Foundation type to use on garage build

[kdrymer]

Hi all - Looking to build a 28x32 stick-built garage, 2x6 construction with attic trusses for additional storage in Northeastern Pennsylvania. I am starting to get estimates for the foundation work and have had a few guys tell me they would do a monolithic slab foundation, and another would do a stem-wall down to the frost level at 42 inches below ground. We get plenty of cold weather in the winter so I am concerned about frost protection.

I've attached a picture of the land contours. It slopes in two directions as indicated by the arrows (box is approximately where garage would be built, obviously not to scale). Based on this is there one foundation I should be considering over the other?

I know a stem-wall would work regardless, but has additional cost with it. With a monolithic pour, I'm not sure if the surrounding land has to be perfectly flat, or if this setting would be appropriate with how it slopes. I would have a swale put in on the two upper sides of the garage regardless to divert water away from the building.

Please let me know if I left out any other details that would be helpful. Thanks in advance!

 

[LopezBart]

If you're going to heat the place, I think you'll want an isolated slab floor with insulation underneath and between the stem wall and slab.

 

[kdrymer]

I don't have any plans to heat it.

 

[ConCretin]

Monolithic slabs are a perfectly viable option for a structure such as yours. I have several myself and we have pretty severe winters up here in Maine. There is an increased risk for frost movement but if you have decent soils, the risk is minimal and can be mitigated almost completely with the use of rigid insulation under and around the slab to create a frost protected shallow foundation. None of mine are frost protected and I've never seen any signs of movement.

There should be considerable costs savings for a monolithic slab over a frost wall/slab on grade, some of which would disappear with the addition of insulation. There is one downside to a mono-slab in that your wood framing would sit closer to the ground then it typically would with a frost wall. You can however add a curb on top of the slab and still pocket considerable savings. It's always a good idea to see what's allowed by your local codes if you have any before getting too far along.

With a sloped site, you'd remove all organic matter and then build a level gravel pad for the monolithic slab. Check out my Guide to Floor Slabs in the link below for some additional thoughts.

 

[billconner]

The garage I built several years ago is also 32 28 and not heated. I did a rubble filled trench with surface bonded block stem walls on top of it. It was inset into a slope. All choices were based on diy the whole thing.

By code - IRC - I don't think you can do larger than 600 SF slab on grade without frost protection. It's probably done all the time, as ConCretin suggests.

If yours is not well drained soil - and my family has been in NE PA since early 1800s and I know there are iffy areas there - I would either do the footing at frost depth or do a frost protected shallow foundation - probably 2" of rigid foam to 3 or 4 feet outside perimeter. 45 pieces 4x8.

 

[u2slow]

Yeah, building rules here forced me to do stem wall on my 750sqft shop.

I did NOT do attic trusses because i wanted a high ceiling for tall vehicles. The attic-trussed shop 6 doors down showed me how that plays out.

I've cut in around the core vehicle area with tall shelving and mezzanine now.

 

[The Cobbler]

I'm not sure if the surrounding land has to be perfectly flat, or if this setting would be appropriate with how it slopes.

your slope doesn't look too bad. in my case I had to go 16": below grade at the lowest point of the land so I believe I went about 32" below grade to the bottom of the pour at the deepest point . the base was 8" wide & 45° up to where the bottom of the 6" pad met so there was a lot of concrete

 

[mikegt4]

I don't have any plans to heat it.

Come the first Jan/Feb after the garage is completed you may change your mind. Some insulation under the concrete floor is easy to install and not much in the total cost of the building and it will increase the value of the building when you sell.

 

[The Cobbler]

Some insulation under the concrete floor is easy to install and not much in the total cost of the building.

good point. I insulated my floor with 2" XPS high density foam when I poured mine

 

[kdrymer]

What are we talking % wise in cost increase between monolithic and a stem/frost wall foundation? 40% more, 50%?

 

[The Cobbler]

I'm not sure what my cost difference would have been. I didn't explore the footing/stem wall option

 

[finn]

I built the 32’x54’ with a monolithic (thickened edge) in about 2003. No issues to date, and our weather in the UP is pretty extreme.

The inspector asked that we add a course of block to keep the sill elevated. We elected to go with three courses to get a little higher ceilings, for not much additional expense.

We used split face blocks for aesthetics.

We went with attic (actually storage) trusses, which was a waste of money. Access is poor with pull down stairs, and permanent stairs waste too much floor space

 

[kdrymer]

I built the 32’x54’ with a monolithic (thickened edge) in about 2003. No issues to date, and our weather in the UP is pretty extreme.

The inspector asked that we add a course of block to keep the sill elevated. We elected to go with three courses to get a little higher ceilings, for not much additional expense.

We used split face blocks for aesthetics.

We went with attic (actually storage) trusses, which was a waste of money. Access is poor with pull down stairs, and permanent stairs waste too much floor space

Did you insulate around the foundation? I've seen where foam sheets are placed up to the outer edge and then buried, but I worry about long term durability with that.

Without insulating, the whole structure can move with the ground freeze/thaw cycles?

 

[Stuart in MN]

There is one downside to a mono-slab in that your wood framing would sit closer to the ground then it typically would with a frost wall. You can however add a curb on top of the slab and still pocket considerable savings. It's always a good idea to see what's allowed by your local codes if you have any before getting too far along.

My garage is on a slab, with one course of blocks around the perimeter on top of that to keep the framing up out of the dirt.

 

[Yankeefarmer]

You haven’t mentioned your use plans other than “storage.” When I built mine, I wanted high ceilings to accomodate a lift and so my mini-ex could fit inside for maintenance. My architect was concerned about how to achieve my stud wall heights wrt code requirements, and it turned out that adding to the stem wall height addressed that issue at minimal cost. That wouldn’t have been possible with a monolithic slab.

Others have mentioned that attic trusses have downsides. Perhaps for some applications. I used attic trusses for about half the building, and scissor trusses for the other half, to provide storage area and ceiling height.

 

[jack stand]

Aside from the actual foundation selection, how much of the up slope real estate do you own?
It appears to be of a lot of drainage area that could involve your building site.
You can't build swales or change your natural water flow around your site if you don't own it.
Deciding on the elevation of your site can correct this but it's a serious decision that needs serious attention. More often than not, owner builds are too low and it's not easily corrected.
If you own clear up to the church don't worry about it.

 

[ConCretin]

What are we talking % wise in cost increase between monolithic and a stem/frost wall foundation? 40% more, 50%?

That's hard to say because so many factors are local but I'd expect you're in the ballpark and maybe a bit low. Some quick back of the napkin math using local numbers suggests the concrete cost is about half.

The site work only requires one operation for a mono-slab to clear, grub, fill and grade whereas the frost wall requires additional pricey excavation and backfill operations separated into two mobilizations around the foundation work. The concrete is a single placement for the mono-slab while the frost wall/slab requires three. Add in the extra material cost for frost wall/slab option and you should see considerable savings.

There is a little extra cost due to the sloped site for both options but they should be similar. The mono-slab will require more gravel for the pad and sloped aprons while the frost wall/slab option will need similarly profiled backfill and interior fill. You could extend the walls at the deep end and save some exterior fill but you'll pay for taller walls.

You'll obviously need to get local bids to know for sure.

A couple additional thoughts; Your sloped site is a plus for a mono-slab option because natural drainage will help keep your base free of moisture and thereby more frost resistant.

You seem understandably concerned with frost movement. Another term for a mono-slab is floating slab. A well constructed mono-slab can move up and down with frost conditions and you'll never even know as long as soil conditions are generally uniform under the slab. If this bothers you, it might be worth the extra bucks for a standard frost wall foundation. Keep this in mind however, your frost walls will be frost protected but in an unheated building, your slab won't be unless you protect it too.

 

[Skooterj]

I built my 24x30 on a monolithic slab with one row of block to get me an extra 8 inches of ceiling height and to keep the wood elevated. 720 SF (24x30) is the largest I could do on a monolithic by code in my jurisdiction. It was almost twice as much to do a stem wall. My building is not insulated yet, and the only heat is a torpedo heater that only runs when I'm actually in the building. It has worked great for storage.

 

[u2slow]

Come the first Jan/Feb after the garage is completed you may change your mind.

Or from the other extreme.... 13 years later my building (ceiling mainly) is still not closed up enough to heat.

If a stem wall setup is chosen, you dont have to rush on the slab choice. I was overdrawn at that point and just happy to have a basic/minimum 6" slab.

 

[Skooterj]

Or from the other extreme.... 13 years later my building (ceiling mainly) is still not closed up enough to heat.

If a stem wall setup is chosen, you dont have to rush on the slab choice. I was overdrawn at that point and just happy to have a basic/minimum 6" slab.

I knew a guy who ran out of money before he could finish his barn and worked on the gravel fill for years until he could afford the slab inside his stem walls.

 

[finn]

Did you insulate around the foundation? I've seen where foam sheets are placed up to the outer edge and then buried, but I worry about long term durability with that.

Without insulating, the whole structure can move with the ground freeze/thaw cycles?

Yes and no. The plan was to throw up a wall and use half the building (actually 30’) for daily drivers and cold storage. That side wasn’t insulated around the perimeter or under the 4” slab.

The other side (24’) got pex tubing, perimeter and underfloor insulation, and a thermal break along the non insulated side.

I bought a nearby building before I ever did the hydronic heat or threw up the dividing wall.

In retrospect, I should have done the entire pour with perimeter and underfloor insulation, but the budget didn’t allow it. I should have omitted the storage truss. What I have works fine, though.

 

[Sumboodie]

Monolithic slabs are a perfectly viable option for a structure such as yours. I have several myself and we have pretty severe winters up here in Maine. There is an increased risk for frost movement but if you have decent soils, the risk is minimal and can be mitigated almost completely with the use of rigid insulation under and around the slab to create a frost protected shallow foundation. None of mine are frost protected and I've never seen any signs of movement.

There should be considerable costs savings for a monolithic slab over a frost wall/slab on grade, some of which would disappear with the addition of insulation. There is one downside to a mono-slab in that your wood framing would sit closer to the ground then it typically would with a frost wall. You can however add a curb on top of the slab and still pocket considerable savings. It's always a good idea to see what's allowed by your local codes if you have any before getting too far along.

With a sloped site, you'd remove all organic matter and then build a level gravel pad for the monolithic slab. Check out my Guide to Floor Slabs in the link below for some additional thoughts.

No clay on your land?

My Dad was saying when they put in a fuel tank at the school up the street in the 90s, they had to dig out 15ft of clay.

 

[kdrymer]

Aside from the actual foundation selection, how much of the up slope real estate do you own?
It appears to be of a lot of drainage area that could involve your building site.
You can't build swales or change your natural water flow around your site if you don't own it.
Deciding on the elevation of your site can correct this but it's a serious decision that needs serious attention. More often than not, owner builds are too low and it's not easily corrected.
If you own clear up to the church don't worry about it.

I own all that area in the picture and beyond that as well almost up to the church, our property sits almost at the top of a hill where that church is.

 

[kdrymer]

Our soils are composed of primarily Mardin, as shown below:

Map Unit Symbol	Map Unit Name	Acres in AOI	Percent of AOI
MaC	Mardin channery silt loam, 8 to 15 percent slopes	0.1	97.8%
VoC	Volusia channery silt loam, 8 to 15 percent slopes	0.0	2.2%
Totals for Area of Interest	0.2	100.0%

 

[billconner]

Having been going to that area a lot since I was an infant 74 years ago, that sounds right - even though I didn't that it was called Mardin.

Google "building on Mardin soil". Not encouraging.

 

[ConCretin]

Not being familliar with the term 'Mardin channery silt loam', I fired up the Google machine only to end up in the the 'dense fragipan'. At this point, I threw in the towel and would encourage you to do the same. lol.

I'd seek out some local knowledge about your soil conditions before deciding on a foundation system. In geotechnical terms, even poor soils can easily support the loads of your structure. As long as the soils drain freely, you should be fine. Even clay type soils are fine as long as they aren't expansive and the water can run off rather than pool on top.

 

[billconner]

I thought I read that expansive clay was commonly found with Mardin soil.

 

[kdrymer]

So would this sway you one way or another?

 

[billconner]

If the building department allows I'd be tempted to do a deep base - 18 inches? - of maybe 1 1/2 or 2" gravel, with drains to daylight down hill, and a slab. If let base extend a couple of feet outside building perimeter and probably add insulation for a frost protected shallow foundation.

I feel a stem wall on footings would be more susceptible to movement and the slab heaving.

 

[larry4406]

With a monoslab I think you are committed to making a flat spot with swales around it for drainage or building a nearby retaining wall to address the grade differences. So some of the foundation "savings" get spent on perimeter grading and possibly a retaining wall.

With a stem wall, it solves the grade issue easily. The excavated spoils are used as backfill around the perimeter foundation. No other swales or possibly retaining wall needed. Yes this is 3 pours typically (footing, wall, slab) and likely involving waterproofing and draintile depending on how much backfill is against the walls.

I would be inclined to do the stem wall method. Typically the slab can go in after the structure is framed.

 

[billconner]

BTW a gravel pad to frost depth is a frost protected foundation. 36" or so and a single pour slab.

Grade level entrance at top, bottom, or side?