foundation questions

[woodydiver]

is anyone have any experience on digging and pouring a foundation. i am building a 24 by 28 foot gargage/workshop. I am pretty sure on how i want to do the walls and trusses, but any help with the concrete and footers would be greatly appreciated.

[JimmyC]

The only experience that I had was in NJ. I built a 18'x36' outbuilding. NJ code wouldn't let me build a floating slab that big, so we had to go down to frostline for the walls (36"). One way was to dig a trench deep enough and lay block, we used a backhoe and dug 36" then just filled it with concrete which brought it to ground level. Then concrete block was layed for 3 courses, and a floor was poured.

You might want to wait, or PM, Travis Porter or toolferone, they both built shops down here and might be able to help you.

Lots of luck,

Jimmy

[toolferone]

My footers are 12" wide and 12" deep (check your local code). My area was pretty flat so I poured a monolithic slab, meaning the slab and footers are poured together. If you have much of a grade it is better (cheaper) to pour footers and build up with block and pour the slab inside of it. Click the pic for more pictures!

[walnutjerry]

Originally Posted by woodydiver

is anyone have any experience on digging and pouring a foundation. i am building a 24 by 28 foot gargage/workshop. I am pretty sure on how i want to do the walls and trusses, but any help with the concrete and footers would be greatly appreciated.

I would check with the building inspectors office if inspection is required in SC. Requirements may vary with the type of building going up, soil your building on etc.

Jerry

[DIYGUY]

Around my neck of the woods most of the soil is clay. Standard way of doing a foundation is a block wall over a footing and then leaving an earthen crawl space. This won't work for a shop with a floor of course, but some parts of the technique may work for you, depending on the soil. Clay has only two states - it is either hard or soft. When it is hard you can dig a trench and use the ditch as a form - no need for anything else. Just pour your footings and let them set up. Then you can put your concrete block on the footing. The usual depth here is about 2' or so, seldom deeper, occasionally a tad or so if that. The footing should be about a foot, check with your code as previously mentioned.

Pouring a slab is best left to a professional in my opinion - you will be glad you did. They not only have the tools (power floats, bull floats, etc) but also the expertise. You should also pay attention to the concrete mix you order. Your sub will help you with that. It might be a good idea to use both rebar and fiber in the mix. Also, have you thought about dust collection and power? Lay out your shop and consider putting PVC pipe under your slab as a nice way of moving sawdust. Another thought - why not put a small outbuilding on the back of the shop to house your dust collector? That way you don't have to listen to it when it is running and changing the bag won't contaminate the shop.

Well, that's my thought for the day - now I must relax ...

[JimmyC]

As far as electrical and dust collection goes, you might want to put an electrical conduit on inground DC in the middle of the floor for the tablesaw. I wish my shop had it.

Before the slab is poured, think about conduits for electrical, cable, phone, water, sewage, and DC, if you are going to use any these in your shop.

Good Luck,
Jimmy

[Gotcha6]

Be sure to remove any organics from the subgrade. You can do the monolithic yourself if you get some help placing it and rent a power trowel, but for the best finish a professional should be used. 600 square feet is considered the upper limit of what one man can finish with hand tools alone. Concrete has a mind of its own when it goes to set and the hardest part about finishing it is knowing when NOT to work it. Also, use some rebar in the perimeter 3" off the bottom and reinforcing wire in the slab 1" off the bottom. The rebar will hold the foundation together and the wire will help control shrinkage cracks. Wire is recommended for any slab greater than 15 x 15. Concrete will crack. Count on it. Use control joints if necessary. Also use anchor bolts for the framing sills. 1/2" x 12" spaced 4' apart should do. Leave them out of the concrete 2 - 2-1/2". Your local inspector may also require a sloped slab if you have a garage door or any door big enough to park a car or lawn mower in the building so as to allow spilled liquids (gas) to drain out. Good luck.

[Nativespec]

I did one for a 1.5 car garage (1 car and workspace/office). I had a neighbor and some friends help. It was a monolithic slab (a 4" slab with footers about 18" deep). As the previous poster mentioned, use rebar in the footers and wire mesh to help with cracks and place your bolts in while you can.-the steel is actually for temp. control in the slab. Also and luckily, my neighbor had done a lot of this in his lifetime, and he had two steel pipes he would lay in the concrete to use as a guide for the scree and take them out, move them over and do it again. You will need a bull float on a long handle and some hand floats. Also, you have to order just enough concrete (I had about a wheel barrow amount left over) because the concrete companies do not carry any back with them. I would also reccomend tall rubber boots because concrete will take your skin off.

And finally, I will never pour a slab (of any meaningful size) again. It is hard work and I had no experience and if it had not been for the neighbor, the truck driver and a friend (all had experience), it would have been a disaster.

Thanks,

David

[Bernhard]

My house (74x50)is build on a monolithic sandwich slab and my garage/workshop (30x55) is build with strip foundation/concrete block and poured slab. I build both within the last 3 yrs.
All the hints, tips and cautions listed so far are right on the money.
Depending on your location and use, you may have to get a building permit and all the associated inspections.
If you have a level area, a monolithic slabis the most cost effective and effcient way. Tom's picture is a nice and neat illustration.
General surface rep needs to include removing topsoil and any organic matter, grading, compacting (if fill was used), 2" crushed stone layer for capillary break, vapor barrier. I would use 2 # 4 rebar on chairs in the thickend part of the edge of the slab froming a ring beam. Webing/rebar on the area does not increase strenght only prevents slab separation; I prefer to use fiber-re-enforced concrete.
With a 20x24 building, you want to use a rotary laser to level your form boards.
You may want to consider placing conduiy, water line, or even radiant floor heating under the slab. Now is the time to do that. Even if you are not planning to use it all, it is cheap and easy to do it now!
That area will require about 6.5 yards. Is the site accessable to the truck? If the chute does not reach the slab, you will need at least 4 wheelbarrows and 5 people. Remember you have 90 min after the concrete is mixed before it starts to set up. Do NOT under any circumstance add water to the concrete at the site! this will weaken the the concrete. Specify the mixture with about a 4" slump. You may want to order the mix with a retarder (to give you amble time to finish) and a super plastizer/water reducer (making the concrete still fluid without adding extra water). Prepare a little area for overflow (i.e. do you need a little pad of concrete for something else?), leave a little concrete in wheelbarrow until the slab is screeded....just in case.
Tools needed: concrete rake, bull float, shovel, wheelbarrow, edging tool, trowels, powertrowel , screed board, boots, rubber glaves, running water, portable lights in case you running behind (finishing concrete in the dark by candle light is not fun).
Place the concrete slightly higher than the form board and have 2 people screed to level as you fill the forms. A third person can start bull floating the slab after the screed. Floating will push the aggregate below the surface and bring the 'cream' to the top for a smooth, hard finish. once all bleed water is evaporated, you can start troweling, this does take some practise. In case you rent a powertrowel test it out on a lawn (When I did my workshop, the rented power trowel had a bent float paddle and couldn't be used: But concrete is very unforgiving, it doesn't wait for you. I and 3 other guys finished the floor by hand...we finished at 2 am).
The powertrowel should not sink and the concrete should just supoort your weight without sinking, just an impression of your foot print.
You may want to palce foundation achors in the wet concrete to attache your stud walls.
Curing the concrete is also very important. Contrary to popular belief, concrete does not dry, but forms hydrates. So it is very important to keep the concrete wet during curing. Ponding is best but mainly impractible. Springling water on the finished slab and covering it with plastic works well, too. Remover the plastic every day and hose down the slab to keep it wet for about 4-5 days.
Oh, one more thing: Control joint: Concrete will shrink as it cures. So you need to saw control joints. Ideally you want square areas, but with 24x20, dived the slab in 4 quadrants. You can place concrete control joints in the the wet concrete somewhere befor troweling, or form the joint or cut it the day after placing the concrete. I prefer the placing the control joints befor troweling. Any concrete supply house will sell you these nifty little t-shaped sticks. There placed in the concrete and after curing, the top of the T is riped off and discarded and underneath is aneat joint.
My preference is stay away from spray curing membranes and such: No paint or epoxy will stick to it (in case you plan to finish the floor later).

And last but not least: It is doable, just takes some prep work and at least one person that has done it before.

Cheers,
Bernhard

[scsmith42]

There are a lot of great responses in this string; in particular Bernhard's response is spot on.

A few observations. First, the ultimate strength in concrete is primarily dependant upon the amount of cement used per yard, and the amount of water per bag of cement. Usually a 4 or 5 bag per yard mix will result in around 4000 psi concrete at 28 days. However, the more cement, the faster it sets up so you have to take this into account as well. Use of the retarder admixture referred to above will help here.

The folks who are working the cement typically will want it to be "wet", ie a low slump / high water content to ease their job. DON'T DO IT! Every gallon of water added will reduce the ultimate strength and abraision resistance, and increase your liklihood of long-term cracking. I like to specify a 3.5" - 4.5" slump at the site, and a 4000 psi strength at 28 days to the concrete company.

Ask the concrete company to provide you with some plastic cylinders to for use as test samples, and then fill them first when the truck arrives (place 8" of concrete in the cylinder, and then rod it 28 times with a 5/8" diameter steel rod, then place another 8", etc). Even if you don't have the cylinders tested, it will send a message to the concrete company that you are serious about your concrete and they usually take extra steps to make sure that the mix is correct - else they could have to pay the cost to break it up and replace it.

Cement has a maximum working limit - it's typically 1.5 hours from the time it was mixed at the batch plant. You need to be ready when the trucks arrive and move fast to get it placed before it starts setting up. Ideally, you want it all placed before it's 1 hour old.

A vapor barrier underneath the slab is important for two reasons. First, long term is helps prevent moisture from wicking up throug the slab. Second. it helps keep the moisture in the slab while it's hydrating, and prevent it from curing too quickly.

Wet concret is HEAVY. All above-grade forms need to be well supported - typically a 2" thick form is supported every 18" - 24". Inadequately supported forms CAN AND WILL blow out on you when your pouring, and if this occurs... it sucks the big one...

If you decide to go with radiant heating in the slab, make a drawing as to where your tubing is placed and then place the tubing so that it doesn't move much when pouring. Use that drawing as a reference long term so that you know where the tubing is in the even that you ever want to drill into the slab for machine anchors, etc.

Also, if you plan on using radiant heating consider using Tekfoil as your vapor barrier. It will increase the efficiency of your radiant heating significantly. (www.farmtek.com)

Get rid of all topsoil and roots that would be underneath the slab. Be sure that all remaining material is very well compacted.

And, from a planning perspective, make sure that when all is said and done and your building complete, that you have good drainage for at least 5 away from the building in any direction.

Plan on renting a concrete vibrator for use during your pour. Do not over-vibrate (can separate the rock from the sand/cement and weaken the slab). Typically I will vibrate in the same place for 1 - 2 seconds and then move on.

Re ordering concrete, it's better to have extra than not enough! Ditto the comments about having alternate projects prepared to accept any extra (such as sidewalks, stoops, etc).

Placing concrete is hard work, and if you choose to do it yourself you will want several people on hand to assist (I would recommend at least 5 for a slab your size), and a good quantity of shovels, wheel barrows, and the finishing equipment. Plan on having a serious backache for a few days afterward!

Scott

[Nativespec]

Very important-fiber reinforced concrete is not a substitute for wire mesh. The wire transfers the changes in temp-the fiberglass mixed in with concrete cannot do this. If you use the fiber reinforcement, I would advise using it in addition to the wire.

David

[junquecol]

Having poured thousands of cubic yards of CONCRETE in a previous life, I agree with most of the coments made here with a couple of exceptions. A 3000 pound mix is more than adequate. It is often refered to as a "six bag mix". This means that six bags (one bag equills to 94#. or one cubic foot) of cement was used to make mix. With currently available adimixtures, less cement is required. Concrete companies are basically honest on strength of mix. They can't afford to not be. As for sloping garage floor, code doesn't require it. It says floor shall be sloped, so as to prevent entry of liquids into adjoining living surfaces. Most inspectors only read first part of statement. Exactly how deep will the water have to be before it goes into the house? I think the hydraulic pressure will blow the doors out first. If inspector gives you a hassle on this point, go to Dept of Insurance as they regulate the building code. Pour a level floor, with a stepdown of at least 3/4" where the doors are. This will keep rain from blowing under roll up doors. If you run block up from the footing, you can use header block for the top course. This way you don't have to mess with expansion material, which should be called contraction material. A slab is at it's largest during the heat of hydration. It actually gets smaller as it sets. Remember that cement, Portland Cement to be exact, is used along with course and fine agrigates (stone and sand), water, and other adimixtures to make CONCRETE

[Mark Anderson]

from R309.3 (pg 47 norht carolina residential code)

"The area used for parking of automobiles or otther vehicles shall be sloped to facilitate the monement of liquids to a drain or toward the main veicle entry door."

the loop hole being it dose nto say how much. i wnet 1/2" in 36', withc is level for all practical purposses, yet it dose slope to the door.

also although the code book dose mention a "drain" i am prety sure this is agianst federal law, due to posible ground water polution.

[Nativespec]

It is also to keep flamables from entering the house and to drain to the outside. I had no Idea that we could not use a drain-I would like one in the kitchen.

I am pretty sure you will have to slope your garage. The code not only protects ourselves and family, but protects future users/owners of the property. The problem you could run into is a pain in the rear home inspector working for the buyer of your home sometime in the future(assuming you are not getting a permit to build your building and having to build to code).

David

[Gotcha6]

The part about the drain says sloped OR drain. If a drain is used it then comes under the plumbing code and must incorporate a separator to prevent oil contaminating your septic or sewer system. Dumping it out on the ground is taboo too. The key part about the slope issue is the USE. If the area is not to be used to park 'automobiles or other vehicles' then it is not required. If it is, other items also come into play such as ignition sources within 18" of the ground (explosion hazard area if your lawn mower leaks gas) such as compressor contactors, well contactors, gas heaters, etc.
Just leave it as a woodworking shop & forget about the slope. Put in a pair of 3-0 doors and they can't claim you'll be parking a car in it. You should be able to get most woodworking projects out that unless you're into boats.
Minimal slope if you do put it should be 1/16" per foot. Few slabs are poured closed enough to grade to avoid puddling at slopes less than this. 1/8" per foot would be better.