A parapet is the short wall that rises past a flat or low-slope roof, and its top is the part of the roof system that many homeowners never look at. Water hits that top edge from above, runs down both faces, and every seam or joint in the cap is a place where it can get in. The piece that covers the top is called the coping cap, or simply the coping. On a St. Louis building it is usually metal, and the metal, its thickness, its joints and its overhang decide whether the wall top sheds water for years or starts leaking after the first hard freeze.
This page is for a homeowner who has a brick or block parapet on a porch, a bay, an addition or a garage, or who is comparing bids that include one. You do not need to know how the cap is fabricated. You need enough to write down what the crew will install, and to notice when a quote leaves the top of the wall out.
Material and spec
Coping caps are usually aluminum, painted galvanized steel, or a prefinished steel that comes with its paint system already applied. Aluminum resists rust and bends easily on a brake, which is why it shows up on so many parapets. It also expands and contracts more than steel across the same temperature swing. A St. Louis wall can see surface temperatures change by many dozens of degrees between a January morning in full sun and a freezing night, so an aluminum cap of any length needs joints that can move. Steel moves less, but it rusts at a cut edge if the coating is scraped, and a cut edge is what a crew makes at every joint.
The gauge or thickness belongs in the spec as a number. Aluminum coping on residential parapets is often written in thousandths of an inch, and the figure on a bid should match what the fabricator stocks as standard for that width. A quote that says only "metal coping" gives you nothing to hold the crew to. Ask for the alloy or steel grade, the thickness, and the finish. If the finish is a paint system, name the system and the color the same way you would name a shingle shade, because an unnamed color will be whatever the fabricator has on the shelf.
The cap also has to be sized to the wall. Its width should match the masonry plus the overhang on each side, so the top of the wall is covered end to end with metal that reaches past the face. The usual overhang on each face is an inch or two, and the spec should give a number, not the word "standard." The overhang keeps water from running down the face of the brick. A cap that is too narrow for a double-wythe wall leaves the top course exposed to every rain.
Slope and drainage
A coping cap should slope so water leaves the top of the wall and does not sit on it. Many caps pitch to one side, and others pitch to both sides from a center line. The spec should say which. A cap pitched toward the roof side sends water onto the low-slope membrane, which then has to carry it, so the membrane must be finished against the cap correctly. A cap pitched to the outside sends water down the face of the wall, which is acceptable if the drip edges are right and the wall below is built to take it. The choice belongs to the person who designed the detail, and the crew should not make it on site.
Drip edges on both faces should be named. A drip is a small downturned lip along the bottom edge of the cap that breaks the water away from the wall. Without one, water creeps back under the edge and runs down the brick, and staining at the top course is usually the first sign.
Joints and the membrane underneath
The joints in the cap are where most failures start. Some caps come in long pieces with a lap joint, where one piece overlaps the next and the overlap is either sealed or left to shed water on its own. Others come in shorter lengths with a formed joint or a slip joint designed to move. A caulked lap is only as good as its sealant, and sealants have a cold limit printed on the data sheet. A bid that calls for caulking a joint in late November may be asking for a sealant that will not cure properly. Ask whether each joint is lapped, sealed, or designed to move, and in what order the sealant goes on.
Under the cap, the membrane covering the flat roof should run up the inside face of the parapet and turn over the top of the wall or tuck behind a receiver. This is the base flashing. The cap's roof-side leg then sits over or against that membrane, so any water that gets past the cap lands on the membrane instead of the masonry. If the bid does not say how the membrane meets the cap, ask. The membrane type and its termination belong in the same spec as the cap, because the membrane you choose decides how it has to be finished at the top of the wall.
Fasteners and cleats matter too. Caps are held with hidden cleats or hooks at intervals, with exposed fasteners only where the design allows them. An exposed screw in a cap is a leak path unless it has a washer and sealant, and the washer will age faster than the metal around it. Ask whether the cap is attached with concealed clips that allow movement or with fixed fasteners. Fixed fasteners at every joint leave the aluminum nowhere to move, and the cap will buckle or split at the first warm week.
What it costs
Coping is priced by the linear foot of wall, and the price moves with the material, the width of the cap, the number of corners and the number of joints. A cap that turns three corners and has a dozen short joints costs more to fabricate and more to install than the same length of straight wall with one long run. Corners are the expensive part. Each one is a miter or a formed piece, and each miter is a seam that has to be sealed. If your parapet wraps a porch or a bay, ask the fabricator how many pieces the cap will be made from and where the joints will fall.
Sealant, cleats, end caps and drip edges are small line items that can disappear from a quote. Ask whether each one is inside the coping price or listed separately. Ask whether removal of an old cap is included, and what happens to the old metal. A parapet that had a cap before may have one that is bent, rusted or pulled loose, and the crew may need to remove it before the new cap can be set. That removal is part of the work, and it should be written down.
Membrane work at the parapet is often priced apart from the cap. If the flat-roof membrane has to be terminated, turned up or replaced where it meets the wall, that is a separate scope from the metal. A bid that prices the cap but not the membrane at the top of the wall has left a gap that someone will pay for later. Read the quote for both lines before you compare totals.
What to ask
- What is the cap made of, and what is its thickness in thousandths of an inch or as a gauge?
- What is the finish, and which paint system or coating does it carry?
- How wide is the cap, and what is the overhang on each face?
- Which way does the cap slope, and where is the drip edge on each side?
- Are the joints lapped, sealed, or designed to move, and which sealant is used?
- How does the membrane meet the cap, and is that termination in the same scope?
- Are the cleats concealed, and how many fasteners will be exposed?
- How many pieces will the cap be made from, and where are the joints?
- Is removal of the old cap included, and who hauls the old metal away?
Ask for the answers in writing, on the scope itself, so the cap is described the same way on the bid and on the job. Keys Roofing puts a written scope with photos and measurements in your hands, and the office number is (314) 220-2333 if you want to walk through the parapet on your building before you choose.