Nine Skylights, Two Hatches, and a Brake That Stops at Four
The membrane is on order, the insulation is stacked and strapped on the deck, and the only thing standing between the crew and a Monday start is a sheet that has to reach the metal shop before somebody locks the brake up for the weekend. Nothing on that sheet is hard. All of it is the kind of arithmetic that goes wrong quietly, because the boxes drawn as eleven small rectangles on the roof plan are, to a fabricator, eleven perimeters, eleven blank widths and forty-four corners — and the plan gives you none of those three.
Curbs are one of the few quantities on a low-slope roof that have no relationship at all to the roof's area. You can take the field off in squares, get the insulation and the cover board and the fasteners right to the pallet, and still not have counted a single millimetre of the metal that goes around the objects standing in the middle of it. That metal comes off the boxes themselves, face by measured face, and the boxes are rarely the size the drawing gave them. A unit gets substituted for a model one frame size up. A curb gets built out of whatever timber was on the truck. The hatch drawn at 750 by 900 turns up as 762 by 914, because that is what the manufacturer actually makes.
So what you send is a list of measured objects, not a derivation from a drawing. The rest of this is what belongs on that list, in the order a shop wants to read it: what each perimeter actually is, how wide the blank has to be cut, which pieces are lengths and which are counted items, and the two dimensions the shop cannot guess and will not think to ask about.
A curb detail taken apart
- Skylight unit or hatch lid — the factory item whose published outside curb dimension fixes the box below it, and which arrives whatever size the manufacturer makes rather than whatever the drawing asked for
- Unit skirt or integral cap flashing — the down-turned metal the unit brings with it; how far it reaches decides how much counterflashing you actually have to buy, and on some hatches it removes the item entirely
- Metal curb counterflashing — wraps the curb head and hangs down over the membrane turn-up with a hemmed free edge clear of the roof, so the two shed by overlap rather than by sealant Skylight Curb Perimeter Flashing Length Calculator
- Membrane base flashing — the waterproofing itself, turned up all four faces and mechanically held near its top edge; everything above it is a cover, not a barrier
- Insulation and cover board — the build-up packed around the base of the box, every millimetre of which is height the curb loses above the finished surface Roof Insulation Cover Board Calculator
- Curb and trimmed deck — the box and the framed hole it stands on, which are two different rectangles and get confused with each other on almost every order Skylight Rough Opening Calculator
Measure the Box, Not the Hole
There are three rectangles at every skylight and somebody on site calls all three of them the opening. The rough opening is the hole framed in the deck, dimensioned to the framing members. The curb is the box built up off that framing, and its outside dimension is larger than the rough opening by two curb wall thicknesses plus whatever the setting-out gave away. The unit's own frame is a third figure again, published by the manufacturer as the outside curb size the unit is built to sit down over. Flashing wraps the middle one. Take metal off the rough opening and every side arrives short by two curb wall thicknesses, which on a curb stick-built out of nominal two-by stock is a little over three hundred millimetres of missing perimeter per box once all four sides are added.
Chase the manufacturer's published curb size before anything else, because the whole detail hangs from it, and it is what changes quietly when a specification gets value-engineered. ASTM E2112 Standard Practice for Installation of Exterior Windows, Doors and Skylights is the practice document the trade works to, and AAMA/WDMA/CSA 101/I.S.2/A440 describes what the unit is rather than what your curb measures — neither will tell you the outside dimension of the box on your roof. Only a tape does that, and only after the curb is built.
Measure at the top of the curb, on all four faces, on the roof. A curb measured at its base and flashed at its head loses whatever the timber has twisted, and curbs twist: they are tall, thin, open boxes that spend a week in the weather before anyone goes near them. Run the diagonals while you have the tape out. Out-of-square barely moves the perimeter, but it is exactly what stops a fabricated corner piece from sitting down flat, and a corner that will not sit down is the one you will be sealing rather than lapping.
The perimeter arithmetic assumes a rectangle, and plenty of curbs are not one. Ridge skylights with a chamfered end, hatches with a return built out for a ladder head, two units set on one long shared curb — all of those get measured face by face and added, not put through a length-and-width formula. Where the box genuinely is rectangular, doubling the sum of the two sides is the whole calculation, and it is worth doing once per box in writing rather than eleven times in your head at the top of a ladder.
Two tape figures off the built box give the raw run for one skylight — and note that it is raw, with no waste in it, so it is the number you add allowances to rather than the number you order.
The length of the skylight curb.
The width of the skylight curb.
Flashing length needed
14 ft
Add a waste allowance for corner cuts and laps per the flashing manufacturer's installation instructions — this calculator gives the raw perimeter length only.
They open the calculator with your figures already in it
Skylight Curb Perimeter Flashing Length Calculator: 14 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- One number for four different pieces. Curb flashing is an apron at the downslope side, a side pan or step flashing up each slope, and a head piece with a counter over it upslope, and they are not interchangeable lengths — on a shingle roof the side dimension converts into a COUNT of step pieces keyed to the shingle exposure rather than a run of material.
- Gives no girth, and girth is what makes the joint watertight. How far the metal turns up the curb face and how far it laps out over the roof covering sets the coil width or the profile of the pre-formed piece; a perimeter says nothing about either, and the right length of the wrong-width stock is the usual way this order goes wrong.
- A curb much wider than 600 mm (2 ft) across the slope generally needs a cricket or saddle behind it, and that is separate metal with its own framing. Without one, water and debris bank against the flat back of the curb and stay there — the flashing is not failing, it is being asked to hold a pond.
The Width Nobody Writes Down
A shop cannot cut anything from a perimeter. What it needs is the developed width of the blank — the flat dimension the strip is sheared to before it goes near the brake — and that is a figure almost nobody puts on an order because it never appears on a drawing either. It is built up, not measured: the leg that returns over the curb head or tucks under the unit's frame, plus the face down the outer wall, plus the hem at the free edge, plus whatever the shop's own bend allowance takes at each fold.
Two of those are yours to decide and neither is obvious. The face height is the curb's height above the finished roof surface less the gap you deliberately leave at the bottom, because a counterflashing that touches the roof wicks water back up behind itself and holds silt in the gap for the life of the roof. The hem is what makes the free edge shed instead of running back along the underside, and it costs its own depth in blank each time the metal folds. Get the developed width wrong by fifteen millimetres and nothing is visibly amiss until the last box, when the flashing hangs low enough to sit in water.
The blank width is also what decides how much sheet you buy, which is a different question from how much length you need. Architectural flat stock is commonly supplied around 1219 mm — 48 in — across, so a blank in the region of 240 mm yields five strips across a sheet with only the trim cut to spare. Widen that blank by twenty millimetres to cover a taller curb and the yield drops to four. The run has not changed by a single metre and the sheet count has gone up by a quarter. Metal for curbs is bought as sheets and cut as strips, and the two numbers only meet at the girth.
| Element of the blank | What fixes it | Worked example |
|---|---|---|
| Return over the curb head, or leg under the unit frame | curb wall thickness and what the unit's own frame leaves clear | 50 mm / 2 in |
| Face down the outer wall | curb height above the finished surface, less the gap at the bottom | 175 mm / 6 7/8 in |
| Gap held above the roof surface | kept open so the hem sheds rather than wicks — subtracted, not added | 25 mm / 1 in |
| Hemmed drip at the free edge | adds its own depth to the blank once per fold | 15 mm / 5/8 in |
| Developed blank width | the return, the face and the hem, before the shop's bend allowance | 240 mm / 9 1/2 in |
| Strips from one 1219 mm / 48 in sheet | blank width into sheet width; five of these leave 19 mm over | 5, or 4 where both edges get trimmed |
Corners Are Counted, Not Measured
Perimeter buys you straight metal and nothing else. Every box has four external corners, and in sheet metal a corner is a fabricated unit — mitred and soldered, welded, or made as a one-piece pressing — because a corner folded out of a continuous strip leaves a pinhole at the base of the fold that sealant closes for a season and then stops closing. Eleven boxes is forty-four of those, priced individually, and they are the line most often missing from an order that was built from a length.
Splices are the other counted item. A run of stock length divided into a perimeter almost never lands evenly, so decide where the joints fall rather than letting the offcuts decide: mid-face, never within a hand's width of a corner unit, lapped in the direction water runs and left free enough to move. Single-ply systems add a parallel count of their own, because the membrane corners at a curb are moulded pieces rather than something cut on the roof, and that is four per box before anybody opens a roll.
A Hatch Is a Curb With a Lid and a Ladder Underneath
The arithmetic at an access hatch is identical and the life of the detail is not. A plant curb gets flashed and then ignored for twenty years. A hatch curb gets knelt on, leaned against, stepped over and dropped onto every time somebody comes up with a bag of tools, and the membrane in the first metre around it takes abrasion that nothing else on the roof suffers. The same detail that lasts the life of the covering beside a chiller wears through beside a hatch that opens twice a week.
Check the hatch schedule before you cut anything, because a large share of hatches ship as a factory unit complete with an insulated curb and an integral cap flashing already turned down over it. Where that is what has been bought, the metal you were about to order is already on the delivery, and what you need instead is the membrane turn-up beneath it and the fastening that holds it — which is a different line, a different supplier and a different lead time. Ordering counterflashing for a hatch that came with its own is a small, invisible waste that repeats on every hatch on every job until somebody reads a submittal.
Allow for waste explicitly at a hatch rather than mentally. Four corners each need cut and folded material, the run has to lap where it closes on itself, and the last stock length always leaves something short of useful. That is why the hatch quantity here is a perimeter carrying an allowance while the skylight one is a bare perimeter: the two answers are not comparable side by side, and reading them as though they were is how a job ends up with waste counted twice on some boxes and not at all on others.
One more thing belongs on the hatch line and is not flashing. An open roof hatch is an unguarded floor opening, and fixed guarding around the opening with a self-closing gate at the ladder head is a requirement in most jurisdictions rather than an upgrade — OSHA 29 CFR 1910 Subpart D Walking-Working Surfaces covers it in United States general industry, with ANSI/ASSP A1264.1 Safety Requirements for Workplace Walking/Working Surfaces and Their Access giving the fuller treatment of roof openings. It is not your metal quantity. It belongs on the same order sheet all the same, and it is what gets discovered once the scaffold has gone.
Feed the walked perimeter of the hatch curb with an allowance that reflects its four corners and its closing lap, and keep the answer on its own line rather than merged with the skylight figures.
SettingsSettings for this calculation
Waste is set to 8% by hand. Pick a tier above to replace it, or keep your own figure.
The perimeter of the roof access hatch curb.
Extra material to allow for corner cuts and overlaps.
Hatch curb flashing needed
21.06 ft
A roof hatch is a curb that people stand on, open and climb through, so its flashing is exposed to mechanical wear that a plant curb never sees. The quantity follows the hatch size; the detail has to survive traffic.
They open the calculator with your figures already in it
Roof Hatch Curb Flashing Calculator: 21.06 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Excludes the hatch unit itself, its insulation, and the safety railing or grab post that most access regulations now require around a roof opening.
- Does not cover the ladder or stair below, or the structural trimming of the deck opening.
- Where the hatch sits in a walkway route, a protection layer or paver walkway around it is a separate item and is what stops the membrane wearing through.
The Route People Take When They Come Out of It
Watch where the boots go. Everyone who comes through the hatch walks the same line to whatever they came up to service, and after a few winters that line is visible from the ladder: compressed insulation, a scuffed surface, and a shallow depression that holds water for a day after rain. The flashing around the curb is rarely what fails there. The membrane a metre away from it is.
Protection along that route is cheap, it is ordered from the same supplier on the same day as the metal, and it is the one line on the sheet that prevents damage rather than repairing it. Size it to the path people actually take, not to a tidy rectangle around the hatch, and run it to the plant rather than stopping at the edge of the curb detail.
Take the path as an area — the width two people pass in, along the line the boots really follow — and convert it to panels at whatever the product's own sheet says one panel covers.
The total area of the protective walk path to be covered.
The surface area covered by one walk pad panel.
Walk pad panels needed
25 panels
They open the calculator with your figures already in it
Roof Walk Pad Protection Panel Calculator: 25 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Treats the path as solid coverage, and walk pad is not laid solid. Pads are set with gaps between them so water crossing the roof passes through the run instead of damming behind it — a continuous strip laid across the drainage path ponds water on its uphill side, which is the exact condition the pads were there to keep off the membrane. Those gaps also mean a given route needs fewer panels than a straight area division suggests.
- Says nothing about whether the pad will bond to your roof. Walk pad is chemistry-matched to the membrane: a PVC pad will not heat-weld to TPO, and an adhered pad on a mechanically attached or ballasted roof needs the membrane manufacturer's own attachment detail. This is not merely a loose pad — putting a foreign product onto a warranted membrane is one of the routine ways a roof warranty is voided.
Eleven Perimeters Into One Order
Now the sheet assembles itself, and the discipline is to keep unlike things apart. Metal is ordered by profile, and a profile is a blank width plus a set of bends. Two curbs standing at different heights above the finished surface need different blank widths, which makes them two profiles however similar they look on the plan, and their lengths do not add together. Sort the eleven boxes into groups that genuinely share a girth first, then total the run within each group.
Add the laps before dividing, not after. A run that closes on itself needs an overlap where it meets, and every splice along the way needs one too, so the figure that goes into a stock-length division is the bare perimeter plus those overlaps — not the perimeter alone. Dividing first and adding laps afterwards is how an order comes back one length short on the largest box, which is always the one with the least tolerance for a joint in an awkward place.
Then convert to what is actually sold. Stock arrives in fixed lengths, commonly around 3.05 m or ten feet for brake-formed pieces, and a run that needs three and a bit lengths needs four. Doing that conversion per profile rather than across the whole job is the difference between an order that arrives complete and one that arrives with enough total metal and the wrong pieces in it.
- Walk the roof with the schedule and record each box by name, not by type — hatch A, skylight 3 — with its four measured faces.
- Establish the finished roof surface at each box, after the new build-up, and take the curb height from that surface rather than from the deck.
- Group the boxes by the blank width their curb height produces, and treat each group as its own profile.
- Total the perimeters within a group, then add the closing lap and every splice lap to that total.
- Divide the lapped total by the stock length the supplier actually cuts, and round up within each group.
- Count the corner units and any splice plates separately, four corners per box, and price them as items.
- Add the material specification, the gauge and the finish once per profile, and state which face is the exposed one.
Run each profile's lapped total against the stock length the shop cuts from, one group at a time, so the answer is a number of pieces the fabricator can pull rather than a length somebody has to interpret.
The combined length of all flashing runs needed on the job.
The length of a single flashing sheet or cut coil piece as supplied.
Flashing sheets needed
5 sheets
They open the calculator with your figures already in it
Exterior Flashing Layout Trim Sheet Calculator: 5 sheets — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Rounds up on the total when each run has to come out of one sheet. Five separate 3.5 m (11.5 ft) runs from 3.05 m (10 ft) stock take ten pieces, not the six that dividing the combined 17.5 m (57 ft) suggests, because offcuts do not join end to end. Run this per length of flashing rather than per job total.
- Says nothing about coil width. Flashing is brake-formed from flat stock, so the stock has to cover the developed girth — every leg, plus hems and the metal taken up in each bend — and coil comes in fixed widths. The right number of pieces in stock too narrow to fold the profile is not usable material.
- Blind to the metal. Aluminum against pressure-treated lumber or in contact with a copper gutter corrodes at that contact, gauge decides whether a long run stays flat or oil-cans, and pre-finished coil carries a coating that will not survive being folded the wrong way. A sheet count chooses none of that.
The Two Dimensions the Shop Cannot Guess
The first is the height of every curb above the finished roof surface, which moves during the job without anybody deciding to move it. A curb detailed to a deck level and then re-roofed over with a hundred millimetres of new insulation and cover board has lost a hundred millimetres of everything that mattered: the face the metal covers, the room the membrane had to turn up, and the clearance between the free edge and standing water. NRCA's low-slope guidance and the membrane manufacturer's own details both set a minimum height for the base flashing measured from the finished surface, not the deck; BS 6229 does the equivalent job for flat roofs in United Kingdom practice, and the residential code sets its own minimum curb height for curb-mounted skylight units unless the unit's instructions supersede it. Whichever of those governs on your job, it is measured from the surface the crew will be standing on when they finish, and it has to be re-derived rather than carried across from the old roof.
The second is what the metal is. Gauge, alloy or coating designation, finish and which side is exposed all have to be stated, because a shop with a blank width and a length will otherwise cut whatever is on the rack. ASTM A653/A653M covers zinc-coated steel sheet, ASTM A792/A792M the aluminium-zinc coated equivalent, ASTM B209/B209M aluminium sheet and plate, and ASTM B370 copper sheet and strip for building construction; the gauge, the cleating and the joint spacing come from the SMACNA Architectural Sheet Metal Manual and the fabricator's tested details rather than from preference. Compatibility is the trap on a re-roof, where the new curbs meet old metal: copper runoff onto galvanised or aluminium, or fasteners of one metal through another, produces a corrosion problem that looks nothing like a flashing failure and gets diagnosed as one.
Handedness sits between those two and belongs on the same sheet. State which way the hem faces, whether the return goes over the curb head or under the unit flange, and — where the roof has any fall at all — which face of each box is the upslope one, because that is the face whose detail matters most and the one nobody can reach once the unit is set. Where a box is wide enough across the fall to hold water against itself, the diverter behind it is a separate fabricated item on a separate drawing and is sized from the width and the slope, not from the perimeter you have just taken off.
Before It Goes to the Brake
Read the sheet back against the roof one more time, out loud if there is anyone to read it to. Eleven boxes named individually, each with four measured faces and a curb height taken from the finished surface; a profile group for each distinct blank width; a lapped run and a piece count per group; forty-four corner units; the membrane corner pieces; the pads for the walk route; and the specification of the metal stated once per profile. Anything on that list you are quoting from memory rather than from a tape is the thing that will be wrong.
Then keep it. A curb is worked on again years later by somebody who was not there, and the fact they most need — what the developed width was and what height it was cut for — exists nowhere on the finished roof, because it is folded up inside the detail. Photograph each curb before the counterflashing goes on, with a tape in the frame showing the height above the surface, and file those with the cut list. A minute per box on the day is what separates a future repair that matches from one guessed at from the outside.
- Confirm every curb height was measured from the finished surface, after the build-up, and not from the deck or the old roof.
- Check each hatch submittal for an integral curb and cap flashing before ordering counterflashing for it.
- State the blank width, the bend schedule and the exposed face for every profile, not just the length.
- List corners, end pieces and splice plates as counted items with their own quantities.
- Note which face of each box is upslope, and flag any box wide enough across the fall to need a diverter behind it.
- Photograph each curb with a tape in shot before it is covered, and file the images with the cut list.
The sheet the metal shop needs before Friday
Three quantities and one specification. The perimeters come off the boxes, the blank width comes off the curb height, the corners are counted, and none of it comes off the roof plan.
- Perimeter of every box, measured at the curb head — Four faces added rather than a length-and-width formula wherever the box is not a plain rectangle; diagonals checked while the tape is out.
- Curb height above the finished roof surface — Taken after the new build-up is on, per box, because it sets the blank width, and a re-roof changes it without announcing anything.
- Developed blank width, per profile group — Return over the head, plus the face, plus the hem, before the shop's bend allowance — and it is what decides the sheet count, not the run.
- Lapped run and piece count within each group — Closing lap and splice laps added before dividing by the stock length, then rounded up group by group rather than across the job.
- Corner units, end pieces and membrane corner patches — Four external corners per box in metal, counted and priced individually, with the moulded membrane corners counted alongside them.
- Metal specification stated once per profile — Gauge, coating or alloy designation, finish, exposed face, and a compatibility check against whatever old metal the new work touches.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.