Doors don’t just welcome people into homes. Properly installed, they also keep out unwanted air and water. Experts say one of the most important pieces of this protection is also one of the most commonly botched: the sill pan. By code, this needs to be a pan flashing sealed or sloped to the exterior to allow water to drain to the exterior.

Improper or nonexistent sill pans allow air and water to get underneath the doorsill and into the subfloor, which can quickly lead to damage of hardwood floors and set off a mold time bomb.

Here are three of the most common mistakes builders make when it comes to sill pans — and how to correct them:

1. Using flashing tape. Manufacturers make ready-made sill pans with proper slope and drainage that cost around $30. But often, builders don’t think about the sill pan until they’re installing the door. So rather than hold up construction, they’ll often simply fashion a sill pan out of flexible flashing tape. This approach to sill pans doesn’t include a back dam necessary to prevent air and water infiltration. Flashing tape sill pans also don’t have the proper slope to allow water to drain to the outside. They are also susceptible to damage when an installer slides the door into place.

2. Improper sealant. To effectively keep out air and water, a sill pan needs to be properly sealed. Even if they have a proper sill pan, builders may be unsure about the right sealant to use for the most effective barrier and to meet code. Using the wrong caulk or sealant can lead to air and water intrusion. Look to the door manufacturer’s instructions for specific sealants to use with sill pans.

3. Not following code. Today, more and more states are adopting codes that specifically require properly installed sill pans. In fact only nine states have not yet officially adopted either the 2012 or 2015 International Residential Code. Specifically, section R703.8.1 requires “pan flashing” to be installed on doors and windows either to manufacture instructions or to code. And even within states that have not formally adopted those codes, some municipalities have. The safest bet for builders? Assume code requires pan flashings at door sills.

The best way to avoid all these mistakes is to turn to manufacturers. As water and mold issues become increasingly important, manufacturers have taken the guess work out pre-hung door installs with kits that contain all the elements needed to get the job done right and prevent faulty installs. Along with sill pans and sealants, the kits contain the screws, flashing, shims and foam builders need to install doors that will welcome visitors — and keep out moisture.

For more on DAP’s Quick Kit pre-hung exterior door installation kits, go to