How To Make A Chimney Cap Out Of Sheet Metal
Content Menu
● Chimney Cap Purpose and Types
● Q&A
Introduction
Every chimney needs a solid cap to keep water, leaves, and animals out while letting smoke escape. A store-bought cap might fit poorly or cost more than it should. Making one from sheet metal gives a perfect fit, better strength, and the chance to use scrap or leftover stock. The process follows basic sheet-metal shop practice: measure the flue, lay out parts on flat stock, cut, bend, join, finish, and mount. Done right, the cap lasts twenty years or longer with little upkeep.
The work uses the same steps found in small HVAC shops or custom metal yards. Start with a clean drawing, mark the sheet, cut straight or curved lines, bend at set angles, fasten seams, and seal edges. Each stage builds on the last. A small error in layout shows up as a gap at the roofline. Attention to gauge, alloy, and joint detail keeps the cap tight through heat cycles and storms.
This guide walks through the full build for a single-flue masonry chimney, then shows variations for round metal flues and multi-flue stacks. Examples come from real shop jobs: a square cap for an 8×12-inch brick flue, a cone for a 6-inch insulated pipe, and a louvered box for a factory exhaust. The same tools and rules apply to every version.
Chimney Cap Purpose and Types
A cap blocks rain, snow, birds, and debris. It also stops wind from pushing cold air down the flue. The top must stay open enough for full draft. Most caps fall into three shapes: flat lid with side skirts, sloped pyramid, or round cone. Each shape uses the same sheet-metal operations; only the layout changes.
Flat-lid caps suit square or rectangular flues. Sides drop 3 to 4 inches and flare out 2 inches beyond the tile for drip edge. Pyramid caps add a peaked top for faster runoff. Cone caps fit round metal pipes and roll from a single sector. Louvered boxes work on large commercial stacks where side vents improve flow.
Draft loss comes from undersized openings or sharp inside corners. Keep total vent area at least equal to the flue cross-section. Mesh screens of 1/2-inch grid stop embers without choking flow.
Material Choices
Galvanized steel in 24 or 26 gauge handles most homes. The zinc coat resists rust for 15–20 years in moderate climates. Use G90 coating for best life. Stainless 304 in 0.024 inch thickness lasts longer in salt air or with condensing appliances. Copper 16 oz (0.021 inch) ages to green patina and bends easily but softens above 400 °F. Avoid plain carbon steel; it rusts in weeks.
Gauge selection depends on wind load and span. A 24-inch square lid in 26 gauge sags under snow; step to 24 gauge. Round cones in 28 gauge hold shape if rolled tight. Always buy certified sheet with mill markings.
Tools and Shop Setup
Basic hand tools cover most caps: aviation snips (left, right, straight), 36-inch metal brake, rivet gun, cordless drill, carbide scribe, steel rule, and center punch. Add a bench grinder for edge cleanup and a MIG welder for stainless seams. A 4×8-foot plywood bench with clamps holds stock flat. Good light and a shop vac keep metal chips out of the way.
For curves, a slip-roll former or simple pipe mandrel works. A plasma cutter speeds large blanks but is optional. Safety gear includes gloves, safety glasses, and hearing protection.
Measuring and Layout
Climb the roof with a tape and notepad. Record inside tile dimensions, outside tile dimensions, and any offset. Add 2–3 inches to each side for overhang. Sketch the base rectangle, then the four side panels as trapezoids. Mark bend lines 1/2 inch in from edges for hems.
Transfer the sketch full size to butcher paper. Cut the paper pattern and test-fit on the chimney. Adjust until the paper sits level with even overhang. Tape the pattern to the sheet and scribe around it. Mark fold lines with dashed scribe marks.
Cutting the Blank
Secure the sheet to the bench with clamps. Cut straight runs with red-handle snips, keeping the top blade on the waste side. For inside corners, drill a 1/4-inch relief hole, then nibble to the line. Curved cuts follow the same rule: short bites, steady pressure. File all edges smooth.
Nest parts on the sheet to save material. A 16×16-inch cap with 4-inch sides fits on a 24×30-inch blank. Label each piece after cutting to avoid mix-ups.
Bending the Sides
Set the brake to the first bend line. Lift the sheet, align the mark under the clamp bar, and lower the bar. Bend to 90 degrees for base flanges, then 30–45 degrees for slope. Springback adds 2–3 degrees; over-bend slightly. Use a square to check corners.
For pyramid tops, bend the peak panels last. Clamp two sides together and bend the ridge in one pass. Hem edges by folding 1/4 inch twice; this stiffens the lip and hides sharp edges.
Assembly Methods
Rivets join galvanized parts fast. Drill 1/8-inch holes every 4 inches, insert aluminum rivets, and set with a hand squeezer. Stainless seams need TIG or MIG welds. Tack corners first, then run short beads. Cool between passes to limit warp.
Fold-and-crimp seams work without fasteners. Notch corners, fold tabs, and crimp with a seamers tool. Seal all joints with high-temperature silicone or butyl tape.
Adding Spark Screen
Cut 18-gauge galvanized mesh 1 inch larger than the opening on all sides. Bend the extra inch down to form attachment tabs. Spot-weld or rivet tabs to the inside frame. Screens clog with creosote; make them removable with screws or clips.
Surface Finish and Coating
Grind weld beads flush on stainless. Brush galvanized with zinc-rich primer on scratches. Spray two coats of 600 °F silicone paint for color and UV protection. Copper needs only clear lacquer if the owner wants bright metal; otherwise let it weather.
Mounting the Cap
Clean the flue crown with a wire brush. Set the cap centered and level. Drill for 1/4-inch masonry anchors every 12 inches around the flange. Bed the flange in roofing cement or silicone for a watertight seal. For metal pipes, use adjustable band clamps.
Common Fixes
Gaps at corners come from short flanges; add 1/2-inch strips and reseal. Warped lids flatten on a sandbag with a rubber mallet. Rust spots on galvanized mean the zinc is gone; wire-brush and cold-galvanize spray.
Variations
Round caps start as a flat sector. Calculate radius and arc length, add 1-inch overlap, roll on a slip former, and rivet the seam. Multi-flue caps use one large base with separate tops for each tile. Louvered sides cut from one piece and bent outward give extra draft on factory stacks.
Conclusion
A sheet-metal chimney cap is a straightforward shop project that solves a real problem. Measure carefully, cut clean, bend true, and join tight. The finished piece sheds water, stops birds, and improves draft for years. Skills learned here—layout, forming, fastening—carry over to ductwork, flashing, and enclosures. Start with galvanized 26 gauge for the first cap, then try stainless or copper once the steps feel familiar. The roof stays dry, the fire burns clean, and the shop gains another useful product.
Q&A
Q1: Will 28-gauge steel hold up on a 24-inch square cap?
A: No, it flexes under wind or snow. Use 24 gauge minimum for spans over 18 inches.
Q2: Can I solder copper seams instead of riveting?
A: Yes, with 50/50 lead-free solder and flux made for copper. Keep heat low to avoid burning the metal.
Q3: How much overhang is enough for rain protection?
A: Two inches past the tile edge on all sides gives a safe drip line.
Q4: What mesh size stops embers but allows draft?
A: 1/2-inch galvanized hardware cloth works; 3/4-inch is too open for sparks.
Q5: Do I need to paint galvanized steel?
A: Not required, but high-heat paint adds color and extends zinc life in harsh sun.