Garage Addition - Slab Foundation Questions

[scootermcrad]

Hey everyone!

I'm working on some layout work for a large addition to our existing garage and have some questions regarding the concrete work. I'm putting some basic plans together for the structure so I can clearly convey the addition plans when I have contractors come to quote the work. I want to make sure I'm educated on what would be best for my addition before people start coming out.

The current garage is an older 2-car garage (approximately 24 wide by 21 deep overall). The slab has a slight slope to it, towards the garage doors and appears to be what local code would require. We are adding to the front AND back of the garage (this works out for our lot layout). 16 feet to the back of the garage with a single bay side entrance and 12 feet to the front of the garage and moving the doors.

The questions really come when I start thinking about the slope of the floor on the rear 16 foot deep addition. This area will have the side entrance for a single car and then the rest of the space will be used for my machining tools. Is this portion of the addition best to be flat with a center floor drain, slope with the existing slab, or should the direction now change to slope out the side door? Additional thing to note, the rear 16 foot addition will be a heated slab, with PEX tubing set in during the pour. Does this change the answer?

I'm assuming the front 12 foot addition can just continue with the slope, same as the current slab. Would that be correct?

It probably doesn't matter, but the construction of the rest is full brick veneer over stick framing.

Anything you concrete experts can share, would be appreciated.

Thanks!

 

[readhead]

I would call the back part storage on the plans and make it flat. Be careful in back if you need to bolt anything down.

 

[Bondo]

This area will have the side entrance for a single car and then the rest of the space will be used for my machining tools

Ayuh,.... That's the Beauty of concrete,.... changin' grades,....

Do yer lay-out, 'n make the machine areas dead level,....

In the car parkin' area, put a swale to the door,... 1/2" will drain,....

I'm assuming the front 12 foot addition can just continue with the slope, same as the current slab. Would that be correct?

That would depend on the pitch, 'n where/ how it meets the driveway,....
Is it an inch in 10',..?? more,..?? less,..??

 

[matt_i]

Depending on what sort of machine tools you have, how heavy, I'd skip the floor drain in that part. Two reasons. 1) will weaken the floor from a standpoint of compromising pallet jacks, Hillman rollers or forklifts rolling over the grating. 2) machine tools like low humidity environments without a lot of outside airflow (which induces large humidity changes as well as temp changes like warm moist air blowing on massive cold machines making them sweat all over). A floor drain would be an instant path to moisture and possibly airflow thru the trench into that section of the garage that could bring a lot of ambient air in.

I'd shoot for level on the machine tool section, I'd shoot for a 1" grade change in the car parking area, front to back. Blended transitions, you don't want any inconsistencies when rolling heavy stuff around.

Also, if you have heavy non-benchtop machines, I'd go to a 6" floor in that area. 1/2" rebar on 16" centers tied on 2" chairs.

 

[scootermcrad]

Ayuh,.... That's the Beauty of concrete,.... changin' grades,....

Do yer lay-out, 'n make the machine areas dead level,....

In the car parkin' area, put a swale to the door,... 1/2" will drain,....

That would depend on the pitch, 'n where/ how it meets the driveway,....
Is it an inch in 10',..?? more,..?? less,..??

Yep! I would say that is a good estimate. 1" over 10-ish feet. The driveway will get completely redone where it meets the garage. It's in great need of an overhaul, so I'm going to let the building dictate what it needs to be. It will be a slight slope away from the garage no matter what. The whole garage is at the highest part of the lot, so it works out great.

I like the idea of leveling out the machine area. Even a slight pitch is okay, as the machine tools all have leveling feet.

What should I expect the method will be for joining the new slab to the old slab? Drill and place rebar?

 

[scootermcrad]

Depending on what sort of machine tools you have, how heavy, I'd skip the floor drain in that part. Two reasons. 1) will weaken the floor from a standpoint of compromising pallet jacks, Hillman rollers or forklifts rolling over the grating. 2) machine tools like low humidity environments without a lot of outside airflow (which induces large humidity changes as well as temp changes like warm moist air blowing on massive cold machines making them sweat all over). A floor drain would be an instant path to moisture and possibly airflow thru the trench into that section of the garage that could bring a lot of ambient air in.

I'd shoot for level on the machine tool section, I'd shoot for a 1" grade change in the car parking area, front to back. Blended transitions, you don't want any inconsistencies when rolling heavy stuff around.

Also, if you have heavy non-benchtop machines, I'd go to a 6" floor in that area. 1/2" rebar on 16" centers tied on 2" chairs.

Great thoughts here! And good points!

I have a lathe, mill, large cast iron band saw, and who knows what the future will bring, but probably nothing larger than the mill (bridgeport J1) which weighs in around 1800 lbs, I suppose.

So, I suppose I should keep the PEX out of the machine areas too, or would I still be okay to go forward with the heated floor? I don't anticipate bolting any of this equipment down, so not worried about drilling into anything. Thoughts?

 

[matt_i]

I don't know which part of NC you are located in, if its mountainous the PEX heat would probably be great. If its going to be air conditioned 6+ months out of the year, it might not be worth the investment as its not just something you turn on for a couple hour shop session. I was going to build a shop in middle TN and had ruled out the PEX heat for these reasons above. Now in Michigan I think it would be quite nice but it is an investment up front in time and materials to source the rigid foam with enough compressive strength to do what I wanted to do. Also the shop building that came with my house already had forced air heat...

 

[scootermcrad]

I don't know which part of NC you are located in, if its mountainous the PEX heat would probably be great. If its going to be air conditioned 6+ months out of the year, it might not be worth the investment as its not just something you turn on for a couple hour shop session. I was going to build a shop in middle TN and had ruled out the PEX heat for these reasons above. Now in Michigan I think it would be quite nice but it is an investment up front in time and materials to source the rigid foam with enough compressive strength to do what I wanted to do. Also the shop building that came with my house already had forced air heat...

More good points. I'm north of Charlotte, so it's fairly mild. Probably overkill. And yeah, I suppose just getting it properly insulated woulud be good enough.

I probably wouldn't put in a real elaborate HVAC system. I was planning on insulating the **** out of it and just heating it in the winter with a wood stove. The rooms above the garage will have a small wall unit, as it will be used for an office or work room.

 

[wssix99]

A 1" per 4' or a 1" per 8' slope would be common. They are there to flow heavier than air gasoline fumes to the doors and out of the space. Flat floors in places where you have cars are generally not advisable and if the garage is attached to a house - it's definitely not to code. The safest option for your machine tools would be to elevate your flat portion of the floor on a step/gas curb so that the levels your cars are parked are below the step and gas fumes can't flow up to where you have the tools.

What should I expect the method will be for joining the new slab to the old slab? Drill and place rebar?

Not if you do PEX. Rebar pins would be a pathway for heat to your cold un-insulated and unheated slab. If you heat the new sections with PEX, I'd isolate them with an expansion joint and just let them float separately from the other floor sections. You can fill the tops of those joints with liquid or hard plastic fillers to make them look nice.

 

[scootermcrad]

A 1" per 4' or a 1" per 8' slope would be common. They are there to flow heavier than air gasoline fumes to the doors and out of the space. Flat floors in places where you have cars are generally not advisable and if the garage is attached to a house - it's definitely not to code. The safest option for your machine tools would be to elevate your flat portion of the floor on a step/gas curb so that the levels your cars are parked are below the step and gas fumes can't flow up to where you have the tools.

I definitely see how this would be a good idea, however it would make moving the machines around difficult with a pallet jack. I would like to leave the option open for moving the machines around easily, in the event I add or replace a machine tool and have to rearrange some things. I also want to leave it "non-specific" in layout in case I decide to move the machines out of the garage all together in to another building (this is a future possibility). I would probably feel more comfortable having a gentle slope in these areas and just leveling out the machines with their leveling feet.

Not if you do PEX. Rebar pins would be a pathway for heat to your cold un-insulated and unheated slab. If you heat the new sections with PEX, I'd isolate them with an expansion joint and just let them float separately from the other floor sections. You can fill the tops of those joints with liquid or hard plastic fillers to make them look nice.

Also a good point! Didn't think of that. I'm leaning towards ditching the PEX anyway...


SOOO... Since we're on that subject (and I'm sure it's covered in detail elsewhere here on the forum, so may as well keep it short), how much of a real advantage will I see in the North Carolina mild winter months with a heated slab over simply just making sure the slab is well insulated from the ground? I'm starting to think this is an effort not worth it. And it would only be a small area as well, so in the end, probably not much gain. Frost line is like 10-inches in my area, I think. Lows rarely in the single digits and teens, and only for short periods of time.

Thanks everyone! Keep the input coming! Learning a bunch here!

 

[scootermcrad]

Another question, sort of off the initial topic, but certainly related. The existing slab that is there now and being added to is an older slab didn't have expansion joints put in, so it has a couple random cracks. Is this something I could expect them to repair with good results? Just a case of cutting proper expansion joints and filling with sealer?

 

[scootermcrad]

I did a quick sketch at lunch to better explain what's going on here. The middle cross hatched area is the existing structure. Shown are the three car entries and the basic layout.

 

[Bondo]

Ayuh,.... In yer lay-out, the machines go along the left wall, correct,..??

If so, thicken the slab in that area to support the weight, 'n vibration,...

Then blend into a pitched swale that'll run from the stairs to the car door,...
The car area could be a 4" slab, while the machine area a 6" or 8",...

On the other end, ya just thicken the edges, continue the pitch to the door, 'n go for it,...

 

[matt_i]

If your existing slab is "old" with no expansion joints, I don't think there is any cure at this point for the randomly generated cracks, that's just along the weakest point/line of it. You could of course cut new joints into it but the concrete reaction has probably long ago stabilized to equilibrium.

You could also seal or vee out and fill the cracks if it helps you feel better but I don't think there is any functiontal gain at this point.

Along the lines of heating, if you insulate well enough, you can probably heat with 4 x 100w incandescent light bulbs, if left steadily on during colder months.....

Just to clarify, my comments against the PEX floor heat are not against its effectiveness or usefulness, just against the inital cost vs. something like a hanging forced air heater, e.g. modine hot dawg.

 

[scootermcrad]

Ayuh,.... In yer lay-out, the machines go along the left wall, correct,..??

If so, thicken the slab in that area to support the weight, 'n vibration,...

Then blend into a pitched swale that'll run from the stairs to the car door,...
The car area could be a 4" slab, while the machine area a 6" or 8",...

On the other end, ya just thicken the edges, continue the pitch to the door, 'n go for it,...

Yep! Left wall (back wall) will be where the machine tools go. 6" slab in that area wouldn't be too much extra cost, and should be plenty, I would think. Basically it works out to about a 5 x 23 area. So that's only about 20 additional cubic feet of concrete to the 4" slab.

 

[scootermcrad]

If your existing slab is "old" with no expansion joints, I don't think there is any cure at this point for the randomly generated cracks, that's just along the weakest point/line of it. You could of course cut new joints into it but the concrete reaction has probably long ago stabilized to equilibrium.

You could also seal or vee out and fill the cracks if it helps you feel better but I don't think there is any functiontal gain at this point.

Along the lines of heating, if you insulate well enough, you can probably heat with 4 x 100w incandescent light bulbs, if left steadily on during colder months.....

Just to clarify, my comments against the PEX floor heat are not against its effectiveness or usefulness, just against the inital cost vs. something like a hanging forced air heater, e.g. modine hot dawg.

I really like the idea of keeping it simple. I keep forgetting that there are ways to insulate a slab. I've been hanging around too many OLD shops with cold floors.

 

[scootermcrad]

So... I should probably ask an additional question, since I shared the floor plan. What's not been mentioned is the fact that I would really like to put in a 4-post lift in the 12-foot addition, in one of those bays. 2 of the posts will be in the new section, but the other 2 post would sit on the old portion. I don't know how thick the existing slab is, but I suspect it's only 4". Most manufactures say you can get away with a 4" slab, it seems, however, should I go ahead and put in footers? Just not 100% sure where I want it to be exactly.

Thoughts?

 

[Bondo]

What's not been mentioned is the fact that I would really like to put in a 4-post lift in the 12-foot addition, in one of those bays. 2 of the posts will be in the new section, but the other 2 post would sit on the old portion. I don't know how thick the existing slab is, but I suspect it's only 4". Most manufactures say you can get away with a 4" slab, it seems, however, should I go ahead and put in footers? Just not 100% sure where I want it to be exactly.

Ayuh,.... Decide where ya want it, 'n put thickened footers where ya need 'em,....

Cut a hole in the existin', 'n deepen the footers there,....

 

[scootermcrad]

Ayuh,.... Decide where ya want it, 'n put thickened footers where ya need 'em,....

Cut a hole in the existin', 'n deepen the footers there,....

10-4

 

[wssix99]

how much of a real advantage will I see in the North Carolina mild winter months with a heated slab over simply just making sure the slab is well insulated from the ground? I'm starting to think this is an effort not worth it. And it would only be a small area as well, so in the end, probably not much gain.

IMO - It's probably not worth the effort/expense in your situation unless you are willing to rip up and redo the entire floor and want to heat the garage 24/7.

If you have one part of the slab that is heated/insulated and the rest isn't, it will just make your feet really comfortable in that one area. Functionally, with the rest of the slab un-insulated, it will be like having all the windows and doors open on a 55 degree F - or colder - F day. (That un-insulated slab will always be 55 degrees or less and will constantly be sucking away whatever heat you are putting in to the room's air.)

No matter what you do, having part of the slab un-insulated will lead to a very inefficient heating situation - so I'd think it best to go with a spot/space heating solution that you can turn on and use where and when you need it. (I wouldn't think a radiant floor would be very good for a space heating solution.)

Another question, sort of off the initial topic, but certainly related. The existing slab that is there now and being added to is an older slab didn't have expansion joints put in, so it has a couple random cracks. Is this something I could expect them to repair with good results? Just a case of cutting proper expansion joints and filling with sealer?

Those cracks are likely from shrinkage of the slab when it initially cured, so they should not get worse. So, there should not be a need to take any other preventative measures. As mentioned above, you should be able to get good results by grinding them out and properly filling them.

In my garage, the cracks were limited to our saw cut joints, but we filled those with Ardex and with an epoxy coating, they are invisible.

Most manufactures say you can get away with a 4" slab, it seems, however, should I go ahead and put in footers?

No. No. No.

You aren't "getting away" with anything because a "footer" is not required. The lifts are designed to be installed on flat, floating slabs. The thickness of the slab (coupled with the continuous support of the ground underneath) is where the strength comes from and is also required for anchoring.

If you don't have the required thickness, you will need to either replace the floor or install a new thickened slab (keyed or pinned to the old slab) under the two posts. If you need to do this, its probably best to wait to do this until you know where you'll want to put the posts vs. deal with it during your construction. (Although it will be a little dusty/messy.)

^ A lot of people get confused when they see the instructions for these thickened repair slabs and call them "footers." That's not what they are and one would notice that the lift manufacturers don't call them that, either.

 

[scootermcrad]

IMO - It's probably not worth the effort/expense in your situation unless you are willing to rip up and redo the entire floor and want to heat the garage 24/7.

If you have one part of the slab that is heated/insulated and the rest isn't, it will just make your feet really comfortable in that one area. Functionally, with the rest of the slab un-insulated, it will be like having all the windows and doors open on a 55 degree F - or colder - F day. (That un-insulated slab will always be 55 degrees or less and will constantly be sucking away whatever heat you are putting in to the room's air.)

No matter what you do, having part of the slab un-insulated will lead to a very inefficient heating situation - so I'd think it best to go with a spot/space heating solution that you can turn on and use where and when you need it. (I wouldn't think a radiant floor would be very good for a space heating solution.)




Those cracks are likely from shrinkage of the slab when it initially cured, so they should not get worse. So, there should not be a need to take any other preventative measures. As mentioned above, you should be able to get good results by grinding them out and properly filling them.

In my garage, the cracks were limited to our saw cut joints, but we filled those with Ardex and with an epoxy coating, they are invisible.




No. No. No.

You aren't "getting away" with anything because a "footer" is not required. The lifts are designed to be installed on flat, floating slabs. The thickness of the slab (coupled with the continuous support of the ground underneath) is where the strength comes from and is also required for anchoring.

If you don't have the required thickness, you will need to either replace the floor or install a new thickened slab (keyed or pinned to the old slab) under the two posts. If you need to do this, its probably best to wait to do this until you know where you'll want to put the posts vs. deal with it during your construction. (Although it will be a little dusty/messy.)

^ A lot of people get confused when they see the instructions for these thickened repair slabs and call them "footers." That's not what they are and one would notice that the lift manufacturers don't call them that, either.

All good points! Thank you! I'm assuming there is no insulation under the existing slab, however, it doesn't ever seem "miserable" in the winter. I assume that even if the existing slab is not insulated, it would still be beneficial to insulate the new slabs, correct?

With regards to the bit about the lift, so you're saying there's no need for footers. Are you also saying that 4-inches is indeed a common thickness worthy of supporting a 4-post? If not, and my existing slab is only 4-inches, it's not worth cutting the slab and creating a thickened area for the two posts? Maybe this IS what you're saying, just making sure I'm understanding what you're saying.

It's easy enough for me to select a reputable make of 4-post and plan out it's location. No big deal. I'm a big planner anyway, so I'm sure I will long before the slab goes in. HA!

 

[wssix99]

I assume that even if the existing slab is not insulated, it would still be beneficial to insulate the new slabs, correct?

It would be "beneficial" but I'm not sure if it would be cost effective. To figure out if you'll get a return on your investment, you'll have to run some numbers and make a lot of assumptions. The insulation isn't cheap. Since you won't be insulating the whole thing, you'll get diminished returns from that investment also.

I'd think of your situation like having a house with a bunch of open windows. Your garage slab is the collection of your open windows and the cold ground underneath is the cold "outside air." If you insulate part of the slab, its like closing just one window of the "house" - but you still have a bunch of other open windows/areas where heat can escape. The heat won't go through your insulated slab very quickly, but it will still easily find its way outside through the un-insulated portions.

If you aren't going to heat the garage 24/7 or if you will never tear up the existing slab or if you don't ever plan on partitioning the new garage, then I wouldn't think that you'll see a return on your investment in slab insulation.

If you think you may tear up the old slab and re-pour it some day, may partition off the new area, etc. then maybe insulating it now will be a good idea so you can have a completely insulated "envelope" at some later time?


I'm not an expert in this kind of thing but you may want to look in to it: (BTW - There are a lot of witch doctors and mad scientists out there who think they are - but aren't, so you need to search a little to find a reputable and real expert.) You can insulate the edge of your foundation and put insulation horizontally at the base of your foundation to help protect heat loss from your floor. Even without insulation directly under your slab, this arrangement should create a "bubble" around your garage that will slow down heat loss at the point that the dirt under the floor radiates your heat outside the foundation. This, in turn, should keep the dirt under the floor warmer and your slab warmer.

With regards to the bit about the lift, so you're saying there's no need for footers.

100%

Are you also saying that 4-inches is indeed a common thickness worthy of supporting a 4-post?

No doubt.

A lot of people have a hard time believing this, but if you walk through this simple thought experiment, it makes perfect sense - particularly in the case of a 4 post lift:

- The weight of your car is transferred to the slab via four points: the contact patches of the four car tires. When the car is on a 4 post lift, the weight is transferred similarly: through the four base plates of the lift. Since the surface area of those base plates is similar to the surface area of the tire contact patches, the concrete doesn't see much difference in a car sitting directly on it vs. a car on top of a lift. The total weight is spread out across the same-ish amount of surface area.

 

[matt_i]

Agree on the thickened slab. The biggest liability issue that I see as a machine designer is the lift getting loaded off-center and tipping over, when raised. So, the pullout strength of the anchor bolts is what keeps this from happening. If one used a wedge anchor, it forces a "funnel" shaped volume of the concrete into load. The larger the funnel, the more load it would take to tear it out, and the only way to get a large funnel is with a deeper section of concrete. I post that one should make the thick section 1 foot larger in every direction, measured out from the footprint of the lift. This way you can move the lift around some and not go outside the area of the thick section.

I recommend epoxy anchors because I think they are stronger and more durable than wedge anchors. Take the case of a live load like an industrial robot. No amount of wedge anchors can hold it down without eventually getting loose. But 4 epoxied threaded rods hold it just fine. The epoxy typically has a 6-7ksi tensile strength if you believe the manufacturer. Which is stronger than the concrete itself.

 

[scootermcrad]

Thanks everyone! Lots of info there! When I get a little further with the slab layout, I'll post some more info. Please don't hesitate to keep the input coming. It's appreciated!