This section is not meant to provide you with the necessary instruction to pour your garage foundation slab. This is not because it is beyond the abilities of the novice builder, although many do contract out this job. Pouring a slab is within most people's abilities. However, unlike wood framing, which can be corrected if improperly constructed, your work on the slab is "set in stone"—literally. Because of this and the fact that so many local codes and practices apply to concrete slabs, we have only touched on the subject.
Should you decide to undertake this project, we suggest you talk with local professionals to be sure you know what you are doing. Have a professional inspect the forms before the concrete is poured and hire a professional to work with you during the pour to be sure you get everything right the first time.
A slab foundation is made by building wooden forms and pouring the concrete inside. A footing trench is dug around the perimeter to provide added thickness along the edge of the slab which would otherwise be stress areas and the weakest point.
Sand or gravel is spread below where the concrete is poured so the under-slab area will not compact under the weight of the concrete and cause it to sink. Then, a vapor barrier is installed above the sand or gravel (in some areas below) to stop moisture from traveling up into the slab through capillary action. Wire mesh follows this layer, placed in the center of the slab, and rebar (steel rods), laid in in the footing areas, to add rigidity to the concrete to prevent cracking.
Metal foundation anchors or anchor bolts are embedded in the fresh concrete to secure the garage walls to the foundation. Many garage slabs also have a center drain for car or boat washing.
Local code will dictate such things as thickness of the slab, thickness of the footing, width of the footing, sizing of the wire mesh, thickness of the layer of gravel or sand, size and location of rebar, and location of bolts or foundation anchors.
Sill (Bottom) Plates
Most Common Mistakes
- Not checking to see if the slab foundation is square.
- Using badly bowed stock.
- Not leveling sill plates.
- Not cutting the top plate so that it breaks at the center of a stud.
The sill plates for a garage are usually 2x4 pressure treated or redwood horizontal framing members that are bolted or attached by foundation anchors or anchor bolts. These are also the sole or bottom plates of the stud garage wall. The purpose of the sill plate is to anchor the garage framing to the foundation, to level the foundation wall, and to keep the untreated framing members out of contact with the concrete where standing water could cause them to rot.
The metal foundation anchors or anchor bolts will have been encased in and should be protruding from the foundation after the concrete was poured. These devices will anchor the sill plate, and thereby the wall, to the foundation. Local code will specify placement of sill anchors or bolts. Usually foundation anchors are embedded a minimum of six inches into the concrete with the bend anchors over the bottom plate and nailed into a secure upper position, split into 5/8-inch wide tabs which will strap over the 2x4 sill plates. These upper tabs have holes to accommodate special fastener nails 1 1/4-inch in length. Two nails are driven into each side of the sill plate through the holes in the tabs. The upper ends of the tab are then bent over the plate and nailed into the top of the sill with a minimum of one nail on each side.
You can choose the order in which you will build your walls, but we suggest you begin with a wall that has no windows or doors, if possible.
Select your sill plate stock so that it is as straight as possible. When ordering, specify good straight plate stock 12 to 14 feet long. If it is longer than this, it is likely to have some warp or bow. If the walls are longer than 14 feet, you will want to use more than one piece of stock. Do not use pieces under six feet in length if possible.
Note that bottom plate splices may be anywhere but top plate splices may only occur in the center of a stud to ensure the strength of the top plate. Splices in top and bottom plates must be separated by a minimum of four feet on the horizontal and splices in the cap plate (the uppermost of the two top plates) must be at least four feet from any splice in the top plate. Before you choose your sill or bottom plates, again check your local codes to see if redwood or pressure treated stock is required or preferred. Place them with your wall top plates to do the wall layout. The entire wall is built first, then lifted into place before the sill plates are attached to the slab.
The total length of each plate equals the total outside length of the walls minus 3 1/2 inches (the allowance for walls overlapping at the corners). The end of one wall stops at the edge of the platform on one side while the other end stops short by 3 1/2 inches and is overlapped by the intersecting wall which also goes to the end of the platform.