How to Install TPO-Coated Edge Metal: Fastening, Joints, Welding the Membrane and Cover Strips
Clad metal edges let the roof membrane weld directly to the edge, so there is no sealant line. The fastening and the joints decide if it survives the wind.
- Professional
- Half a day to 2 days depending on the perimeter
- Updated
The short answer
TPO-coated edge metal is steel laminated with a layer of TPO so the roof membrane can be hot-air welded straight to it. Fasten the flange to a sound, blocked substrate in a staggered pattern, commonly 3 to 4 inches on centre, leave a joint gap per the maker, cover every joint with a welded cover strip, then weld the membrane to the clad surface at least 1-1/2 inches wide and probe it.
The edge of a flat roof is where the wind wins or loses. Suction is strongest along the perimeter, the membrane ends there and the metal that holds the edge is the structural part of the detail. A well-made edge resists wind, sheds water into the gutter or scupper and gives the membrane a clean, sealed termination.
TPO-coated metal is galvanized steel that has a TPO layer laminated to the surface. The coating is the same material as the roof membrane, so it can be hot-air welded to the sheet. This removes the sealant line used on bare metal edges, which is one of the first things to fail. The welded edge is a fused joint, not a caulked one.
This guide covers clad drip edge and gravel stop. For a parapet with a cap, see how to install parapet coping metal. For the membrane details at the wall, see how to flash TPO walls, curbs and corners.
Tools
- Hot-air hand welder with 1-1/2 inch nozzle
- Silicone roller
- Rounded-tip probe
- Drill or screw gun with depth control
- Chalk line and tape measure
- Metal shears or nibbler
- Hand seamer and brake for small pieces
- Caulking gun
Materials
- TPO-coated (clad) metal drip edge, gravel stop or fascia, usually in 10 foot lengths
- Continuous cleats for fascia-style edges
- Fasteners approved for the substrate, stainless or coated
- TPO cover strips, typically 6 inches wide, and flashing membrane
- Wood blocking, pressure treated or isolated from the metal where required
- Membrane-compatible sealant
- Membrane cleaner
Safety first
- Edge metal work is performed at the open edge of the roof. Tie off before leaning over or working on the eave line, and keep materials away from the edge.
- Cut metal edges are razor sharp. Wear cut-resistant gloves and handle 10 foot lengths with two people in wind, because they act as sails.
- Hot-air welding near a metal edge means the nozzle may reflect heat into your hands. Wear heat-resistant gloves and never weld near solvent adhesive vapours.
Step by Step
Step 1: Check the edge substrate and blocking
Edge metal is only as strong as what it is fastened to. Check that the wood blocking at the perimeter is sound, continuous and firmly fastened to the structure with the fasteners specified in the design, and that its top is flush with the insulation and cover board, so the flange sits flat and does not tent the membrane.
Use treated lumber or isolate the wood from the metal where required. Blocking that rots, shrinks or is loose will pull fasteners out in the first wind storm. Fix it before the metal goes on.
Step 2: Plan the layout and the joints
Edge metal is commonly sold in 10 foot lengths and needs a small gap at each joint, often about 1/4 inch, sometimes a bit more, to let the metal expand and contract with temperature. Steel moves less than aluminum, but on a long run it still moves enough to buckle a joint without a gap. Confirm the gap on the manufacturer's specification.
Mark where corners fall and use factory-formed or shop-formed corner pieces where they are available. Mitred or field-cut corners with open edges need their own cover strip or sealant detail. Plan the start and end so a short length does not land in a corner.
Step 3: Fasten the flange in a staggered pattern
Install the first length against a chalk line, with the drip edge hanging the specified distance beyond the face of the wall or fascia, often about 1/2 to 1 inch and sloped to drain. Fasten the flange through the pre-punched slots or holes, in a stagger, typically at 3 to 4 inches on centre, although the spacing set by the wind design governs.
Edge metal must resist wind uplift rated to the design. Many designs follow published edge-metal test standards, so do not substitute fasteners, spacing or metal gauge. Seat fasteners so the heads are flush and do not cut through the coating. Do not over-tighten.
Step 4: Leave the gaps and set the next length
Use a spacer of the specified thickness to keep the joint gap consistent and straight. Check each piece for alignment, because a length that is out of line shows badly along a whole roof edge and opens the cover strip.
Continue around the perimeter to each corner. At a corner, follow the maker's corner detail so the water running along one face cannot run into the gap at the corner.
Step 5: Weld cover strips over every joint
The gap between metal pieces is a path for water. A cover strip, usually a strip of TPO flashing membrane about 6 inches wide, is centred over the gap and welded to the coated metal on each side. Clean the surfaces, use a hand welder, and keep the weld continuous and at least 1-1/2 inches wide on each side of the joint.
Some systems also use a factory cover plate or a bead of sealant under the strip. Follow the manufacturer's detail. Round the strip corners and probe the welds after they cool.
Step 6: Weld the field membrane to the clad metal
Run the field membrane over the flange and past the edge on the face or drip edge as the detail calls for. Clean the clad surface of dirt and any protective film, make a test weld on a scrap of clad metal, and weld the membrane to the coating with a hand welder and roller.
Metal carries heat away from the weld, so settings that work on membrane to membrane may need adjusting on clad metal. The test weld and peel test will show it. Probe the full edge of the weld after it cools, including the ends and every cover strip.
Watch out: Do not weld the membrane to bare metal. Only the clad surface will fuse. Scratches that expose steel in the welding zone will leave a spot that will not weld and will rust.
Step 7: Finish corners, ends and the drip line
Install the end caps or corner pieces, cover strip them, and seal small gaps with the approved sealant. Check that the drip edge sheds water clear of the face of the wall or fascia, with a clean drip and no standing water behind the metal.
Walk the entire perimeter from the roof and from the ground. Look for gaps, fasteners that have backed out and lifted edges. A long run that is not straight hints at a blocking problem. For gutters or scuppers at the edge, see how to install a scupper.
Why Clad Metal Instead of Bare Metal
A bare metal edge is joined to the membrane with sealant or a cover strip of membrane that has to be adhered to the metal. Sealant ages and cracks, and it is a short-lived part of a detail that has to last as long as the roof. A clad edge changes the joint from a sealed one to a welded one.
- Advantages: the membrane is fused to the metal, there is no sealant line at the edge and the colour matches the roof.
- Limits: the coating can be scratched, the metal must still be fastened correctly and exposed steel at cut edges still needs a cover strip or seal.
- Not a wind shortcut: wind performance comes from the metal gauge, the cleat or flange and the fastening, not the coating.
Wind and the Fraser Valley
Perimeters and corners are the highest wind suction zones on a flat roof, and outflow winds in the valley and storm winds off the Fraser River test them every year. A loose edge metal length that flaps is the first sign of trouble, and one that lifts will peel the membrane with it. Check the fasteners and the blocking after every big wind event.
Common Mistakes
- Loose or rotted blocking. Fasteners cannot hold in weak wood, and wind lifts the edge and the membrane with it.
- No joint gap. The metal expands in the sun and buckles at the joint, tearing the cover strip.
- Fasteners too far apart. The edge flaps in wind and works loose, and the wind design is not met.
- Welding to bare steel. Only the coating welds. Exposed metal at a scratch or cut edge must be covered.
- No cover strip over a joint. Every gap is a path for water into the blocking.
- Skipping the test weld on clad. Metal draws heat away, and settings from membrane to membrane may run cold.
When to Call a Professional
Edge metal is a structural and waterproofing detail. The wind rating depends on the gauge, the fastening and the blocking, and a failure takes the membrane with it. It should be installed to the roofing manufacturer's detail and any design drawings.
Call a roofer if edge metal is loose, lifting or rattling in wind, if there are gaps along the drip edge, or if the membrane at the edge is peeling. Those are all problems that get worse quickly.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Install TPO-Coated Edge Metal: Questions
What is TPO-coated metal?
TPO-coated metal, also called clad metal, is galvanized steel with a layer of TPO laminated to its surface. It is made into drip edge, gravel stop, fascia and flashing. Because the coating is the same as the roof membrane, hot-air welding fuses the membrane to the metal and removes the sealant line used on bare metal edges.
How far apart should edge metal fasteners be?
Common spacing for the flange is 3 to 4 inches on centre in a staggered pattern, but the wind design for the building and the manufacturer's specification control it. Corners and perimeters usually get the tightest fastening. Never substitute the fastener type, spacing or gauge of metal without confirming the wind rating.
Why is there a gap between lengths of edge metal?
Metal expands and contracts with temperature. Lengths are installed with a small gap, often about 1/4 inch, so the joint can move without buckling. The gap is covered by a cover strip of membrane welded to the clad metal on both sides, which keeps water out while allowing movement.
Can you weld TPO to regular metal?
No. The membrane only fuses to the TPO coating, not to bare steel, aluminum or galvanized metal. Bare metal edges use sealant or an adhered cover strip instead. If clad metal is scratched through to the steel inside the weld zone, that spot will not fuse and needs a cover strip of membrane.
Related Guides
All TPO and PVC Roofing 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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