Question about pouring Concrete pad for existing garage

[Shaggy’nScooby]

Hello, my first time posting and hopefully not a question asked to oblivion. I have a 3 door 700 sq ft garage that came with the home I bought, it has a dirt floor as of now. Structural engineer gave it a look and says it’s good. Question is, is it really that simple of just removing dirt and putting a concrete pad it? I had four contractors come out and got quotes ranging from $9,000 - $12,000. I don’t know anything about concrete work but want to make sure I don’t throw money down the drain.

This is what I’m getting quotes for:
Digging 5" of dirt and hauling
Install 10mil vapor barrier
Install rebar #4 2'@ oc each way
Install 5" concrete slab
Additional 3" Digging and hauling and Install 3" gravel

 

[billconner]

That sounds right to me. I'd go for gravel, compacted.

I don't know where you are - climate - but any thoughts of hearing or cooling, justifying foam?

 

[kaymccampbell]

We don't know where you are, so yeah, maybe.

 

[Rusted Nut]

Doesn’t sound out of line. Make sure your existing dirt grade is solid, probably is after sitting there for several years. Would hurt to run a jumping jack over it though.

 

[75gmck25]

It depends on location, but I would add the thickest foam insulation you can afford under the slab if you plan to work in it as a shop. The ground is a huge mass of cold dirt and it's hard to overcome that cold infiltration with moderate size heaters in a garage that large. Good insulation in the slab, walls and ceiling will really make a difference in heating the shop. Once a floor is done properly it doesn't require any maintenance other than a coat of paint.

Hydronic heat in the floor is the next step up in comfort, but also a big step up in cost.

 

[ConCretin]

Honestly your specs are a bit overkill but you'll end up with a very stout floor if constructed properly, which isn't a bad thing. You didn't mention concrete psi. I'd want a min of 3500 and ideally 4000 psi. You don't need the compressive strength from a structural standpoint but the extra cement will provide a more durable surface. The only other thing I'd recommend is making sure the concrete is being batched with a mid range water reducer. This chemical admixture will provide a workable slump with less water. Nothing will ruin a good slab faster than too much water.

If you think you might heat the space at some point, under-slab insulation might make sense although I wouldn't bother unless I was using radiant. It's actually counterproductive for an unheated space since it blocks the heat from the ground, which would otherwise add a little heat to the structure.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on constructing a slab. $15 per square foot would be about right around here. If you were able to get 4 quotes, you should feel pretty good about it. Ask a few questions after you read the guide and pick the guy you feel best about.

 

[Shaggy’nScooby]

That sounds right to me. I'd go for gravel, compacted.

I don't know where you are - climate - but any thoughts of hearing or cooling, justifying foam?

I’ll mention gravel compaction to contractor, I was assuming that is standard. I’m in Connecticut so I don’t know if that changes anything. Heating wise, I am going to throw a wood stove in there asap.
Thank you for your response, highly appreciated.
Let me know if you there’s anything else worth considering.

 

[Shaggy’nScooby]

We don't know where you are, so yeah, maybe.

Connecticut, I should’ve added that

 

[Shaggy’nScooby]

Doesn’t sound out of line. Make sure your existing dirt grade is solid, probably is after sitting there for several years. Would hurt to run a jumping jack over it though.

Dirt grade is definitely solid but I’ll mention to contractor that I hire. Thank you for your response!

 

[Shaggy’nScooby]

It depends on location, but I would add the thickest foam insulation you can afford under the slab if you plan to work in it as a shop. The ground is a huge mass of cold dirt and it's hard to overcome that cold infiltration with moderate size heaters in a garage that large. Good insulation in the slab, walls and ceiling will really make a difference in heating the shop. Once a floor is done properly it doesn't require any maintenance other than a coat of paint.

Hydronic heat in the floor is the next step up in comfort, but also a big step up in cost.

Highly appreciate your response, I’m going to look into the foam insulation for under the slab. That’s my first time hearing about it but I’m located in Connecticut so wouldn’t hurt to insulate since I’m going to use garage mostly for welding/gym.

 

[Shaggy’nScooby]

Honestly your specs are a bit overkill but you'll end up with a very stout floor if constructed properly, which isn't a bad thing. You didn't mention concrete psi. I'd want a min of 3500 and ideally 4000 psi. You don't need the compressive strength from a structural standpoint but the extra cement will provide a more durable surface. The only other thing I'd recommend is making sure the concrete is being batched with a mid range water reducer. This chemical admixture will provide a workable slump with less water. Nothing will ruin a good slab faster than too much water.

If you think you might heat the space at some point, under-slab insulation might make sense although I wouldn't bother unless I was using radiant. It's actually counterproductive for an unheated space since it blocks the heat from the ground, which would otherwise add a little heat to the structure.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on constructing a slab. $15 per square foot would be about right around here. If you were able to get 4 quotes, you should feel pretty good about it. Ask a few questions after you read the guide and pick the guy you feel best about.

Honestly your specs are a bit overkill but you'll end up with a very stout floor if constructed properly, which isn't a bad thing. You didn't mention concrete psi. I'd want a min of 3500 and ideally 4000 psi. You don't need the compressive strength from a structural standpoint but the extra cement will provide a more durable surface. The only other thing I'd recommend is making sure the concrete is being batched with a mid range water reducer. This chemical admixture will provide a workable slump with less water. Nothing will ruin a good slab faster than too much water.

If you think you might heat the space at some point, under-slab insulation might make sense although I wouldn't bother unless I was using radiant. It's actually counterproductive for an unheated space since it blocks the heat from the ground, which would otherwise add a little heat to the structure.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on constructing a slab. $15 per square foot would be about right around here. If you were able to get 4 quotes, you should feel pretty good about it. Ask a few questions after you read the guide and pick the guy you feel best about.

Thank you for an informative response. I’ll ask the contractor about concrete psi since that’s something I never considered. I’ll also ask about the mid range water reducer, another thing I never considered. As well as the the under slab insulation since I’ll be adding radiant heat.

Im feeling more confident about spending a good chunk of money on getting the slab poured so thank you for your response!

 

[NUTTSGT]

Edit your profile to show Connecticut. That will help in the future.


How much property do you have ? Do you have room to dump the excavated dirt ?

How do you feel about sweat equity ? I've dug alot by hand, half the house garage (24x32), trailer pad (10x32), basement floor and more. You could probably save yourself serious cash, digging it out, having stone hauled in and rent a compactor. Set your own rebar and then have a crew come in for a day to pour/finish the concrete work.

There's many guys around here that work on the side from their day job and do a Saturday side job. Stop by your local concrete plant and ask if they know anybody interested in a side job if you're willing to do the sweat equity prep job.

 

[PCustoms]

@Shaggy’nScooby maybe I missed it, but what does the garage currently have for a foundation?

 

[Joemctag]

Assume that your concrete will extend under your buildings frame but not out past it. You don’t want th concrete guy’s work to cause rainwater to get into the bottom of your wood or steel sidig and rot/rust it out or to let the rainwater run over your floor. Think also about sloping the whole floor slightly or at least just at the doors, so that water doesn’t get in or stay in. If your slab won’t be above grade (dirt) 6” to 8” but at least, say 4”, then maybe you could think about directing water away. These are all issues that MANY of us have faced.
Good luck!

 

[rancherbill]

Are you going to pour concrete under the garage? I don't see an amount for lifting. They are not that heavy and for the future the structure will last longer.

 

[finn]

5” of concrete and all that rebar seems excessive for a residential garage.

Look into thickening the slab only where you might put a lift, if you see a lift in your future.

I skipped the rebar and just used wire mesh and 4” in the existing lean to I enclosed and added a door for a 16’x50’ cold storage area. Did the same thing, otherwise, ie pulled out probably four or five inches of sand, then compacted, etc.

Still looks good after probably ten or twelve years. There’s currently an 11000 lb F450 dumptuck with 4000 lb of bagged concrete mix parked on the slab, plus an 8800 lb CTL and an empty F250 Super Duty on that slab.

 

[carcruse]

Check with your city for the gravel base requirements. 3" is a little light, should be a minimum of 4 to 6 inches. My city requires a minimum of 4" compacted gravel base. Also, I would include in the contract that all work shall be in accordance with city code. I agree with finn, use wire mesh instead of re-bar.

 

[NUTTSGT]

@Shaggy’nScooby maybe I missed it, but what does the garage currently have for a foundation?

Good question. Since he said garage, I assumed a stick frame with a CMU foundation. I thought that was odd to have a dirt floor but you never know if previous owner not got around to finishing out the interior.

The more I think about it, I wonder if it's a post frame.

Some pics would also help.

 

[PCustoms]

Good question. Since he said garage, I assumed a stick frame with a CMU foundation. I thought that was odd to have a dirt floor but you never know if previous owner not got around to finishing out the interior.

The more I think about it, I wonder if it's a post frame.

Some pics would also help.

See I read dirt floor and immediately thought pole barn....

 

[RMW1]

Edit your profile to show Connecticut. That will help in the future.


How much property do you have ? Do you have room to dump the excavated dirt ?

How do you feel about sweat equity ? I've dug alot by hand, half the house garage (24x32), trailer pad (10x32), basement floor and more. You could probably save yourself serious cash, digging it out, having stone hauled in and rent a compactor. Set your own rebar and then have a crew come in for a day to pour/finish the concrete work.

There's many guys around here that work on the side from their day job and do a Saturday side job. Stop by your local concrete plant and ask if they know anybody interested in a side job if you're willing to do the sweat equity prep job.

How do the concrete contractors feel about do-it-yourself forms, dirt compacting, gravel spreading, vapor barrier installation, rebar installation, where the concrete guy just does the pouring and finishing? They certainly would be reluctant to take any responsibility for cracks that show up later, not to mention the loss of revenue for less work done.
Or are there actually contractors who would do just the pour as a "side job" for a little extra income?
Do contractors take any responsibility for cracks even when they do all the prep work?

 

[Stick-man]

I bought a home with a new pole barn and gravel floor. I dug out some gravel and I had concrete poured, and it turned out to be a great shop.