fiber mesh but would encourage you to look at steel fibers. Not sure they are available everywhere but I believe superior crack resistance.

 

[ConCretin]

Maybe I'm just old school but I'd be reluctant to substitute fiber for steel reinforcing in a free-standing exterior slab. Unlike interior slabs which are generally protected from movement by the foundation around them, monolithic slabs are subject to more potential movement especially where frost occurs, which can cause and/or displace cracks.

There are high end macro and steel fibers that act like rebar and actually restrain cracks but they tend to be pricey and have other drawbacks such as clumping and finishing issues. If you tell the batch plant you want fiber and leave it at that you're going to get the standard fiber that has some benefits but holding cracks together isn't one of them.

IMO the average homeowner would be better served by less expensive and proven steel reinforcing.

 

[akpolaris]

I have radiant heat in my 30' X 30' shop. Good idea. No less than 4" slab, 6" would be preferable. Is it with footers or a monolithic slab? I used mesh and rebar both in my floor but still have some cracks, but we neglected to cut in expansion jopints

 

[txvwnut]

4 inch minimum, even though you are just going to be doing woodworking in it you need to take into consideration the weight of your machinery and and the vibrations it can impart into the slab.

 

[NUTTSGT]

A 3" slab for a sidewalk would be absolutely the bare minimum I would do for a sidewalk.

You're talking a woodshop with equipment that will hum and vibrate....3" nah ...3" minus 1/2" for the radiate tubing in the concrete ? Absolutely no way would I even consider it.

 

[thammel]

Go for 5" to 6" slab. You won't regret it.

 

[The Cobbler]

24 x '40

to go another 2 " would be about 6 yards .
whatever your per yard cost is , it should be under $1500 . for a shop, I would beef up the pad thickness

 

[rixtrix1]

I'm not a more and bigger is better fan. I think it would be OK. People talking about lifts and vehicles but I thought you said wood working, no vehicles? I suspect most people recommending 4"+ don't have the 14-16" compacted base. Seems like a smart plan to deal with the clay. Did he mention drain tile?
ps: as article above says "4" is more than adequate" .

vapor barrier? you want one I believe between foam and base.

The main reason for a thicker slab is to add mass to hold the radiant heat from the tubing. The compacted base UNDER the foam does nothing to add for radiant heat and the foam itself is somewhat unstable with all the seams and the fact that the subbase cannot be made perfectly level. for 100% contact with the insulating foam. Tying the tubing to the mesh pretty much guarantees the mesh will be below the slab, where you want the tubing to be, but not adding stability. Using " Lego" mat looking foam secures the tubing on the bottom and gives the concrete something to bite into to hold the foam securely, then adding rebar on cages places it at the optimum level for strength. Fiber and wire were originally designed to strengthen lightly loaded driveways, pathways and sidewalks that are outside and experience expansion and contraction, leading to separation. Sure, you can go with less anytime, but the cost of making a permanent structure more stable is minimal. Much cheaper than ripping out and replacing a cracked floor, especially one with radiant heat tubing installed. Did you notice he's in Saskatchewan, where temps swing wildly from super cold and above putting more stress into the slab( depending on whether or not the edges of the slab are insulated too)?

 

[billconner]

That's one view but hydronic radiant heat also works well in 1 1/2" of gypcrete. Just based on science, I suspect the slab mass and water temp can be adjusted to work well. Also it seems in a slab in a shop that the time to warm it up quickly is inversely proportional to the mass. Some might say keep it at a comfort 24/7/365 but I'm sure that used more energy than setting it back as much as possible. Heres another opinion on slab thickness from a radiant floor contractor.

"The thickness of a concrete slab for radiant floor heating depends on the type of system and the size of the pipe being used:

Hydronic systems
These systems use conventional concrete and typically have a thickness of 1.5–6 inches.

Electric systems
These systems use self-leveling concrete mixes and have a total thickness of 3/4–1 inch, including the heating.

Radiant panels
The minimum recommended thickness for the slab is 1-1/4 inches for 3/8 inch pipe, 1-1/2 inches for 1/2 inch pipe, and 1-5/8 inches for 3/4 inch pipe.

Topping slabs
These slabs should be at least 1.5 inches thick to accommodate the tubing and reinforcing mesh.

Non-reinforced slabs
These slabs should not be more than 4–5 inches thick, otherwise the system will not run efficiently. "