Building the Sun Cottage

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

SLAB

The first course of block (it's called the foundation wall) will provide an excellent form for the slab pour, so you can go ahead and lay one row down over the starter bars and into a mortar bed. (Take this opportunity to correct any unevenness in the footing by adjusting the thickness of the mortar bed to insure that the walls will be square, straight, and level.) If there's any chance that water may become trapped underneath the slab, leave out the mortar in the vertical joints on the front foundation row. That way, water can flow out from beneath the building and won't develop pressure under the slab.

Concrete post pedestals should now be positioned. These may consist of groutfilled eight-inch-square blocks slipped down over the 3/8" steel bars that stick up and serve as dowels (see Fig. 4).

At this point, larger subfloor services (such as warm-air ducts and waste pipes) should be roughed in. Use sleeves made from a larger-diameter pipe to prevent damage from settlement forces at the points where these runs must pass through the foundation walls. To accurately position the plumbing lines, mark the partition locations on the perimeter string lines, and use additional strings to cross over pipe centers. Cap these openings temporarily to prevent dirt or concrete from getting in them.

You'll need to create a porous layer beneath the slab ... both to provide drainage (thereby preventing groundwater from exerting pressure on the underside of the concrete) and to form a cushion that will prevent the cracks that could be caused by unequal bearing pressure during the slab's settlement. To form this "base course", spread 4 "(or 12" if your footings are set on top of the excavation) of gravel or riverbed sand inside the foundation walls. Smooth the surface of this layer and level it ... referring to chalk lines snapped halfway up the block foundation course.

Once the base course is prepared, lay down heavy-duty polyethylene to form a vapor barrier. Overlap the joints between separate sheets by at least six inches, and turn the plastic up at the walls and trim, securing it temporarily with a bead of mastic or sealant just below floor level. Keep in mind as you're positioning the poly, that you need to leave some slack so that the sheets won't be stretched when the concrete is poured.

Now, secure the vapor barrier to the walls by nailing 4"-wide construction—joint filler to the block with concrete nails. Be certain that the tops of these strips are level with the floor, because they'll be used when you "screed" (even out and smooth) the slab. Note too, that this construction joint is particularly important in our passive solar design, since it will allow the slab to expand under the heat of the sun without putting lateral pressure on the walls. (It also insures that the slab won't hang up along the edges and crack during settlement.)

The slab reinforcement consists of No. 10-10, 6" X 6" welded-wire mesh suspended two inches above the vapor barrier on brick fragments, stones, or hardware called "high chairs" (see Fig. 5). Be very careful when you're unrolling the wire ... it's all too easy to tear the vapor barrier or cut yourself on the springy, sharp mesh. This is a nasty job, and you should wear gloves an goggles while doing it.

RSS | Comments | E-mail | Print