Building the Woodshop: Part V - The Foundation

Part V 

One of my favorite lines in a movie was in one of the Naked Gun movies (with Leslie Nielsen) where Ricardo Montalban was playing the villain.  When asked by Priscilla Presley how he could be so evil his response was something like:

"You forget I spent two years as a building contractor!"

Of course that's in jest (?).  You know - there are, of course, good and bad contractors, and I've dealt with my share of each - and the concrete contractor I used for this project was quite unfortunately of the latter persuasion.  I was really disappointed in his work - and I even gave him a second chance the next year to pour the slab out front of the shop for me - but that's for a later entry...  If you are looking for a general contractor, or even just a "sub" contractor, make sure you check references and investigate your choices with the local builders' association.  Above all - you will need patience and perseverance to be successful.  Don't expect perfection - but be ready to stand your ground when needed - and pick your battles well.  Know what's important, and what's not. At the time, there was a shortage of available contractors for me to hire, and I was not patient... 

But first, I should go over the design of the foundation in a bit more depth...

The Design

There were two different foundation systems I looked into using.  My first consideration was to pour what's known as a "monolithic" slab - that's where the footings, stem wall (if there is one) and the finished floor are all poured at once, in one big pour...  The section through such a system looks something like this:

Footing Detail

There are several advantages to a system such as this.. It is the most efficient system, combining the footing, wall, and slab all into a single, large system.  There is considerably less labor involved in the pour, but the earthwork needs to be done more precisely to save fill requirement.  And finally if termites are an issue in your part of the country this type doesn't allow anyplace for the nasty little buggers to penetrate from underneath.  It does have a couple of disadvantages as well...  One is that the bottom of the wall is more prone to water damage, as the top of the concrete can be only slightly above grade level...  and because of that, it also does not work as well on a sloped lot.  Though it's hard to see in the photos below, my lot slopes down from right to left (east to west) about a foot in the width of the shop.  It might not seem like much, but it is a lot of fill to bring in.   I prefer to work with the slope of the land rather than fight it...

Another disadvantage - at least for me - is that I wanted to have the entire floor of the slope 1/8" per foot from the back end to the front - a drop of about 4", so it would drain and I wouldn't have water puddling up in the center of the floor if I parked a snow-bound car inside.  That would make it difficult to frame up so it is level... This might not be an issue for you, but my experience has been to keep the space as flexible as possible - and that means uses other than woodworking.  The next owner of my shop might want to work on cars, for example - and so will I, for that matter.

The final disadvantage is I wanted to be able to hose down the slab in the main section of the shop if for some reason it needed it...    I can seal a rubber base down for some walls, but doing all would be too much.  So - it meant I would opt for the more expensive version (of course) - a stem wall type foundation.  Here's a typical section for that style of  foundation:

Footing Detail

You can see that now the foundation wall is independent of the floor slab, so with this system I can slope the floor and not worry about framing the exterior walls plumb and level.  The slab at the far back of the shop would be about an inch below the top of the foundation wall - and about 5 inches below it at the front.  This works out well for the driveway slab to be poured out front of the shop - it will work out to be about the right height so the foundation wall can be above finish grade level by about 8".

You can also see why more labor is required to make it...  Instead of one pour for the entire foundation and slab, the work now has to be divided into three separate pours...  First you must form up the strip footing so it is below frost depth and pour it - then you must form up the stem walls and pour them; and then, finally, you pour the floor slab.  Three separate pours...  It uses the same or more concrete, and your contractor must have the form work to be able to pour the stem walls.  There are many concrete contractors out there that do only flat work - they don't want to invest the money into the forms required, and most often they can get away without investing in too much heavy machinery.  If you do it yourself, you can easily make your own forms, and rent your own machinery... but it's an added expense, and not a small one.  You could use the forms afterward as sheathing - but you need to coat the forms with a release agent (usually an oil) that can make it less than desirable for use as sheathing.  My neighbor rented a backhoe to do his shop, and by the time he was done with it, he'd spent $1100 or so just for it.

What determines how deep you put your foundation is set by the local building department and is what's known as "Frost Depth" - the depth at which the ground does not regularly freeze.  Freezing ground is bad for a foundation. Water expands when froze, and as a result lifts and stresses the foundation, and over time that stress can result in the foundation failing.

Frost depth in my area is about 2'-0" below finish grade.  With an 8" deep strip footing at the bottom, that means we can use a 2' deep stem wall - which will give us two feet below grade if we keep the finish grade at a maximum of 8" below the top of the stem wall.   Frost depth varies around the country... in warmer climates, there isn't one, just a requirement the footing be on inorganic undisturbed soil or compacted structural fill.  Where I grew up in the Dakotas, it was a full 4' below finish grade.  It's one reason you will see so many basements in the north vs. the south - you are required to go so deep anyway to get down to frost depth with your footing, there's no good reason not to make it just a couple feet deeper to make it a basement.

Optimally, I would have used the detail above, with a 6" wide stem wall.  Structurally, it's plenty strong to hold up the building. it also keeps you from having problems with applying the finish material - when you install the drywall (or plywood, or whatever finish material you have) the inside of the wall will line up with the concrete.  What I ended up with was more like this, however:

 Footing detail

Note the foundation is 8" wide, not 6".  One other difference that is shown on that detail that I don't have - insulation.  I got none.  Nada.  It's a sordid tale - I'll let you know as I go through the photos of the construction below just how I arrived with that detail.

Strip Footings

The first job after you dig the trench for the footings is to form up for the strip footings.  It's pretty simple, really - your biggest concern is setting them at the right height.   You want the footings resting on undisturbed, inorganic soil (or compacted structural fill), just below frost level.  Your building department will be able to tell you just how much your soil can support, and thusly how big your footing needs to be.  You can check with them to find what frost level is in your part of the country.  You then want to form so the top is level and gives the footing a minimum depth (in my case 8").  You can use the excavated soil for the sides of your forms at the very bottom - but it's best to have most of it formed with footer boards to make sure the edges are held in place correctly, which will allow you to estimate just how much concrete you need.  It's always better to overdo the forms rather than under-do them, as nothing sucks more than a blowout during a pour.

The easiest way to set the level is if you have a builder's level or transit and a pole, which can be rented for fairly little money.  You set stakes in the ground at strategic points and with the aid of a helper, shoot the top of the stake, then drive it down with a sledge until it's at the right height.  Do this with each successive stake, then using a string line, a plumb bob, and batten boards, drive a pair of stakes in about every 24" or so (more or less depending how good your soil is) and nail the footer boards on the inside of the stakes - which you've placed just for that purpose, using one of the sides as a guide.  Once in place, you can reinforce the footer boards by putting nailers across the top to help keep them from spreading apart while pouring, as this crew did:

Strip footing formwork

For reinforcement, mine has two #4 rebar (#4 means 4/8" or 1/2" diameter) running the length of the tooting.  An additional rebar can be added perpendicular to these every four feet or so, but I am fortunate enough to have soil with a great load carrying capacity, so it wasn't necessary.  The next step is to pour them, obviously...  but I wanted to show the photo below for another reason:

Pouring the footings

 Look at that nice, green lawn.  By the time the concrete was done - they had torn the living snot out of my yard.  I tried to keep them from doing too much damage, and I even roped off the area where my drain field was located to keep them from driving their 574 ton trucks over it - which could easily crush the pipe.  Or should I say did?  For some reason, I'm telling you - it was like trying to keep flies of of s__t.  I came home after bringing dad to a doctor's appointment to find they drove right over my little tape barrier like it wasn't even there.  Not only did this happen once, but it happened several times over the time they were there.  My drain field still works, but it's capacity has been cut about in half from what it was - no more "super" loads in the washer for us...  and a healthy repair bill someday when I do half to replace it.  The one guy I was most worried about - the kid with the skid hoe - was easily the most careful and professional and did by far the least damage of any of them.

Stem Walls

So after the footings were poured and allowed to dry for several day, the concrete guy's crew comes back and sets the forms for the stem walls:

Stem Walls

It was at this point that I went out back and measured them only to find out they had set them for an 8" wall and not a 6" wall, like was in the drawings.   The concrete guy wasn't to be found anywhere, this was entirely done by his crew...  Ugh.  Is it really that hard to do something I ask?  It's not like I was being disagreeable, or hard to get along with, or anything.   I bring it up to him, and he's like, well - we can tear it down and do it at 6", but then I'll have to schedule it for later because that crew's already on another job.  Jeez, can't these guys come up with something more original?  In the end, I said screw it, let it be 8".  I'll just deal with it later, and let it remind me of why I should never hire bozos like this guy again (yet I don't even listen to myself - more on that in a later entry in this series).

Reinforcement was just as described in the detail above - a #4 rebar every 4'-0" vertically, and a #4 within 6" of both the top and bottom of the wall.  Anchor bolts were placed every 6'-0" and within 12" of each end of each wall.

 After they poured the stem walls, the kid with the skid hoe came back and backfilled around the stem walls, stripped the topsoil in the center, and no - I don't know why he did it that way, but it didn't take him all that long to get the sod out and down to good soil, so what the heck?  As long as it got done right, I wasn't concerned.  There wasn't enough good fill to use on the inside of the walls to support the slab, so I ordered structural fill brought in to bring the sub-base up to grade.  They brought in a type of fill that doesn't require compaction, which I think is interesting - it's a product I was familiar with but had never seen it done before, a material called CLSM (Controlled Low-Strength Material).  It looks mostly like really sloppy wet sand, and it is brought in in a concrete truck and poured out and leveled almost like concrete. 

It's usually made up of a mixture of portland cement, fly ash, and aggregate mixed with water.  Lots of labor saved in that you don't have to compact it, and it can be put in place so very quickly...  I did need 2-1/2 truckloads of it, each costing $200 though (I ended up having to buy 3 truckloads full)  I had the remainder put out so I could use it as fill for under the driveway slab that would eventually be placed out front of the shop.


So, I think in my case, I paid dearly for it.  But at least it was ready for the floor.  I did have one problem - there was a lot of soil left over in a big pile - sod, rocks, organic soil - pure garbage to me and I had nowhere to put it.  The kid with the backhoe offered to take it - seems he was filling some ditch on his property out in the back woods, and could put it there. He loaded it and hauled it out himself, so there was some consolation. I rewarded him by getting him to put in a gravel driveway back to the shop and to redo the one out front.  He did a particularly splendid job, for a fair price.

For more information on CSLM, visit the Portland Cement Association's web site.

The Floor Slab

So - another appointment for dad, and I knew they were going to pour the floor - when I got back, this was the scene:

Pouring the slab

It was a good thing I got back when I did...  I noticed two things were completely wrong.  First - there was no insulation laid down around the perimeter.  Second - he hadn't dug out for the two pier footings I would need for the two columns I had designed for the structure.  You know, I don't think he actually ever looked at the drawings...  Guess there might have been too much information there for him to handle.  Anyway, it was already too late for the insulation, but I did at least get him before the concrete had reached where the pier footings were to go - and stood there while he dug them out and made sure they were the right size, even as the concrete was flowing closer to the pit.  Essentially, the pier footings can be poured as a part of the floor, like a monolithic slab would be - so it's fine if done that way.  It would have sucked if he hadn't done them though - I wouldn't have known about it, and would have wondered why the floor slab cracked so badly around the columns later...

The finished floor

At long last - I had a foundation on which to build my shop.   Woohoo!  The rest would be all up to me and me alone.


Up next - walls!