Keeping Water out of a Building: Openings, Forces that Move Water, Roofs

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Buildings are amply provided with openings through which water can pass. Some, such as movement joints, joints between pieces of cladding materials, and cracks around doors and window sashes, are intentional. Other openings are unintentional but inevitable:

shrinkage cracks in concrete, lapses in workmanship, defects in materials, holes for pipes and wires, and cracks and holes created by the deterioration of building materials over time.

Forces That Can Move Water

Water can be driven to penetrate a building by any of a number of forces. Gravity constantly pulls water downward and causes hydro static pressures where water is allowed to accumulate to any depth. Air pressure differentials caused by wind action can drive water in any direction, including uphill. Capillary action can pull water in any direction through porous materials or narrow cracks. Even the momentum of a falling raindrop is sufficient to drive the drop and its spatter deep into an opening.

The problem of preventing water penetration is aggravated in many cases by the wintertime freezing of water into ice. Ice may clog normal drainage paths and cause ponding of water on roof or ground. The expansion that occurs during the transformation of water into ice can open paths through the building enclosure and is a common factor in several kinds of building deterioration.

Let us examine a building, starting from the top and moving to the bottom, to see what sorts of strategies are employed to prevent water penetration.


A roof that is dead level or that rises at a slope of less than one in four sheds water slowly and therefore offers maximum opportunity for water penetration. Such low-slope roofs are commonly covered with a continuous, impervious membrane. This may consist of layers of felt bedded in tar or asphalt, metal sheets soldered together, sheets of synthetic rubber or plastic heat-fused or cemented tightly together, a synthetic rubber compound applied in liquid form, or, in primitive buildings in relatively dry climates, a thick layer of clay soil. These continuous roof membranes, seemingly the simplest and most fool proof way to keep out water, constitute in reality the least reliable roofing mechanisms that we can employ. They are highly susceptible to puncture, especially from materials or tools dropped during construction. They are exposed to extreme thermal stress from summer sun, winter cold, daytime—nighttime temperature shifts, and air temperature differentials between indoors and outdoors. Some times they crack from the resulting thermal movement. They are incapable of passing water vapor, which sometimes leads to blistering and rupture of the membrane. A slight hole in the membrane caused by any of these mechanisms is likely to admit prodigious quantities of water into the building, because water drains slowly, if at all, from a low-slope roof, and slow drainage presents maximum opportunities for gravity and capillary forces to do their work. Nevertheless, there is often no alternative to a membrane roof, and if care is taken during construction, thermal movement joints are provided at appropriate intervals, and a warm-side vapor retarder is installed to avoid problems of vapor pressure, a membrane roof can give long and satisfactory service.

A steep roof, one that slopes at a substantial angle, is much easier to waterproof than a low-slope roof. The steeper the slope is, the faster the water will run off, the less likely it's that the wind will drive water up the slope of the roof, and the easier it's to keep out water. Almost any material will shed water if it slopes steeply enough, as can be proved by holding a washcloth or flat sponge under a water faucet at various inclinations. Whatever material is used on steep roofs, it's usually installed in small units known as shingles.

Shingles of many materials — slate, limestone, wood, asphalt-impregnated felt, fired clay, sheet metal—are used in different parts of the world. Each shingle is a small unit, easily handled and applied by the roofer, easily replaced later if defective, and free to adjust to thermal or moisture stresses in the roof structure. Each shingle allows water to pass off three of its four sides under the forces of gravity and wind, but the shingles are laid in such a way that the next lower shingles catch and drain the water quickly in turn, and soon to the lower edge of the roof (ill. 1-12). The weakness of a shingle roof is its susceptibility to water driven up the slope or across the slope by a very strong wind. This is countered by a twofold strategy of providing a sheet material beneath the shingles, usually asphalt-impregnated felt paper, to block the passage of air through the roof plane, and by sloping the roof steeply enough that a larger component of gravity is brought into play against the upward flow of water. Through experience, safe minimum roof slopes have been determined for shingles of various materials under various wind conditions, and leakage is unlikely even in a heavy storm if these criteria are satisfied. In windswept locations—seashores and mountaintops—it is often wise to use even steeper slopes than those recommended or to increase the overlap of the shingles.

ill. 1-12: Wood Shingle Roof: Felt; Nails; Length of Shingle; Exposure to weather.

The adhesive force of water is utilized as the major water-resisting mechanism of a thatch roof (ill. 2-12). If a thick layer of straw, leaves, or reeds slopes at a sufficient angle, the drops of water adhere to the fibers and run downward along them to the lower edge of the roof just as water can be transferred from a beaker to a test tube in the laboratory by pouring it along a glass rod. A thatch roof absorbs considerable water and must dry out between storms to minimize decay. Therefore, thatch is never laid over a solid roof surface but instead is tied to spaced, horizontal poles or strips of wood over a well-ventilated attic. The sheer thickness of the layer of tightly packed stalks is sufficient to dissipate wind energy that might other wise drive rain through without the use of a sheet material beneath.

ill. 2-12

On one- and two-story buildings, broad roof overhangs can shelter walls and windows from rain. The walls of taller structures can't be protected from direct attack directly by rain, making roof over hangs superfluous.

Edges of roofs are particularly problematic. Water may creep under the roofing material or penetrate the tops of parapet walls. The problem is worse on steep roofs and some low-slope roofs because all the water gathered by the roof drains to the edges of the roof for disposal. In most cases, steep roofs simply lap over the walls, so that the roof water drips well outside the wall, a simple and effective expedient. The dripping roof runoff erodes the soil below, however, often enabling water to penetrate into the basement, washing away soil from around and beneath foundations, and spattering earth onto the building walls. At a minimum, a trench filled with gravel should be provided beneath the eave to prevent erosion and provide drainage (ill. 3-12).

ill. 3-12

An alternative approach is to catch the roof-edge runoff in gutters. The gutters slope slightly to drain into vertical pipes called downs pouts or leaders, which in turn discharge either onto splash blocks, into a municipal storm sewer network, or into dry wells (gravel-filled pits) to be absorbed by the ground. But such systems tend to clog with leaves, dirt, pine needles, and other debris and are a nuisance to clean, a problem that can be alleviated somewhat by installing coarse screening over the gutters.

Snow causes particular problems at the edges of steep roofs in cold climates. One problem is caused by snow’s tendency to slide off, tearing away gutters and endangering people and objects below. it's better to reduce the danger by holding the snow on the roof with small metal or wood fences installed for that purpose (ill. 4-12). Furthermore, snow is a fairly good thermal insulator, and heating fuel can be saved by retaining it in place. The supporting roof structure must be strong enough to hold a considerable depth of snow without distress.

ill. 4-12: Snow logs on Alpine barn roof

A second problem is caused by the melting of snow on steep roofs, especially where the thermal insulation of the building is inadequate. Heat passing from the warm interior of the building gradually melts the snow from beneath. The snowmelt water runs down the roof until it reaches the overhang and the gutter, which are much cooler than the roof over the interior spaces, and in many cases colder than the freezing temperature of water. The water refreezes into ice on the overhang, clogging the downspout and gutter. A pool of water collects above this ice dam (ill. 5-12). Unfortunately, shingled roofs are not resistant to standing water. The water penetrates around and under the shingles and is often first noticed when it discolors the interior wall surfaces and drips from the heads of the window openings just under the roof. The remedies for ice dams are improved attic insulation and also ventilation openings under the eaves and at the roof ridge, to carry away quickly any heat that gets through the insulation (ill. 6-12). If for some reason these measures are impossible or ineffective, the last few rows of shingles at the lower edge of the roof may be replaced by an impervious membrane, or electric melting cables may be installed at the eaves.

ill. 5-12

ill. 6-12

Edges of low-slope roofs may or may not overhang the walls, or they may join a parapet wall (figs. 7-12, 8-12). In any case, it's important that the edge of the roof membrane be raised at least a few inches (100 mm or more), in order to keep water from spilling over it and down into or onto the structure beneath. The membrane is turned in two folds of 135 degrees each with a cant strip to avoid having to make a crack-prone 90-degree fold. The vertical edge of the membrane is protected with overlapping sheet metal flashings or is tucked into a reglet. Rainwater may be carried away by interior roof drains evenly spaced across the expanse of the roof or by scuppers (ill. 7-12) and downspouts around the exterior walls.

ill. 7-12: parapet; coping; scupper; Alternative detail using reglet block instead of flashing

ill. 8-12

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