Page 229 - Construction Waterproofing Handbook

4.4 CHAPTER FOUR
BELOW-GRADE WATERPROOFING/BASEMENTS

Chapter 2 presents all the appropriate techniques for adequately waterproofing the most
common below-grade residential room—the basement. All too often residential contractors
will substitute damproofing for waterproofing in their choice of products and installation
techniques because since very few of the waterproofing systems presented in Chap. 2 are
readily available at any local building supply store used by most homebuilders.
Damproofing products are not intended for below-grade applications and are rarely
appropriate for any below-grade applications. Damproofing applications are intended for
areas subject to the presence of moisture only (such as cavity wall construction) and not
water under hydrostatic pressure—which is the case for construction below grade.
Residential basement water leakage follows the 90%/1% principle, where the leakage
is usually caused not by the waterproofing system or materials themselves but by improper
application and substrate conditions.

Substrate
Most waterproofing systems require a fairly high-strength concrete mix, usually approximately
5000 psi, to ensure against excessive cracking in the substrate. Even though a residential base-
ment may not be installed at elevations as low as commercial projects, they still require the
same substrate soundness to ensure that it is adequate to receive a waterproofing treatment.
Most residential concrete mixes arrive at the site averaging approximately 3500 psi. This
lower-strength concrete, in combination with poor placement techniques (e.g., poorly built
formwork, no vibration of the concrete during placement, and little, if any, reinforcing steel),
typically will produce a substrate that insufficient for any quality waterproofing installation.
Manufacturer recommendations for substrate preparation should be followed and
supervised carefully during actual placement. This may require techniques and systems
rarely used in residential construction, such as water stop detailing at concrete junctures.
Recommendations for construction and/or expansion joints in the substrate also should be
reviewed and designed carefully by an engineer to prevent cracking in the substrate from
movement or settlement that will exceed the capacity of the waterproofing system.
Soil compaction prior to slab placement is rarely completed in residential construction,
and this often leads to excessive cracking of the completed slab. Most waterproofing system
have specific soil compaction requirements when concrete is used as a substrate. Contractors
should be aware of such guidelines and ensure that only concrete subcontractors that are
capable of compacting the soil to meet the waterproofing substrate guidelines be used.
Rarely do contractors prepare below-grade slabs for treatment with waterproofing sys-
tems. In many instances, a mud slab is required under the basement slab to effectively apply
the waterproofing system. Instead, residential contractors will limit their “waterproofing”
application beneath a basement slab to laying down a layer of visquene (again, available at
the local building supply store), which is a damproofing system, not a waterproofing system.
This is the reason for so much leakage in residential basements—water infiltration
through the slab or transition at the slab to vertical wall joint caused by use of only a layer
of visquene beneath the slab and no termination or transition detailing between the slab
and wall. While the contractor may have installed a waterproofing system adequately on
the vertical portions of the wall because the substrate was readily accessible for treatment,