Demystifying the Drainage Debate

May - June 2003

by Sean Davies

As a whole, the window industry has never truly understood the issues surrounding water damage and window drainage. Many misconceptions still exist regarding how to best handle drainage issues.

Myth #1:
A horizontal slider that tilts in for cleaning is a worst-case scenario for water drainage.

Most of the window industry believes that in order to achieve good water test results, you have to compromise air performance. This is inaccurate, yet is still widely accepted as fact because water that leaks into a window has to be redirected outside. This requires holes to be cut into a window so water can be released. But these holes also allow air in-- the more holes there are, the worse the air performance is.

Weep holes are required for proper drainage of any slider window. Many manufacturers tend to use sub-standard weep covers (or none at all) in order to reduce production costs. Unfortunately, they often risk their company’s reputation for quality windows for the sake of a few pennies.

Proper weep covers should incorporate a one-way valve in the weeper assembly that snaps snugly in place over the hole. The valve is key to preventing air from being forced into the window from outside winds, while allowing water to drain out from the inside track of the sash.

Myth #2:

It takes magic to pass the water test.
The standard water test is graded by pressure--one-tenth of the structural pressure of a window. That means for Level 20 (30 pounds per square foot structural), a window would have to pass the water test at three pounds per square foot of pressure. When water leaks into the window, it accumulates in the sill until it builds up enough pressure to open the one-way valve. Therefore, more pressure must be exerted inside the sill than exerted on the exterior of the window by external forces like wind.

By merely factoring the density of the water at 62.5 pounds per foot and multiplying it by the height of a window sill, manufacturers can determine how high their sills need to be in order to overcome outside wind pressure. If you have proper drain holes channelled in the right way, you will ensure that your window passes the water test. There’s really no magic involved--it’s just regular old physics.

When a window is tested, the amount of water sprayed on it is quite excessive (twice as much pressure as ever recorded in North America). Not surprisingly, this causes water to leak in, because all windows will leak to a certain extent. But that water has to be channelled back out. It has to drain out at least as quickly as it’s coming in, or it’s going to start backing up.

When designing weepers we calculate the flow rate and pressure required and determine what size opening is needed to allow the same amount of water coming in to be drained out. If you have a tiny hole and you want the water to exit at a certain speed, more pressure needs to be applied. When you have a bigger hole, more water can be drained at less pressure. Drainage systems should be engineered so that water can flow out under low pressure.

Hanging Drainage Myths Out to Dry
Not all weepers are created equal. The majority of weep-hole covers on the market today are constructed of nylon. This can cause problems for manufacturers and homeowners with windows that incorporate weep covers with one-way valves.

Nylon tends to absorb water which causes it to swell ever so slightly. When we’re dealing with small components with tight tolerances, swelling of the weep cover can cause the one-way valve to jam in the open position, allowing wind and insects to penetrate the sash. To rectify this problem, manufacturers should select a weeper with a non-hydroscopic reed valve designed to work in the low-pressure environments of window and door sills.

To prevent the reed valve from being forced shut by water streaming down the outside of the sash, proper weep covers should incorporate a hood over the valve.

By taking an engineering approach to design, hardware manufacturers like ours are evaporating old drainage myths and giving window manufacturers the opportunity to minimize water damage and costly service repairs without sacrificing air performance for water performance.

Sean Davies is the chief executive officer and director of engineering for Preferred Engineering Products based in Vaughan, Ontario.