SLAB FOUNDATIONS

Perhaps the least expensive and labor intensive foundations is the cement slab. There is not a lot of digging involved and much less materials are needed. One thing to keep in mind, is that the slab sit on higher ground to prevent water seepage caused by heavy rains on the surface of the slab itself. But lets talk about the layers of a good slab foundation.

Ground Preparation: If the ground surface is even, uneven or below the grade that surrounds it, the land beneath the slab must be built-up higher than the dirt that surrounds it. This will prevent problems of flooding. It should also be an evenly level surface that is tamped down. Next is to dig trenches for sewer and water pipe, HVAC ductwork and electrical wiring and cable.

Forms: Mark the slab foundation with slab forms and string, marking the ground below with spray-paint, lime powder or flour. If digging by hand, its best to mark it after the perimeter forms are in, if by backhoe, mark the perimeter initially, waiting until after the trenching to put up the form boards. Make sure the forms are level and braced. Forms need to be at least 8" above grade. The inside face of each form should be under the perimeter string. The depth of the footing trench must meet the requirements of the local building codes.

Screeds & Guides: Cement slabs must be "struck-off" or even with the top of the forms using a long board called a screed. Keep in mind, any length longer than 20' (such as for a house or garage) will require the use of permanent dividers made of treated or rot-resistant lumber or installation of temporary screed guides to both level off the the gravel base and strike off the slab.

Gravel Fill: The size and type and depth of fill must be done to local building code regulations. Use a gravel rake to even fill four inches under the top of the form. For thicker slabs, adjust accordingly. When using a rake, measure down from the twine. Keep in mind, where allowed, more and more builders are opting to line the inner and exterior perimeter with rigid board insulation - ask your building inspector about this technique.

Rigid Board Insulation: Cement is well known to draw body heat, making you feel colder when walking across a floor, bare footed, or even with shoes. More builders are offering to put rigid board insulation like XEPS (ExTruded Polystyrene) over the gravel fill which is waterproof and can be used below grade. The thicker (within reason) the insulation, the better! Also an insulated floor will help cut down on heating costs.

Vapor Barrier: Vapor barriers are essential to prevent ground moisture from leaching through the concrete slab. Without it, carpets would get wet, mildew and yucky stuff like forming mold - essentially, a dry floor is a better floor! The polyethylene sheeting should be at least 6-mils or thicker, spread over the rigid board insulation and overlap by at least a foot - do not puncture or tear the plastic sheeting. Over the sheeting should be a layer of sand.

Rebar: Steel reinforcing bars known as "Rebar" must be planted in the footing trench in two levels - 3" above the bottom of the footer and 2" below the surface of the finished slab. Where rebar's' meet, a 24" overlap must exist for a 0.5" bar and 30" for a 5/8" bar. Use tie wire to join the rebar together. Place bottom of bars on dobies resting on the bottom of the trench. Wire to rebar's hung via nails in the form. Rebar must be 2" from the form boards and 3" from the sides of the trench.

Reinforcing Mesh: Reinforcing steel mesh is to be installed 2" above the vapor barrier and two inches below the surface of a 4" slab. Use 2" dobies to support the mesh. If you need more than one length of mesh, additional ones should be wired together. Plan for connection between slab and footing in climates requiring deeper footings.

Concrete Slab: House & Cottage encourages you to use a cement mixed with fiberglass strands which can eliminate the need for steel reinforcing mesh - ask your building inspector if it meets code in your municipality. Before you can pour, the building inspector will need to approve the work on the form. At the time of the pour, its a good idea to have two wheelbarrows and four or more helpers on hand. Initially, pour the cement in an area farthest from the truck and working back towards the truck. Be careful not to allow the cement to push the reinforcing rods out of alignment. Pour the cement in sections. As each section is finished, have it struck off with a screed. As work progresses, have a helper rap the sides of the form boards with a hammer. After the pour, strike off the cement and let it begin to set. In hot weather, its important to begin smoothing off the surface with a bull float if it begins to harden. Insert anchor bolts for the mudsill after striking and smoothing. After surface starts to harden, use a bull float for initial smoothing, using knee and toe boards while finishing with a wood float, followed by a steel float. Finish up by using a trowel to cut between the batter board and the cement. Let slab cure for two days, covering it with a sheet of plastic with edges and seams weighed down to trap moisture.