Miami-Dade HVHZ Water Management

The Hidden Hero: Sill Pan Flashing

Hurricane rain does not fall straight down. It drives sideways at 100+ mph, forcing water past window seals. The sill pan is your last line of defense, catching infiltrated water and draining it back outside before it rots your wall cavity.

Get Window Specs Installation Guide

Rain Intensity

1.5"

Min End Dam Height

6-10°

Outward Slope

100%

Corner Seal Required

Proper Sill Pan Installation

Follow these steps to ensure your sill pan actually protects the wall cavity from wind-driven rain infiltration.

Prepare the Rough Opening

Clean the rough sill surface. Remove any debris, nails, or irregularities. The surface must be flat and level side-to-side. If needed, shim to create the 6-10 degree outward slope before installing the pan.

Install the Back Dam First

Apply flexible flashing tape or liquid membrane up the back of the rough opening at least 6 inches. This creates the back dam that prevents water from reaching the interior wall. Overlap with the sill pan material.

Set the Sill Pan with End Dams

Install the formed sill pan with end dams at least 1.5 inches high. The pan must extend past the exterior wall face by 1/2 inch minimum. Seal all corners completely - this is where most leaks occur.

Seal and Test

Apply compatible sealant at all pan joints and where the pan meets the rough opening. Before installing the window, flood test the pan by pouring water into it. Water must drain to exterior only - no leaks to interior.

Approved Sill Pan Materials

Choose the right material for your application and budget. All must be compatible with your window system.

Formed Metal Pan

Pre-formed aluminum or galvanized steel pans with integrated end dams. Most durable option. Requires proper sealant at window frame interface.

Rigid PVC Pan

Plastic pans with molded end dams. Lightweight, corrosion-resistant. Some have built-in weep channels. Check UV stability for exposed installations.

Flexible Membrane

Self-adhered flexible PVC or butyl membrane formed into pan shape with tape end dams. Conforms to irregular openings. Requires careful corner work.

Sill Pan Flashing FAQs

Common questions about sill pan requirements in Miami-Dade hurricane zones

What is a sill pan flashing and why is it required in Miami-Dade?

A sill pan flashing is a waterproof tray installed under windows to catch and drain any water that penetrates past the window frame. In Miami-Dade HVHZ, sill pans are required because hurricane-force winds drive rain horizontally at over 100 mph, forcing water past even the best window seals. The sill pan acts as a secondary defense, channeling water back outside before it damages the wall cavity.

How high should end dams be on sill pan flashing?

End dams on sill pan flashing should be at least 1 inch high in Miami-Dade HVHZ, though 1.5 to 2 inches is preferred for hurricane zones. The end dams prevent water from spilling out the sides of the sill pan into the wall cavity. Higher end dams provide more protection during prolonged wind-driven rain events typical of hurricanes.

What slope is required for sill pan flashing?

Sill pan flashing should slope toward the exterior at a minimum of 6 degrees (approximately 1/8 inch per inch). This slope ensures water flows outward rather than pooling under the window. In Miami-Dade, many inspectors prefer 10-15 degrees of slope for faster drainage during heavy rain events.

Can I use flexible flashing tape instead of a formed sill pan?

In Miami-Dade HVHZ, flexible flashing tape alone is generally not acceptable as a sill pan. Code requires a formed pan with end dams that can hold water and drain it to the exterior. Flashing tape is used to seal the pan to the rough opening and create the back dam, but a rigid or semi-rigid pan material is needed for the main drainage surface.

What materials are approved for sill pan flashing in hurricane zones?

Approved sill pan materials for Miami-Dade HVHZ include: formed metal pans (aluminum or galvanized steel), rigid PVC sill pans, flexible PVC membrane pans with formed end dams, and liquid-applied waterproof membranes meeting AAMA 711 specifications. All materials must be compatible with the window frame and surrounding building materials.

Get Your Window Wind Load Specifications

Know exactly what wind pressures your windows must resist. Complete specifications for Miami-Dade permit approval.

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