Section 13: BELOW-GRADE WATERPROOFING
System Description. Basic considerations for below-
grade waterproofing are the same as for plaza waterproofing
(refer to Section 14). The below-grade location adds the extra
requirement to handle probable static ground water pressure.
Therefore, the primary objective is to drain away the water and
prevent the build-up of hydrostatic pressure at the surface of
For both horizontal and vertical surfaces, multiple-
layer systems are superior. These systems may be applied hot or
cold. Fluid-applied and single-ply sheet applications can be
considered if conditions will not be severe and if access to the
system in case of failure is relatively easy. Bentonite clay
panels are sometimes used for waterproofing, especially below
slabs and on vertical walls.
Uses. Below grade waterproofing may be required for
basement slabs, foundation walls, planters, mud slabs, tunnels,
sidewalk vaults, above-grade walls at earth berms, etc.
Substrates. Substrates must be free of laitance, sharp
projections, facial imperfections, curing compounds, oil, grease,
and moisture in any form. In all cases, masonry requires priming
with a material compatible with the selected membrane system.
Membrane. Membrane reinforcing must be provided at all
joints, reinforced, non-reinforced, or expansion joints. (Refer
to ASTM C 981, Guide for Design of Built-Up Bituminous Membrane
Waterproofing Systems for Building Decks.) Reinforcing must also
occur at all sharp turns, flashing terminations, penetrations,
pedestals, drains, etc.
Membrane Protection. Membranes must be protected with
protection board of sufficient thickness to withstand anticipated
abuse. Boards conforming to ASTM C 208, Cellulosic Fiber
Insulating Board, ASTM C 726, Mineral Fiber Roof Insulation
Board, or ASTM C 578, Type II Grade 2, are acceptable.
Single-ply sheet waterproofing requires immediate
ultraviolet protection and/or backfilling to prevent blistering.
In direct, hot sunlight, blistering can occur in 20 minutes or
less on some materials.