Figure 1. Flexible flashing at the side of the rough opening should extend over the flashing at the bottom. Here, the side flashing was cut too short, and the installer failed to lap the filler piece over the bottom flashing. When housewrap or felt paper is later installed, it should be tucked under the horizontal flashing at the bottom of the window.
Figure 1. Flexible flashing at the side of the rough opening should extend over the flashing at the bottom. Here, the side flashing was cut too short, and the installer failed to lap the filler piece over the bottom flashing. When housewrap or felt paper is later installed, it should be tucked under the horizontal flashing at the bottom of the window.
Figure 2. Fasteners should be installed tight, unless the manufacturer directs you to leave them proud.
Figure 2. Fasteners should be installed tight, unless the manufacturer directs you to leave them proud.

Comparing installation instructions from different vinyl window manufacturers reveals a jumble of contradictions. Here are some of the most striking examples:

Should the sill be shimmed? Several manufacturers' instructions recommend shimming between the rough sill and the window frame in two or three locations, just as with wood windows. Others recommend that their windows be installed on a continuous, level rough sill. One manufacturer introduces a third option, instructing the installer to use shims as with a wood window, then to remove the shims after nailing fins are fastened — leaving the window hanging by the nailing fins.

Should the fasteners be driven home or left proud? The instructions from one manufacturer recommend leaving the fasteners proud because this will not restrict the window from expanding and contracting. Installers of another window brand, on the other hand, are instructed to use roofing nails driven all the way in.

Should the nailing fin at the head of the window be left unfastened? Some manufacturers recommend fastening the nailing fin at the head of the window, while others warn against the practice. Some installers avoid the problem by driving a nail above the fin, then bending it down to pinch the fin. This procedure is also part of some companies' installation instructions. But the nail head in this case can tear the building paper and interfere with other flashing and siding.

Among the variety of reasons given for not fastening the head fin is one that might be called “the myth of the settling header,” which claims that the window header will settle and distort the window. A variation states that if the house settles, the sill will drop but the header will stay, again distorting the window if the head fin is nailed.

Given a properly sized and installed header, this concern seems misplaced. Although headers may shrink a bit, this movement is far less than that of the vinyl window itself due to thermal expansion.

Should a vinyl window be fastened at the corners? This question, too, produces no consensus. In some instructions, the installer is told to fasten the nailing fins at the corners. Other manufacturers warn against nailing near the corners because when the corners are fastened tight, that's where the window can crack because it won't have room to expand.

Installing a Vinyl Window All of this conflicting advice from window experts is confusing for installers. It can also be costly, since the warranty will be void if you fail to follow the manufacturer's written instructions. This is a particular problem for installers who work with several brands of window, since it is easier for them to confuse instructions or fail to notice subtle differences in recommended procedures.

Finally, some of the recommended procedures may run counter to reliable methods that installers have developed over the years. If the manufacturer's written instructions seem ill-conceived, your best recourse may be to buy windows from a manufacturer whose instructions you can live with.

The installation procedures listed below are consistent with those of many, but not all, vinyl window manufacturers. These procedures also incorporate best practices for avoiding leaks when installing flanged windows of any kind.

Avoid cold-weather installation, when a hammer can shatter the brittle vinyl nailing fins. If you must install a window when the temperature is below 20° F, use flat-head screws instead of nails.

Size the rough opening according to the manufacturer's instructions. Most manufacturers specify an opening ½ inch larger than the window frame. The window should then be centered, left to right, leaving a ¼-inch gap on either side. Be careful: If the rough opening is even a little bit larger, the nailing fins might not catch the framing.

Don't shim the sill — instead, install the window on a continuous, level base. Some installers prefer to double-up the rough sill, using stacked 2x4s or 2x6s. This permits easy shimming between the two sills, and also gives wide blocking for stucco installers to fasten their wire lath without puncturing the window's nailing fins.

Install flexible flashing around the perimeter of the window opening. Flexible flashing, which generally comes in 9-inch wide rolls, is made from a variety of materials, including poly-ethylene and rubberized asphalt (see Figure 1).