Flashing for Siding: Z-Flashing, Base Flashing and Laps

Most rotted wall sheathing is not caused by bad siding. It is caused by missing or backward flashing for siding, the thin metal or membrane pieces that catch water that gets behind the cladding and steer it back outside. Every siding type leaks a little in wind-driven rain, so the wall needs a drainage plan, and flashing is the part of that plan that changes the water’s direction at joints, openings and the base of the wall.

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The short answer: you need Z-flashing at horizontal joints in panel siding and trim, base or starter flashing where the wall meets the foundation, and head flashing above every window and door. Each piece must lap over the layer below it and tuck under the housewrap above it, like shingles on a roof. Get the laps right and water drains out. Get them backward and the flashing funnels water straight into the framing.

How Flashing for Siding Works With the Drainage Plane

Think of the wall as three layers. Siding sheds most of the rain. Behind it, the water-resistive barrier (housewrap or building paper) is the true drainage plane that handles whatever slips past. Flashing for siding connects the two at every break, so water riding down the face of the housewrap gets kicked out to daylight instead of meeting a horizontal ledge and sitting there.

The rule that governs all of it is shingle-lapping: an upper layer always overlaps a lower layer. Housewrap above a flashing goes over the flashing’s vertical leg. The flashing’s lower leg goes over the siding or trim beneath it. When a piece of tape, wrap or metal is tucked behind something lower on the wall, water traveling downhill hits that reverse lap and gets pushed inward. Inspectors call this a reverse lap, and it is the single most common flashing error on houses with water damage.

Gravity is not the only force. Wind pressure can push water uphill a short distance, which is why most flashing has a vertical leg of at least 2 inches behind the wrap and why many installers add a small hemmed drip edge on the outer leg.

Z-Flashing at Horizontal Joints

Z-flashing is a strip bent into a Z profile: one vertical leg goes up behind the housewrap, a short horizontal shelf sits on top of the lower siding panel or trim board, and an outer leg drops down over the face of that panel. It is the standard detail anywhere two sheets of panel siding meet horizontally, such as fiber cement panels, grooved plywood panel siding and board-and-batten on tall walls.

You also want z flashing over horizontal trim that interrupts lap siding, including belly bands, frieze boards on some designs, and the top of a band board between floors. Without it, water runs down the housewrap, lands on the top edge of the trim board, and wicks into the end grain and the sheathing behind it.

Common sizes have a 3/8-inch to 1-inch shelf to match siding thickness, and legs from 1.5 to 3 inches. Pick a shelf depth that matches the thickness of the panel or trim it sits on, so the outer leg hangs flat against the face. A z metal flashing that is too shallow gets pried open by the upper panel and leaves a gap; one that is too deep leaves a visible ledge that collects dirt.

Z-flashing installation tips

• Leave a 1/4-inch gap between the shelf and the bottom of the upper panel so water can escape and fiber cement does not wick.
• Lap end joints between strips by at least 2 inches, with the upper strip over the lower on sloped runs and the downwind strip over the upwind one on level runs.
• Nail only through the vertical leg, high, so fasteners end up behind the housewrap and not through the shelf.
• Slit the housewrap horizontally above the flashing, slip the vertical leg up behind it, then tape the slit.

Base Flashing and Starter Details at the Foundation

The bottom of the wall is where all the water that got behind the siding ends up. Base flashing for siding is a strip that sits at the transition from sheathing to foundation, letting that water exit over the top of the concrete instead of soaking the sill plate.

On many homes this is a simple L or Z profile nailed to the bottom of the sheathing, with the outer leg extending 1/2 to 1 inch past the face of the foundation. The housewrap laps over its vertical leg. The first course of siding or a starter strip sits above it, with a small gap so the drip edge stays open. On walls built over a rim joist with a wider foundation ledge, a drip cap with a kickout prevents water from sitting on the ledge.

Keep siding at least 6 inches above finished grade and 1 to 2 inches above hard surfaces like patios, decks and roofs. Siding that touches soil or mulch stays wet and acts like a wick, and no amount of base flashing will fix that. Check this clearance at downspout splash areas too, because regrading and new mulch slowly creep upward over the years.

Window and Door Head Flashing

Windows interrupt the drainage plane on every side, but the head (top) is where water arrives from above. A head flashing or drip cap sits over the top of the window trim, its vertical leg goes behind the housewrap, and its outer leg drops over the trim with a small drip edge. End dams, small upturned tabs at each end, keep water from running off the ends and down behind the side trim.

The housewrap sequence matters more than the metal. The correct order for a typical new-construction window is: sill flashing tape first, then the window, then the side (jamb) tapes, then the head tape, then the housewrap flap folded down over the head tape, then the metal head flashing tucked under that flap. Each piece laps over the one below. If the housewrap above the window was cut and simply taped over the head flange from the outside, water running down the wrap will get behind it at the first tape failure.

For replacement windows in existing siding, slide the vertical leg of the head flashing up behind the siding and housewrap as far as you can without tearing it, then seal the slit you made with compatible flashing tape.

Choosing Metal or Membrane Flashing Materials

Metal flashing for siding is still the most durable option for anything exposed to sun. Your main choices:

• Prefinished aluminum: light, easy to bend on a brake, comes in colors to match trim. Avoid direct contact with pressure-treated lumber and wet concrete, since both can corrode aluminum.
• Galvanized steel: stiffer and cheap, good for base flashing. Use the heavier G90 coating near treated wood.
• Copper and stainless: long-lived and expensive, mostly used on high-end or historic work.
• Vinyl or PVC flashing: used with vinyl siding accessories and some trim systems. It will not corrode but can get brittle in cold weather.
• Flexible flashing tape: self-adhered membrane for rough openings, corners and slits in housewrap. It is not a replacement for metal at exposed horizontal joints.

Match fasteners to the flashing metal. Galvanized nails in copper flashing, or steel screws in aluminum near salt air, set up galvanic corrosion that eats holes in the flashing within a few years.

Step-by-Step: Adding Flashing During a Siding Project

Tools and materials: tape measure, chalk line, tin snips or a portable brake, utility knife, hammer or nailer, flashing tape, cap staples, z flashing and drip cap in the needed sizes, and housewrap tape. Wear gloves, because cut metal edges are sharp.

1. Inspect the sheathing. Once old siding is off, probe the sheathing and sill area with an awl. Soft, dark or crumbling wood needs replacement before any new flashing goes on.
2. Install base flashing. Snap a level line at the bottom of the sheathing and nail the base flashing along it, lapping end joints 2 inches or more.
3. Hang housewrap from the bottom up. Start the first course over the vertical leg of the base flashing. Lap upper courses 6 inches over lower ones and vertical seams 6 to 12 inches.
4. Flash openings. Follow the sill, jamb, head sequence at every window and door, finishing with head flashing tucked under a housewrap flap.
5. Add Z-flashing at each horizontal break. Slit the wrap, insert the vertical leg, tape the slit, then set the lower panel or trim beneath the shelf.
6. Flash penetrations. Hose bibs, dryer vents, lights and electrical boxes get mounting blocks with their own head flashing or tape laps.
7. Install siding with drainage gaps. Keep the 1/4-inch gap at Z-flashing shelves and the required clearance at grade, and caulk only where the siding maker calls for it, never along the bottom edge of a flashing.

Where a roof meets a sidewall, a kickout flashing at the bottom of the step flashing is essential. It is technically a roofing detail, but missing kickouts are responsible for a large share of rotted walls below roof-to-wall intersections.

Troubleshooting Leaks and Moisture Behind Siding

Signs of a flashing failure include peeling paint below a window corner, swollen trim, stains on interior drywall near the floor, a musty smell in a closet on an exterior wall, and siding that bows or feels spongy. Water that appears inside often traveled several feet sideways or downward from the actual entry point, so start your search above and to the sides of the stain.

• Stain under one window corner: usually a missing end dam or reverse-lapped head tape.
• Rot along a band board: missing Z-flashing, or caulk that sealed the drainage gap.
• Wet sill plate: no base flashing, siding too close to grade, or a downspout dumping against the wall.
• Damage below a roofline: missing kickout flashing.

Fix the moisture source before anything else. Remove the siding over the damaged area, let the framing dry, replace rotted sheathing, and rebuild the laps correctly. Covering stained wood with new siding seals the problem in. If you find mold, small patches under about 10 square feet can be cleaned with detergent and water while wearing an N95 respirator, gloves and eye protection, after the leak is fixed. Larger areas, mold inside wall cavities, or anyone in the home with respiratory problems call for a professional mold assessment and remediation.

Cost Ranges and When to Hire a Pro

Flashing materials are inexpensive compared with the damage they prevent. Stock Z-flashing and drip cap in aluminum or galvanized steel typically runs a few dollars per 10-foot piece, and a roll of flashing tape costs roughly $20 to $50. For a full re-side, flashing usually adds only a small percentage to the material budget.

Repairs are a different story. Opening a section of wall to correct a window head or add missing Z-flashing generally runs a few hundred dollars for a small area. When the leak has rotted sheathing, studs or the sill plate, the bill can climb into the low thousands, especially if interior drywall and insulation need replacement.

Hire a siding contractor or carpenter when damage extends into framing, when the work is on a second story or above, or when you are dealing with stucco, stone veneer or brick, which have their own weep and flashing systems. Ask any contractor to describe their flashing sequence at windows and horizontal joints before you sign. A clear answer about lapping housewrap over flashing legs is a good sign; a vague answer about caulking everything is not.