The Case for Sill Pans

by Michael Fallarino

It wasn't all that long ago that the standard method that we carpenters used for protecting the vulnerable sill framing of doors and windows was a flap of the 15 pound felt building paper that was carefully cut and folded into the rough opening. Many carpenters and builders are still doubtless doing exactly the same thing. That is, at least where building wraps are used. I read an article recently which disclosed that about 50% of the new structures erected in the US were fabricated without the use of any building wrap. But that is a whole other issue.

Now that we are into the increasingly sophisticated, building science driven twenty-first century, the available smart options for protecting the most vulnerable areas of exterior wall assemblies abound. And I have to admit that I am all for these sophisticated materials and methods. Many window and door manufacturers are either actively recommending or even requiring the use of self-adhered flashings to sill framing and the installation flanges of their millwork.

Remodelers have the opportunity to reflect on how stereotypical damage patterns occur, and it seems that we now have a multitude of ways to circumvent potential damage to framing, wall assemblies, and newly installed millwork. Most of it is actually pretty simple and logical. It just takes a little bit of thought, a little bit of care, and a little time. I have to admit that I have become a flashing freak, because damage from unflashed or poorly flashed interfaces always seems to be a case where an ounce of prevention would have been worth… well, you know what I'm driving at.

While it is possible to craft pan-like protection on site from self adhered flashing such as the Grace company's Vycor®, and similar products, I have become a fan of going one better and actually installing a bona fide sill pan in addition to self-adhered flashing. As a hybrid solution, the Grace company has also added a rigid corner, the VYCORner™ to their line of flashing products.

Unfortunately, sill pans still seem to be largely non-stock, esoteric items for yards and carpenters, respectively.

A sill pan is a pre-formed adjustable system that consists of corners and straight pieces designed to protect the sill framing of doors and windows, particularly the vulnerable interface where the horizontal sill meets its vertical jack studs. This is often the place where millwork framing rots out and exterior wall assemblies become compromised, as a small persistent leak or chronic condensation attacks framing and wall materials. The pan protects the exposed framing in the wall, and with a 90° return, flashes to the exterior sheathing and other protective components.

Sill pan corners are preformed to wrap from the sill up the jack studs in one monolithic form, making water penetration at the corner virtually impossible. Different sill pan manufacturers use different designs to fill in the sill space between the corners, such as modular straight sections spliced into place with metal waterproof tape, or expandable sections. Pans are purchased based on the actual wall and jamb depth, and can also include a preformed back dam if the installer desires that design feature.

While the installation of sill pans will slightly increase the installed cost of each new window and door, they also offer nearly immutable insurance against the possibility of sill and framing damage in a structure's rough opening, as long as the properly installed millwork is well flashed and protected on the exterior face of the building. There is room for design flexibility in the overall installation sequence.

We must consider the reality that structures commonly change hands, installed doors and windows can and do outlive the exterior finish treatment (siding, trim, etc.), and that succeeding remodelers may not be as meticulous as the original installer. One aspect of building green is that we must accept responsibility now for what might happen in the future. The need for a subsequent excuse to explain unforeseen damage just doesn't cut it.

Mike Fallarino is a contractor in the Albany, New York, area. He can be contacted at

© 2009 Michael Fallarino | All rights reserved.

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