08 September 2009

Water Infiltration through Exterior Walls - Gravity, Part 1 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
Gravity draws free water towards the earth in as direct a path as possible. This path will be a vertical line until the water hits an obstruction, at which time gravity will pull the water along the portion of the obstruction with the greatest downward slope. The diversion principle is used to control water infiltration into a building. The the most common application of diversion is the roof and associated gutters and downspouts.

Although roofs go a long way to keeping exterior walls dry, sometimes we design our walls to extend past the roof in the form of parapets, and sometimes wind-blown rain makes the walls wet. In each case, water is delivered to the wall, and gravity has an opportunity the pull the water through the wall and even into the building.

The first line of defense is to pay particular attention to any wall surface that is not vertical. The most common are parapet copings, window sills, and ledges. Every joint in these surfaces is a potential crack, caused by faulty design or construction or the wear and tear of time, through which water can enter the wall. The opportunities are particularly prevalent when brick is used as the horizontal material at these details. The best design is to design each of these details as little roofs using waterproof materials, generous slopes, and proper lapping of joints.

The second line of defense is to provide adequate flashing within the wall to direct water back towards the exterior. For this reason, flashing is required by code at the perimeters of door and window assemblies, penetrations and terminations of exterior wall assemblies, exterior wall intersections with roofs, chimneys, porches, decks, balconies and similar projections and at built-in gutters and similar locations where moisture could enter the wall.

The third line of defense is to fully drain infiltrated water back to the exterior. To do so requires the prudent use of water resistant barriers to collect the water, sloped channeling of the water to the weeps, and flashings that extend to a drip edge beyond the face of the exterior skin to direct the water back outside. The first and last are well understood and can be easily implemented. The idea of sloped internal channels is not well understood and even harder to implement. It's not well understood because we as designers forget that water can accumulate on any flat surface inside the walls just as it does outside; we carefully design a 4 inch window sill to slope away from the window and forget about the 2 foot long internal gutter between our weep holes. It's even harder to implement because sloped gutters cannot be formed as an integral part of a wall system that has a perfectly horizontal exterior expression; it's particularly hard in masonry construction where we are still trying to master the problem of mortar droppings in the cavity in the first place, let alone figuring out how to slope the top of the mortar droppings so that they drain to the weep holes. Companies such as CavClear, Mortar Net, and ThermaDrain have done much to help us resolve the problem of mortar droppings blocking weep holes. However, we as designers need to challenge them to help us provide positive and complete drainage as Mortar Net has begun with its TOTALFlash system.