Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.19
should slope away from the floor. The level of the finished floor should be at least
6 in above grade. Further protection against ground moisture and possible flooding
of the slab from heavy surface runoffs may be obtained with subsurface drains
located at the elevation of the wall footings.
All organic material and topsoil of poor bearing value should be removed in
preparation of the subgrade, which should have a uniform bearing value to prevent
unequal settlement of the floor slab. Backfill should be tamped and compacted in
layers not exceeding 6 in in depth.
Where the subgrade is well-drained, as where subsurface drains are used or are
unnecessary, floor slabs of residences should be insulated either by placing a granular
fill over the subgrade or by use of a lightweight-aggregate concrete slab covered
with a wearing surface of gravel or stone concrete. The granular fill, if used, should
have a minimum thickness of 5 in and may consist of coarse slag, gravel, or crushed
stone, preferably of 1-in minimum size. A layer of 3-, 4-, or 6-in-thick hollow
masonry building units is preferred to gravel fill for insulation and provides a
smooth, level bearing surface.
Moisture from the ground may be absorbed by the floor slab. Floor coverings,
such as oil-base paints, linoleum, and asphalt tile, acting as a vapor barrier over
the slab, may be damaged as a result. If such floor coverings are used and where
a complete barrier against the rise of moisture from the ground is desired, a twoply
bituminous membrane or other waterproofing material should be placed beneath
the slab and over the insulating concrete or granular fill (Fig. 3.8). The top of the
lightweight-aggregate concrete, if used, should be troweled or brushed to a smooth
level surface for the membrane. The top of the granular fill should be covered with
a grout coating, similarly finished. (The grout coat, 1 ⁄2 to 1 in thick, may consist
of a 1:3 or a 1:4 mix by volume of portland cement and sand. Some 3 ⁄8- or 1 ⁄2-in
maximum-sized coarse aggregate may be added to the grout if desired.) After the
top surface of the insulating concrete or grout coating has hardened and dried, it
should be mopped with hot asphalt or coal-tar pitch and covered before cooling
with a lapped layer of 15-lb bituminous saturated felt. The first ply of felt then
should be mopped with hot bitumen and a second ply of felt laid and mopped on
its top surface. Care should be exercised not to puncture the membrane, which
FIGURE 3.8 Insulated concrete slab on ground with membrane dampproofing.