A Special Slab Combining Floor and Foundation

If your car lacks shelter, if your basement is cluttered with tools and garden equipment, or if someone in the family needs a studio or workshop that will not fit in the house, the best solution may be a large outbuilding that can serve almost any purpose except as living quarters. With the help of a few ingenious professional techniques, you can set up a handsome structure that will make life easier for you and increase the value of your property. A turned-down slab eliminates the need for costly forms or masonry skills; precut stud walls can be built on the ground and erected as a unit; and prefabricated trusses , which require no tricky rafter cuts or ridge beam, make the installation of a sloping roof a simple assembly process.

When planning a substantial structure of this type, consider the requirements that its use will create. A garage needs a driveway route, walkways, a turnaround and perhaps a side door as well as a main entrance. If you plan to store garden tools in it, you will want a door that provides easy access to the garden. A studio or shop should have windows, and you may want a loft for storage. If you will need electricity and water, consult your local utility companies about preparing the structure for their installation.

Draw a floor plan of the building on a map of your property to make sure it's located at least as far from the house and the property lines as the local code requires. Then sketch front and side views of the building on graph paper to determine whether the building will harmonize with the main house and surrounding property. You may wish to plan the roof pitch and overhang of a new structure to match those of the house or of other outbuildings. If you suspect that the building may interfere with a prized view or block sunlight essential to the well-being of a garden, you can check by hanging a series of sheets on lines stretched between poles to create the effect on your property that the building will have.

You will probably need a building permit. Bring along your diagrams and sketches and be prepared to describe the type of foundation, wall construction and roof design that you propose. Some localities require a test of the soil to determine whether it will support the proposed structure. And set up an inspection schedule well in advance so that work will not be delayed.

Make a list of materials needed for each of the three stages of the job—foundation and slab, stud-wall framing and roof framing—and arrange delivery times that will mesh with your working schedule. If you order all the materials at once for staggered deliveries, you can usually get a contractor’s discount—which may sometimes reduce the cost of the building materials 10 to 20 percent.

Ready-made supports for a roof. Prefabricated trusses—the ends of a typical design—can be ordered to suit almost any pitch or overhang you like. The light but strong skeleton of 2-by-4s held together by metal or plywood gussets take the place of rafters and joists. The trusses are delivered to the job site ready to be lifted into place—an easy three-man job—and secured to the walls of the structure with metal anchors.

A Special Slab Combining Floor and Foundation

For a sizable building, the simplest base is a turned-down slab—a concrete slab, cast as a unit around a skeleton of rein forcing steel and wire mesh, with its edges turned down into trenches. The turned-down rim supports both the slab and the walls above it, and protects the slab from frost heaving. The builder need not pour conventional footings or lay a foundation wall of concrete block, since both footings and foundation are pro vided by the rim.

This slab’s most distinctive feature, its turned-down rim, is also its main limitation. The rim requires trenches not much wider than the blade of a shovel, but deep enough to meet footing requirements for your area. Digging a trench that meets these specifications may be impossible in regions where frost penetrates deeper than 2 ft.. Even digging to 16”, the minimum necessary for strength, may be difficult in sandy soil. If your local building department advises against a turned-down slab, use a deep concrete footing and block wall similar to those described.

In all likelihood, you will need the ad vice and consent of the building department on other aspects of the slab and the building that rises above it. Most com munities demand a building permit for any structure large enough to need a turned-down slab, and require that out buildings be located a minimum distance from property lines and from the main building. Generally, before you under take to pour a large slab, you must submit to your building inspector a rough map of your property, showing the main building and proposed addition.

To prepare for construction, plan an expedient route for the heavy trucks that will be bringing in building materials and concrete. Make the delivery spot as close to your building site as possible.

On the job itself you can save time and work more effectively with rented professional tools. A transit level enables you to establish lines and angles quickly and accurately. A power tamper will help you compact the soil before the concrete is poured; a power troweler will speed the job of finishing the concrete. You will also need a cutter and a bender for re bar—steel reinforcing bars— and a 50-ft. tape measure.

The slab has a flat floor. To make a drainage slope for such buildings as garages, use forms at the door openings to form a slope of 1” in every 8 ft. from the back wall. String lines and drive grade spikes between the door forms and the back and side walls to shape a drainage trough that channels water to flow out of the doors.

107

Laying Out the Forms

1 Finding the building lines. Drive stakes to mark the two corners at one side of the slab. Use a transit level to set a third stake at a right angle; string a line to this stake from the stake beneath the transit, and measure along the line with a tape to locate the third corner. Drive a stake there, set the transit above it and repeat the operation to find the fourth corner. Run a boundary of string about 4” off the ground around all the corner stakes.

2 Building batter boards. Drive three 2-by-4 stakes about 2 ft. outside the strings at each corner and nail 1-by-6 boards, each 5 ft. long, to the stakes to form a right angle; set the tops of the boards about 10” above the ground. (A sledge hammer or a large brick makes a solid backing for nailing.) Drive a 1-by-2 sighting stake a few inches outside each set of batter boards so that the sighting stakes stand slightly higher than the batter boards.

3 Leveling the batter boards. Place the transit at the center of the building site and have a helper hold a 6-ft. ruler next to one of the sighting stakes outside the batter boards. Level the transit, sight through it at the ruler and note the measurement seen—in this example, 58”. Subtract 8” from the reading. At each sighting stake, have your helper raise the ruler until you see this calculated figure in the transit, then mark the sighting stake at the bottom of the ruler. Drive the batter-board stakes down until the board tops meet the marks on the sighting stakes; lay a level on the board tops to see that they are even. String lines between the batter boards directly above the lines that run between the corner stakes (Step 1). Remove the corner stakes and strings, and grade the area between the batter-board strings to a flat surface 8” lower than the boards. Tamp the soil thoroughly.

Building Turned-down Slabs

108a 1. Placing level forms. For each section of forms, nail three 16” 2-by-4 stakes to an 8- ft. length of 2-by-8. Set the inner face of the forms along the string lines and drive the stakes into the ground until the top of each form is level with the string line. Nail the forms together at the corners and nail an 8-by-12” ply wood backing between stakes at the points where the form boards butt together.

108b 2. Setting grade stakes. String lines between the back- and front-wall form boards every 4 ft.. Then drive 3/8” reinforcing-bar spikes along the strings at 4-ft. intervals, with the tops of the metal spikes level with the strings.

109a 3. Bracing the forms. Drive a 2-by-4 bracing stake a ft. behind every form-board stake. Nail two 1-by-2 braces between each bracing stake and form-board stake, one on the ground, the other diagonally. To prevent concrete from leaking out between the forms and the ground, shovel a layer of dirt or gravel behind the forms.

109b 4. Digging the trenches. Along the inner face of each form board dig a trench 12” wide and as deep as is required for footings in your area. Bevel 5” from each inner edge.

Every 4 ft. along the trench bottom, drive a pair of ½” reinforcing-bar spikes about 5” apart into the trench bottom, with their tops about 5” below the form top. Set a brick flat on the ground next to every pair, and lay two lengths of horizontal reinforcing bar on the bricks against the vertical reinforcing bars.

110a 5. Interlocking the steel. Wire the horizontal reinforcing bars to the vertical ones about an inch from the tops, and wire the horizontal bars together at the trench corners.

Spread a 3” layer of clean 1” gravel over the main part of the building site—do not let any gravel fall into the trenches—and cover the gravel with sheets of 4-mil polyethylene. Over these sheets lay down 6 x 6-10/ 10 wire mesh ex tending almost to the form boards, and wire the edges of the mesh to the reinforcing steel.

Prepare sole plates of pressurized lumber for installation (Step 7) by drilling %” holes every 5 ft. and 1 ft. from each end of a 12-ft. length of 2-by-4. Prepare enough sole plates to extend around the rim of the slab.

6. Filling the forms with concrete. Enlisting several helpers, pour and spread the concrete, tamp it into the trenches with a piece of 2-by-4 and , using a rake, pull the wire mesh a couple of” up into the slab. Level the surface with a screed of 2-by-4s, making the concrete even with the tops of the form boards and the grade spikes. When it's level, drive the spikes below the surface of the concrete. Finally, smooth the concrete with a darby.

7. Installing the sole plates. Run ½” anchor bolts 12” long through the holes in the sole plates, fit each threaded bolt end with a washer and a nut and set the plates on the wet concrete, ¾” in from the form boards. Work the bolts down into the concrete, tapping the threaded ends with a hammer if necessary. Make sure no bolts are installed in door openings.

111a 8. Finishing the slab. When the concrete is hard enough to walk on but still visibly damp, finish the surface with a power troweler. Set the blades flat against the surface and run the machine over the slab, then readjust the blades to angle up ward and smooth the surface by running the ma chine over it again.

Sprinkle the concrete with water, cover it with sheets of polyethylene and let it cure for 3 days or more before you remove the forms.

= = =

Grade Beams: Stiffeners for a Concrete Floor

111b Concrete is a strong building material, but if a slab is poured on marshy soil, clay or a site that has been leveled with fill, it may settle and eventually crack. When building over poorly compacted soil, many professional masons reinforce a slab with grade beams—trenches filled with concrete and reinforced with steel. The beams serve much the same function as the wooden beams that support the floor of a house.

The grade beams shown at right are the type generally used with a turned-down slab. They are poured in trenches 8” wide and 8” deep, spaced every 4 ft. between the slab-rim trenches. Half” reinforcing rods are laid over bricks set on gravel, then wired to the reinforcing bars in the rim trenches. Then another layer of bricks and two more reinforcing bars are set in and the slab is poured as a unit.