A low-slope roof in St. Louis usually ends at a metal edge that holds the gravel in place and gives the membrane something to finish against. That edge is called a gravel stop, and on many bids it appears as one word inside a larger line, often as 'edge metal' or 'perimeter.' If you are deciding what to do next about material and spec, the gravel stop should be written out as its own item. Its metal, its height and its flange width decide how the membrane behaves at the perimeter for the next several winters.
What the gravel stop is doing
A gravel stop is a formed metal angle or channel set along the perimeter of a low-slope roof. The gravel ballast sits on the field behind it. The vertical leg, called the face, stands up above the gravel line and keeps stones from washing over the edge in heavy rain. The horizontal leg, called the flange, runs back onto the roof and gets covered by the membrane or cap sheet. Water running across the membrane meets the metal and is carried over the edge rather than behind it.
Those two legs fail in different ways. The face takes hits from hail, from snow shovels and from mower decks on the ground side, and it bends. The flange fails where the membrane pulls away from it, usually at a corner or at a joint that was not sealed. That is where a leak can start under an edge that looks dry from the ground. A bid that names the metal but not the flange width leaves the second failure point unspecified.
Material and spec
Gravel stops are commonly formed from galvanized steel, painted steel or aluminum, and each one reacts differently to the membrane and to the air around it. Galvanized steel is strong and holds a face well, but its zinc coating needs to be kept away from copper and away from wet, untreated wood that can leach into it. Painted steel carries a coating that has to survive St. Louis sun and repeated freeze and thaw cycles, so the spec should name the finish system and the color, not only the metal. Aluminum resists corrosion well and bends easily, but it needs a fastener and a membrane contact that are compatible with it.
St. Louis weather also moves the metal. A long run expands in summer heat and contracts in winter cold, and the joints have to allow for that movement without tearing the sealant. Ice that forms along the edge in January can push against the face and loosen the fasteners over a few seasons. Spring hail can dent a face that looked straight at the end of the job. None of these problems is unusual, which is why the spec should name the gauge, the joint method and the fastener rather than leaving them to the crew's habit.
Ask for the metal type, the gauge, the face height above the top of the gravel, the flange width measured from the edge, and the finish. A line that reads 'gravel stop, 24 gauge' is close, but it still leaves height and flange open. The face height should be set with the ballast depth in mind, so the metal stands clear of the stones after they settle over time. A face that sits level with the gravel gives the wind and water little to push against except the stones themselves.
Lengths belong in the spec too, with the corners called out by name. A mitered corner, a welded corner and a butted corner with a lap seal each behave differently when the metal moves with temperature. Expansion joints are needed on long runs, and the spec should say where they sit and what cover goes over them. A joint left out of the drawing tends to show up later as a seam that opens in the first hard freeze.
Fasteners and sealant belong in the spec as well. Name the screw or nail type, the spacing, and the sealant at each joint. Sealant has its own cold limit and its own aging curve, so name the product the same way you would name the membrane. If the sealant is described only as a caulk or a roof sealant, nobody can check it after the job.
What it costs
A gravel stop is usually priced by the linear foot, plus any corners, end caps and expansion joint covers. The total depends on the perimeter length, how many turns the edge makes around parapets, drains and penetrations, and whether the metal is shop formed or bent on the roof. A short run around a flat addition costs differently from the same length around a building with several inside corners. Ask for each of these as a separate count, so the bid can be checked against the drawing or the roof walk.
Watch for a line that says edge metal included with no footage. That leaves you unable to compare two bids, because one roofer may have measured the full perimeter and the other may have measured only the open side. A written count in feet lets two bids sit side by side on the same terms. Metal prices move over time, so ask whether the quote is firm for the metal or whether the number can change if steel or aluminum rises before the order ships.
Removing an old gravel stop adds labor. If the old metal is painted with an older finish, is stuck under a parapet cap or has been fastened through the membrane, that work should be its own line. Ask whether removal and disposal of the old edge is in the price, because it is one of the places where a bid gets thin without anyone noticing.
Sealant, backer material and any cover for expansion joints are small items, but each one should be listed. A bid that leaves them out will usually add them at the end, when the crew is already on the roof and the schedule is set.
What to ask
- Which metal type and gauge is in the bid, and is the finish a named system with a color code?
- What is the face height above the top of the gravel, and does it clear the ballast depth on your roof?
- How wide is the flange, and how is it sealed to the membrane at each corner?
- How many linear feet of gravel stop are counted, and where are the corners, end caps and expansion joints?
- Where do the expansion joints sit, and what cover goes over them?
- What fastener and sealant are named, and what is the cold weather limit on the sealant?
- Is removal and disposal of the old edge metal inside the price?
A gravel stop that is named well in the bid is easy to check after the job. Walk the perimeter with the bid in hand and compare the face height to the gravel, the flange to the membrane and the joints to the sealant. If a line in the bid cannot be checked on the roof, it is not yet specific enough to hold the crew to.