Even though, we here at BMB companies do not pour foundations or slabs, we are asked on a regular basis on how to do so. Usually, it is for one of our buildings, so we do our best to guide them through it. That being said, we always recommend that anyone should hire a certified contractor that specializes in slabs or foundations and should order Site Specific Engineered Drawing for the contractor to use as a guild. However, if you are not wanting to do that, then you may want to continue reading. Keep in mind that skipping any of those recommended steps may save you some money, but may get you into trouble.
First, this in not a instruction manual on how to pour a slab. This article will give you a few tips assuming you already know the basics, or you’ve hired someone to do the actual labor.
Before you break ground, or order any building, you may need to check with your local building codes. You may need to obtain a permit for the upcoming structure. If you do not need a permit, you can probably skip this section. Usually, you can call your city or county courthouse and they will direct you from there. They will tell you what their regulations are for a building and/or a slab or foundation. These codes may not be the same for everyone in one county or even for everyone in one city. Areas with major elevation differences set the codes based on actual address. Building codes will usually tell you that your footers (the outermost part of a slab usually supporting the weight of a structure) need to be set below the frost depth (usually over 18″ but is set by local municipality). The codes will also set a required Wind load, Snow load, and Live load requirements. Wind loads are usually defined as a 3 second wind gust that a structure should be able to withstand without taking damage (90 mph used to be a standard, but many municipalities are increasing requirements). Snow loads are usually referred to as “ground snow loads” and usually measured in pounds per square foot (lb per sq/ft). A rule of thumb is everyone gets 20 lb and if your area gets more snow that most than you probably will need to make sure your building is built to withstand more. Live loads describe the weight of a “live” person on the building. These are usually measured in the same way as snow loads.
“How do I prove that my building will meet the local wind, snow, and live loads,” you ask? A set of site specific engineered drawings will include specs on the materials in which the building will be made of, how they will be put together, how it will be attached to the foundation or ground, and the how the foundation needs to be built. The engineered drawings will have a state licensed engineer’s stamp of approval. This will tell the building inspector and the permitting office that your building is up to code. Of course, the building needs to built according to the drawings.
“How do I get a set of Drawings,” you ask? The easiest way is to ask the company that you’re buying your building from for information in. Most metal building companies will offer “certified buildings.” These are buildings that are pre-engineered for certain wind, snow, and live loads and come with “generic drawings”. In many cases these generic drawings will be enough to get a permit and pass inspection. Your local municipality will let you know if it will be enough before you have to pay anything to anyone. If the generics are not enough, the building company you are dealing will will probably act as the middleman for you and assist you in getting your site specific drawings. You will have a set mailed to you. Then you will need to take those to get any required permits before a building is started. If you’re doing everything yourself, then you will need to contact a state licensed engineer to do your drawings.
Picking a type of Slab or Foundation
There are 3 main foundation options to From which to choose. If your building is very large (like 40′ wide clear span), you will just have to go by drawings you purchased. The question: “How big do I need to make my slab?” is answered here as well.
1. Edge Offset – The slab size is equal to the outside dimensions of the building PLUS 5″ in each direction.
2. Notched Edge Offset – The same as Edge Offset, but the extra 5″ is 1.5″ lower or “Notched”.
3. Edge Flush – The slab size is equal to the outside dimensions of the building.
The Strongest slab option is the Edge offset, but it allows for water to flow under the bottom of your siding. The Edge Flush slab uses the least concrete therefore is the cheapest, and allows your siding to be installed so it overhangs the foundation, thus preventing water from flowing under the walls. The Notched Edge Offset is probably the most expensive, but it is both strong like the edge offset style and allows the siding to set below the top of the slab. The notched edge offset style is our most recommended option.
All 3 types of slabs will need a footer and a slab of at least 4″ thick. The footers depth needs to be the depth of the frost line or as per local code, but no less than 12″ deep. The width of the footer needs to be at least 12″ wide.
Here you can see illustration of all 3 styles of slabs for a BMB built Garage that is a 24′ wide by 30′ long (24×30).
Some of our customers will want to know where to place their anchors. That question is best answered by the engineered drawings. If they do not have drawings, then a good rule of thumb is at each end, AND next every other support post. However, most building companies will offer their own anchoring. And some, like Coast to Coast, add cement anchoring for free. They use self tapping expansion anchors, wedge anchors, or adhesive anchors.
Anchors need to be embedded at least 4″ into the slab, and need to be places no more that 6″ from support posts. All anchors need to be A307 equivalent or better.
The soil bearing Capacity needs to be a minimum of 1500 psf. This means if you are placing it in a soft or squishy area, or somewhere with a lot of sand, you may need to have to soil checked, and, if needed, do additional foundation work to prep the soil. Your local courthouse will most likely be able to guide you through this process.
Also, the concrete will need to be reinforced so that it has a minimum strength of 2500 psi at 28 days. Yes, this is the standard curing time for a foundation or slab. Some acceleration agents can be added to speed things up, however. Extra reinforcement should be in the form of re-bar with wire or fiber mesh.
If you do not need a permit and want to do your slab yourself we suggest Googling “how to pour a slab for a garage myself.”