Detailing a Parapet and the Cricket Behind a Curb

A Cantilever Wearing a Hat

Before any of the metal is priced, establish what the wall is. A parapet has nothing above it to brace against, so it is a vertical cantilever fixed only at the roof line, and it is loaded in both directions. ASCE 7 'Minimum Design Loads and Associated Criteria for Buildings and Other Structures' treats it as a components and cladding element and applies pressure to the windward and leeward faces at the same time — which is why the parapet at a building corner, sitting in the roof's highest-pressure zone, is the length that comes off in a storm while the run along the middle of the elevation stays put.

Slenderness is the other question, and on an older masonry building it is the one that stops the job. An unreinforced parapet standing tall relative to its thickness is a well-documented falling-debris hazard in earthquakes; a height-to-thickness ratio of around three is the conservative screening figure quoted in retrofit programmes, with the limit that actually applies coming from the jurisdiction and the Seismic Design Category rather than from a rule of thumb. TMS 402/602 'Building Code Requirements and Specification for Masonry Structures' governs the design where masonry is being assessed or rebuilt, and BS EN 1996-1-1 Eurocode 6 does the equivalent job in Europe. A bad screening ratio means bracing back to the roof structure, reinforcement, or taking height off the wall — none of them decisions a roofing contractor makes with a skip already on the pavement.

How tall the parapet has to be in the first place is not a roofing question at all. Required parapet height above the roof surface, and whether the wall carries a fire-resistance rating, come from the adopted building code's exterior wall and fire wall provisions — Chapter 7 territory in the International Building Code, not Chapter 15. A parapet that is being raised or lowered as part of a re-roof has crossed out of the roofing scope and into the building's, and the drawing has to say so before the coping is ordered to a length.

Coping Is Measured on the Centreline and Fails at the Joints

Take the coping run off the centreline of the wall head, not the inside face and not the outside. On a rectangle the difference is small; on a building with six returns, a stair bulkhead and a stepped party wall it is not. Walk the whole perimeter and record it as a list of straight runs with a corner between each, because that list — not the total — is what the fabricator quotes from.

Then price the run and count the joints separately, because they are different problems. The run is straightforward: centreline length plus an allowance for cut waste and laps, five per cent being reasonable on long straight walls and more where a parapet is broken by returns, upstands and expansion joint transitions that each generate an offcut. The joints are where the detail actually lives. Coping does not leak along its length; it leaks at every splice, every mitred corner, every end unit and every fastener somebody drove through the face because the cleat would not hold.

Metal moves, and coping moves more than most roof metal because it is the hottest thing on the building. A wall head that reads below freezing in January and seventy degrees Celsius on a still July afternoon is working through an eighty-kelvin swing. Aluminium expands at roughly twenty-three micrometres per metre per kelvin and galvanised steel at roughly twelve, so a three-metre aluminium length grows about five millimetres between those two days and a steel one about three. Fix a run rigidly at both ends and that movement has to go somewhere: it goes into buckled faces, opened mitres and fasteners that elongate their own holes until they leak. Standing seam or covered joints at the spacing the fabricator specifies, with the cap free to slide on its cleat, is the detail; a butt joint sealed with mastic is a maintenance item with a date on it.

Two things about the profile itself are not negotiable. The cap falls inward, toward the roof, so that whatever lands on it is delivered onto a surface built to receive water rather than onto the facade — an outward fall stains the elevation below and, at any open joint, drives water into the head of the wall. And both edges get a drip or a hemmed kick standing far enough proud that surface tension cannot walk the water back underneath. For the metal, ASTM A653/A653M covers zinc-coated steel sheet and ASTM B209/B209M covers aluminium sheet and plate, while gauge, cleat spacing and joint type come from the SMACNA 'Architectural Sheet Metal Manual' and the fabricator's own tested details. Where the coping is stone or precast, the same logic runs through a continuous damp-proof course beneath it, to BS 8215 'Code of practice for design and installation of damp-proof courses in masonry construction' or the local equivalent.

Uplift at the edge is the last thing the quantity has to answer to. ANSI/SPRI ES-1 'Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems' tests the cap and its continuous cleat as one assembly, and a cap fitted to a cleat it was not tested with is an untested edge whatever the two parts cost. Existing cleat does not get re-used under new coping, and coping does not get re-used over a thickened roof, because the face dimension the test assumed has changed.

Where the Membrane Stops Climbing

The membrane turns up the inside face of the parapet and stops, and where it stops is the single most consequential dimension on the perimeter. Base flashing has to terminate above the highest level water will ever stand against that wall — not the level it stands at on a good day, but the level it reaches when the primary drains are blocked and the roof is loading up to the invert of the overflow. Around two hundred millimetres, or eight inches, above the finished roof surface is the customary minimum in NRCA guidance and in most manufacturers' details, and it is a minimum rather than a target: a roof whose overflow invert sits high, or whose build-up has just been thickened by a hundred millimetres of new board, has eaten into that dimension before anybody measured it.

Hold the top edge mechanically. A termination bar run continuously along the top of the flashing, fastened at the centres the membrane manufacturer specifies into a substrate that will actually hold a fastener, with a sealant bead behind or above its top edge, is what stops the sheet peeling away from the wall in the first wind event. The bar is a perimeter quantity in the same sense the coping is — the full roof perimeter plus every return, every reveal at a stair bulkhead, every step in a party wall — and it is one of the items that habitually gets counted as a rectangle and arrives short. Counterflashing or through-wall flashing then laps down over the bar so the two overlap in the direction water travels; sealant across that lap is secondary, never the barrier.

The substrate under the flashing decides more than the fastener schedule. Old masonry parapets are often soft, hollow, or rebuilt in a different unit halfway along, and a bar fixed into a header course that turns out to be a half-brick veneer holds nothing. Where the wall will not take a reliable fastening, the answer is a plywood or treated timber backing fixed back to something structural, or a cant and curb built off the deck, rather than tighter fastener centres into the same poor material. FM Global Property Loss Prevention Data Sheet 1-49, 'Perimeter Flashing', is the reference to reach for when the edge is being designed rather than repaired, and it is worth reading before arguing with an insurer about a blown-off perimeter.

1. Establish the finished roof surface level at the wall first, including any new build-up, before measuring anything vertical.
2. Set the base flashing height from that new surface — above standing water at the overflow invert, and never below the membrane manufacturer's stated minimum.
3. Probe the parapet for a fastenable substrate along the whole run, not at one convenient spot near the ladder.
4. Fit the cant or transition the membrane system requires at the base of the turn-up, where the system calls for one.
5. Run the termination bar continuously, fastened at the specified centres, with the sealant bead in the position the detail shows rather than the one that is easiest to reach.
6. Lap the counterflashing or through-wall flashing down over the bar, then fit the cleat and cap over that — in that order, never the reverse.

A Curb Is a Box Standing Across the Flow

Out in the field, the same vertical arithmetic repeats at every curb. A rooftop unit, a hatch, a skylight or a duct penetration sits on a fabricated box, the membrane turns up that box and terminates near its top, and the unit's own flashing or the hatch's own skirt laps down over the termination. The dimension that matters is the curb's height above the finished roof surface, which is the same eight-inch conversation as at the parapet, and which is why re-roofing over an existing curb without extending it is the most common way a perfectly good roof acquires a permanent leak at its largest unit.

Flashing quantity comes off the box, not off the roof. Measure the curb's perimeter — all four faces, including the upslope face nobody can reach because the unit overhangs it — and multiply by the flashing height plus the lap onto the field and the return over the top. A curb measuring one-point-two by zero-point-eight metres is four metres of perimeter before any allowance, and a roof with nine units on it has thirty-six metres of upstand that never appeared on the area take-off. Add the corner patches, which on most single-ply systems are prefabricated pieces counted individually rather than cut from sheet.

Then look at where the curb sits relative to the flow. A box set square across the direction water runs holds everything arriving from upslope against the one detail on the roof least able to argue — a terminated upstand with a mechanical fastening near its top edge. Long, low equipment rails are worse than tall compact units, because they dam a wider front and the pond behind them stays shallow enough to be invisible until it freezes.

Sizing the Cricket That Takes Water Around It

A cricket is the only slope on a low-slope roof that gets built by hand to a drawing, and its geometry is plane trigonometry with no code figure anywhere in it. The ridge runs up the centreline of the curb's upslope face, and each half of the cricket falls sideways from that ridge to the curb's corner. So the thickness at the curb is half the obstruction width multiplied by the cricket's slope ratio, and nothing else. A two-point-four metre curb with a half-inch-per-foot cricket needs fifty millimetres at the wall of the box; the same curb with a one-in-twelve cricket needs a hundred.

That thickness gets built out of the same tapered stock as the field, which is where the second number comes from. A tapered board of a given length gains board length times the slope ratio across itself, so the courses per side are half the obstruction width divided by the board length, rounded up. It matters because tapered board is ordered as a schedule of panel types, not as an area, and a cricket that needs two courses per side is a different line on the order to one that needs one. Polyisocyanurate stock is specified under ASTM C1289 'Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board'; the taper schedule is a shop drawing produced against it.

How steep to build the cricket is a specification decision and belongs to whoever wrote the roof spec, not to the crew laying it. The reasoning behind building it steeper than the field is drainage speed: the cricket is asking water to travel sideways across a surface that is already barely moving it, over a short run, having lost whatever head it had. A cricket built at the field slope will technically drain and will practically hold a film of water and a line of silt around the curb for the life of the roof. NRCA's membrane roofing guidance describes crickets and saddles behind curbs; the slope number itself is a project decision, which is why it is an input rather than an assumption.

The check that catches bad crickets is what happens at the far end. The ridge has to lose its entire thickness over the run available upslope before it dies out into the field — and the deck beneath it is rising as it goes, because the field slopes down toward the curb. If the ridge sheds thickness faster than the field rises beneath it, the ridge tips backward toward the curb and what has been built is a shallow basin with a diverter's shape. The run available is set by whatever comes next upslope: another curb, a drain sump, a wall, a movement joint. It is a measured dimension on a survey, not an assumption, and it is the input people leave at its default.

Width is the other habitual error. A cricket cut flush to the corners of a curb has not removed the standing water, it has moved it from the middle of the upslope face to the two corners — which is exactly where the prefabricated corner patches sit and where a single-ply system has its most-worked detail. Span the obstruction plus enough beyond each corner for the diverted water to rejoin the field, and where two curbs sit close together across the flow, one cricket over the pair beats two with a dead flat pocket between them.