How to Install Apron (Base) Flashing on the Downslope Face of a Chimney
The first piece of chimney flashing: lapped over the shingles, turned up the masonry and extended past the corners.
- Advanced
- 1 to 2 hours
- Updated
The short answer
Apron flashing is one bent piece of metal on the downslope face of a chimney. Cut it about 8 inches longer than the chimney width, bend it so the roof leg is at least 4 inches and the wall leg 4 to 6 inches, fold the ends, and lay it over the shingle courses. Nail the roof leg only, keep nails out of the wall leg, and seal the corner folds. Counter flashing then covers its top edge.
The apron is the first piece of chimney flashing to go on, and the one that faces the most direct weather: wind-driven rain hits the downslope face head on, and anything that runs down the sides ends up at its ends. It has to cover the full width of the chimney, reach past the corners and turn up the masonry, so that water has no way to get behind it.
It also has to be made to fit. A poorly cut apron with gaps at the corners is one of the most common chimney leaks. Fold the ends rather than cutting and gluing them where you can. For how the apron fits with the other parts, see chimney flashing parts explained.
Tools
- Sheet metal brake or hand seamer
- Tin snips (straight cut and offset)
- Tape measure
- Roofing hammer
- Caulking gun
- Hook blade utility knife
- Cut resistant gloves
Materials
- Galvanized or prefinished steel or copper sheet, 26 gauge or heavier
- Ice and water shield
- Roofing nails, 1-1/4 inch
- Polyurethane or hybrid polymer sealant
- Shingles
Safety first
- Work tied off. The downslope face of a chimney is often on the high side of a steep roof. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Cut sheet metal is sharp enough to slice through a glove seam. Wear cut resistant gloves and file or hem edges.
- A flashing piece of this size catches wind like a sail. Hold it by the edges and keep the roof clear of loose scrap.
Step by Step
Step 1: Prepare the deck and membrane
Shingles should be laid up to within a few inches of the chimney face but not run against it. Clean the face of the masonry of dust and loose mortar. Lay the membrane so it extends about 12 inches onto the roof from the face and up the chimney about 4 inches, pressed into the corner. This is the backup layer if the metal ever leaks. See how to install ice and water shield.
Tie the membrane around the corners of the chimney so there is a continuous layer around the base.
Step 2: Measure the chimney and cut the apron
Measure the face of the chimney and add 8 inches total, 4 inches on each side. The piece should be wide enough to bend into a roof leg at least 4 inches wide and a wall leg 4 to 6 inches tall, plus a hem. Use steel, prefinished steel or copper. Avoid bare aluminum against mortar because it corrodes.
Make the apron from one piece. Do not splice it across the face of the chimney, because a seam on the face is a leak point.
Step 3: Bend the apron and fold the ends
Use a sheet metal brake or a hand seamer to bend the roof leg and wall leg at 90 degrees. At each end, cut and fold the wall leg into a tab that wraps around the chimney corner by 3 to 4 inches, so the side step flashing can lap over it. Make the fold with a hemmed or soldered corner if possible. See how to use a sheet metal brake.
Test fit against the chimney. It should sit tight to the masonry and lie flat on the roof.
Tip: Fold the end tabs rather than cutting a notch. A cut corner leaves a gap that must be filled with sealant, and sealant fails.
Step 4: Set the apron over the shingles
The apron goes over the shingles, not under them. Its lower edge should lie on the shingle surface and cover at least 4 inches of shingle, which lets water run off the metal onto the shingles below. Place the wall leg tight to the masonry.
Adjust the position so the ends of the apron extend equally past each corner.
Step 5: Fasten the roof leg only
Keep fasteners to a minimum. The apron lies on top of the shingles, so any nail through the roof leg stays exposed. Common practice is two or three nails near the lower edge of the roof leg with the heads sealed, or cleats on a long apron so there are no exposed holes at all. Follow the shingle and flashing instructions. The wall leg should be unfastened. The chimney and roof move independently over time, and a fastened wall leg tears. The counter flashing will hold the top edge.
Seal the nail heads with a dab of sealant if they are exposed.
Step 6: Seal and lap the corner tabs
Put a bead of polyurethane or hybrid sealant behind the corner fold where the tab meets the masonry, and press. Sealant is a backup only and does not replace the lap. See how to choose and apply roofing sealants.
The first step flashing piece on each side must sit on top of the apron's tab. If the tab is short or folded the wrong way, the step flashing will lap under it and water will run in.
Step 7: Continue with step flashing and counter flashing
Install the first step flashing piece over the apron's end, then work up each side. See how to step flash a chimney.
The wall leg of the apron is covered by the counter flashing, which sets into a mortar joint about 1 inch above. See how to install chimney counter flashing.
Apron Dimensions at a Glance
- Roof leg: at least 4 inches wide onto the shingles, more on a wide chimney or a steep roof.
- Wall leg: 4 to 6 inches tall, then covered by counter flashing.
- Length: chimney width plus 4 inches each side.
- Metal: 26 gauge or heavier steel, prefinished steel or copper.
- Fasteners: roof leg only, near the bottom edge.
Reroofing Around an Existing Chimney
On a reroof the old apron is usually reused only if it is in perfect condition, which is rare. Remove it, check the membrane and deck below, and install a new piece. See how to reflash a chimney during a reroof for the full sequence.
Common Mistakes
- Splicing the apron. A seam across the face of the chimney leaks. Use one piece.
- Cut corners with no fold. Notched corners leave gaps that depend on sealant.
- Apron tucked under the shingles. The apron must lap over the shingles below it so water runs off onto them.
- Nailing the wall leg. It tears when the roof and chimney move.
- Ends that stop at the corners. The ends must extend past the corners so the step flashing has something to lap onto.
- Bare aluminum on masonry. Wet mortar corrodes it.
When to Call a Professional
Apron flashing is part of a chimney system, and leaving out or misplacing a single piece makes the rest fail. If the chimney is on a steep roof, is tall, or the brick around the base is crumbling, hire a roofer. They can also fit the step flashing and counter flashing in the same visit.
If water is already staining a ceiling near the chimney, have the whole assembly looked at, because the apron is rarely the only fault.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install Chimney Apron (Base) Flashing: Questions
What is apron flashing on a chimney?
Apron flashing, also called base flashing, is a single bent piece of metal on the downslope face of a chimney. The roof leg laps over the shingles by at least 4 inches and the wall leg turns up the masonry by 4 to 6 inches. The ends extend past the chimney corners so the step flashing on each side can lap over them.
Does chimney apron flashing go over or under shingles?
Over. The apron sits on top of the shingles in front of the chimney, so water running down the face of the chimney drops onto the apron and runs onto the shingles below. The shingles above the apron are not under it, because the apron is at the bottom of the chimney. The counter flashing then covers the apron's wall leg.
How long should chimney apron flashing be?
Cut it about 8 inches longer than the chimney width, which lets it extend 4 inches past each corner. The ends fold around the corners so the step flashing can lap over them. The roof leg should be at least 4 inches wide, and the wall leg 4 to 6 inches tall, then covered by the counter flashing.
Do you nail apron flashing?
Only the roof leg, with as few nails as possible near its lower edge and every exposed head sealed, or with cleats on a long apron so there are no exposed holes. Do not nail the wall leg, because the roof and chimney move independently. A fastened wall leg tears. The counter flashing set into the masonry holds the top edge in place and keeps it weatherproof.
Related Guides
All Chimneys guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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