The length goes in the box upside down, and stays that way all day
The mitre box is on a pair of trestles in the middle of the room, a three metre length of gypsum cove lying across it, and the question holding everything up is not what angle to cut. The box offers one angle. It is which way up the cove sits in the box, and which of the two pieces either side of the blade is the one you carry up the ladder. Get the first wrong and you have cut a flawless corner for a room you are not standing in. Get the second wrong and you have the mirror of the piece you wanted.
A mitre box has a vertical fence and a flat base, and those two surfaces are a wall and a ceiling at arm's length — except that the base is underneath you and a ceiling is not. So the cove goes in inverted. The face that will bed against the ceiling lies flat on the base of the box; the face that will bed against the wall stands against the fence; the moulded front points up at the light. Every question of hand that follows is answered relative to that one arrangement, which is the reason it is never varied. Not for the awkward length, not for the piece cut on the landing because the box is two floors down, not once in the whole job.
Which way up the cove itself goes is a separate question, and it catches people out on plain profiles. A simple quarter-hollow is symmetrical and does not care. Plenty are not: an ogee, a stepped C-section, anything with a bead worked into one flat carries a wider land on the ceiling face than on the wall face, and reversing it changes both how far the cove drops down the wall and how far it reaches across the ceiling. Every mitre after that is cut for a different moulding. The habit that prevents it costs nothing — pencil the ceiling edge of every length while it is still on the floor.
The blade matters more than the saw. A fine-toothed hardpoint leaves gypsum cove a clean paper edge, where a coarse one tears the paper on the underside of the cut and that torn edge will not take filler. Polystyrene wants a sharp blade rather than a fast one, and a knife on the small returns. Fibrous plaster is sawn with its canvas backing cut through separately, because a saw drags hessian rather than severing it.
Four corners out of one slot
The whole hand problem comes from one fact: a cove stands off the corner, so its two back edges do not arrive there in the same place. The wall edge reaches the junction of the two walls. The ceiling edge lies out on the ceiling, set forward by however far the profile reaches, and meets its neighbour at a point out in the room. That offset makes the wall edge of each piece the longer one at an internal corner, and the ceiling edge the longer one at an external, where the two lines cross beyond the arris rather than short of it. It is a test you can apply to a piece lying on the floor without measuring anything.
So there are not four settings to remember. There is one — the 45 degree slot — and four outcomes depending on which end you are cutting and which half you keep, every one of them checkable by looking at which edge overruns. An internal that comes out of the box with its ceiling edge long is an external mitre. An external with its wall edge long is an internal. Both are recoverable on the trestles and neither is once it is bedded. How much length each external corner consumes is worked through for sprung mouldings in the crown guide.
The box gives you 45 degrees and nothing else, which is a limitation only if you expected the corner to be square. It will not be. What saves the coving fitter and does not save the joiner is that the moulding and its filler are the same family of gypsum: a 45 cut into a 93 degree corner opens a wedge on the face, and the wedge is filled with what the cove is made of. Measuring the corner and cutting to it is still better work — it is simply not the difference between a good joint and a failed one, which is why a mitre box survives here and would be laughed off a second-fix van.
- Set the length in the box the same way every time — ceiling face on the base, wall face on the fence, moulding pointing up.
- Pencil the ceiling edge of every length while it is still flat on the floor, so the orientation survives being carried.
- Cut with the offcut supported, not hanging, because gypsum cove breaks at the mitre under its own weight in the last few teeth.
- Before anything is bedded, lay the piece down and read which edge runs long — wall edge for an internal, ceiling edge for an external.
- Dry-offer every mitre to the piece it will meet, on the floor if you can, and take the high spots off with a rasp rather than the saw.
- Seal the cut ends of gypsum cove with a smear of the adhesive as you go; a raw paper edge left overnight will not take filler cleanly.
| The corner | The edge that runs long | Where the cut face looks | The check before it leaves the trestles |
|---|---|---|---|
| Internal, first length fitted | The wall edge — it carries on to the junction itself | Backwards, away from the room | Stand it up: the bottom edge should reach the corner while the top edge stops short of it |
| Internal, second length fitted | The wall edge again | Backwards | Its cut face has to land on the first piece's cut face, not on the wall behind it |
| External, both halves | The ceiling edge — it crosses past the arris | Forwards, into the room | The bottom edge stops at the arris and the top edge hangs beyond it |
| Butt joint mid-run | Neither — both ends square | Square across the section | No hand at all, so either offcut will serve and the joint can move to suit the wall |
| One length with an internal at one end and an external at the other | A different edge at each end | Opposite ways | Mark the ceiling edge before either cut, because the piece is not symmetrical and cannot be turned round |
A filled joint and a fitted joint are not the same trade
Anyone arriving from second-fix joinery is carrying a rule that does not apply here. In timber a joint has to fit, because nothing you can put into a gap ages the way the timber either side of it does — the reason internal corners in skirting are scribed rather than mitred, argued out in the skirting and architrave guide. Coving reverses the premise. The cove is gypsum behind a paper or canvas face, and the material closing the joint is a jointing compound or gypsum filler of the kind BS EN 13963 covers. Joint and moulding become one thing under a coat of paint.
That reversal is genuinely load-bearing on this job. It is why a mitre box with fixed 45s is still the standard tool on a wall nobody built square. It is why a running joint in the middle of a long wall is a square butt rather than a bevelled lap — two ends cut off, pushed together, filled, sanded, invisible. And it is why the piece that closes a room does not have to be sprung in: cut it a couple of millimetres short of the measurement, bed it, and fill the two ends.
The reversal has a limit and it is worth knowing where. A hairline closes with one pass of filler and disappears. Two or three millimetres takes two passes and still disappears. Past that you are no longer filling a joint, you are modelling a missing section of cornice out of a material that shrinks as it dries, and it will craze in the first heating season — a crack that runs along the mitre and looks exactly like a structural one to whoever finds it. On a moulded cornice the limit arrives sooner still, because filler will close a gap but it will not reproduce a run of enrichment across it. When a mitre comes out that far open, the honest answer is to cut a new piece rather than to bury the old one.
None of that makes the cut unimportant, because two faults are not fillable. One is a cut out of square through the section rather than in plan: the pieces then sit at different angles to the wall and the profile steps across the joint, which filler cannot hide because the shadow line steps with it. The other is a piece short overall, which drags the error into the next corner along.
The return that has to meet what is already on the wall
The awkward piece in most rooms is the one with an internal at one end and an external at the other — the return round a chimney breast or a boxed stack, cut after the length it meets on the main wall is bedded and going off. It is awkward for a reason that has nothing to do with angles. Both ends are cut to work already fixed, so neither end is forgiving; and because the two ends run long on different edges, the piece cannot be turned round if the first is cut on the wrong hand.
Take the measurement off the work rather than off the wall. At the internal end the dimension runs to the face of the mitre already sitting there, which on that end is the wall edge, so hook the tape at the bottom edge of the fixed piece where it meets the corner. At the external end it runs to the arris along the wall, with the ceiling edge carrying on past it. Write both on the piece with your ceiling-edge mark still visible, and cut the internal first: it is the end that has to land tight on something already there, and a long one can be taken back a millimetre at a time, where the external can absorb a small error into the mitre pair.
The external pair is where the two trades part most sharply. A joiner glues and pins an external mitre into one component on the bench and offers the corner up whole; that answer is not available here, because the adhesive is not a joint aid but the bedding, and nothing will hold two lengths of cove together in mid-air while it grabs. So the external is built on the wall: bed the first leg, let it take, then cut the second to the corner that now exists rather than the one you measured. If the first has settled a millimetre proud, the second is cut to the proud one and the mitre closes.
One trick is worth more than any of this. When you cut the first leg of an external, the offcut is the mirror of the second leg. It will not be long enough to use, but it is an exact template for the hand and the angle of the cut you are about to make, and holding the two together on the floor for two seconds catches the reversed mitre before it costs you a length.
What the cove is made of decides how it goes up
Four quite different products are sold under the same word, and the differences that matter on the day are weight and what the adhesive has to do. Paper-faced gypsum cove — the preformed plasterboard cornice covered by BS EN 14209 — is light enough that a bed of gypsum cove adhesive on both back edges is the entire fixing, with nothing mechanical at all. Fibrous plaster cornice is a different animal: a plaster casting on a hessian and timber backing, heavy enough that it is screwed or plugged into the structure and the adhesive only closes the bed. Putting a run of fibrous cornice up on adhesive alone, because that is how the cove in the last house went up, is the failure that arrives six months later in one piece.
Polystyrene coving is the one people forget is a plastic fixed to a ceiling. Its reaction-to-fire performance is a property of the product — classified under BS EN 13501-1, or from a surface spread of flame test to BS 476-7 in the older British system — and in England and Wales what it has to satisfy is the internal linings guidance in Approved Document B. In North America the classification comes from ASTM E84 and the interior finish provisions of the International Building Code. The class belongs to the product and the maker publishes it; nothing here can tell you what a given cove is, and a decorative moulding is exactly the thing that gets chosen from a picture. The other trap is chemical rather than regulatory: solvent-based adhesives dissolve expanded polystyrene, so the fixing has to be the one the maker names.
| Cove or cornice | What governs the product | How it is held | The thing it will punish |
|---|---|---|---|
| Paper-faced gypsum cove | BS EN 14209 for preformed plasterboard cornices | Bedded on gypsum cove adhesive along both back edges | A substrate that is painted, papered or dusty — the adhesive is the whole fixing |
| Fibrous plaster cornice | BS EN 13279-1 for the gypsum binder; the fabricator's drawings for the profile | Screwed or plugged to structure, with the bed closing the gap | Being treated as if adhesive alone will hold it |
| Expanded polystyrene | A reaction-to-fire class published by the maker under BS EN 13501-1 | The maker's own adhesive — solvent-free, because solvent eats the foam | A fire class that was assumed rather than checked |
| Rigid polyurethane and duropolymer | Manufacturer literature; ASTM E84 classification in North America | Adhesive plus temporary pins while it grabs | Movement — it expands and contracts more than gypsum on a long run |
| Timber and MDF cove | Sawn, machined and nailed like any other running trim | Pinned into a ground or a nailer behind the line | Nothing here — it is a joinery job, and the skirting guide covers it |
The surface it has to stick to
Cove adhesive bonds to plaster. It does not bond to emulsion, it does not bond to wallpaper, and it barely bonds to the loose dust that a sanded skim leaves behind. That is the single most common reason a run of coving comes off a wall, and it is entirely designed out before anything is cut, by setting the line first and clearing the two bands the cove will sit on.
Set the line with an offcut of the cove rather than with a tape. Hold a short piece into the corner and mark where its bottom edge lands on the wall and its top edge on the ceiling, and both dimensions come off the actual profile instead of a catalogue drawing. Mark both at every corner and join them: the ceiling line is the one that gets forgotten, and the one you cannot see from the ladder with three metres of cove in your hands. Then cut the paper along both lines and strip the bands, or score painted plaster across them to give the adhesive something to key into. Fresh, very absorbent plaster brings the opposite problem — the bed dries before it grabs — and whether it wants damping, a primer or nothing differs by product, so the answer is in the maker's literature.
Whether to follow a dipping ceiling or hold a level line is not a coving decision and is worked through in the crown guide; it applies here unchanged. What differs is the remedy. A sprung timber moulding on a nailer needs the nailer packed; a bedded cove carries its own packing, because the adhesive can be run thicker where the ceiling falls away and the cove sits on more of it. That tolerance is not unlimited — a bed much beyond the thickness the maker states slumps under the weight before it sets, and takes the cove down with it in the small hours.
- Hold an offcut of the cove into a corner and mark where both of its edges land, on the wall and on the ceiling.
- Carry both marks round every wall and join them, so the ceiling line exists before you are standing under it with a full length.
- Cut and strip wallpaper along both lines, or score painted plaster across the two bands, and brush the dust off.
- Check the maker's literature for what a fresh, high-suction plaster surface wants — the answer is a product property, not a rule of thumb.
- Bed the first length on the longest uninterrupted wall, where a full length reaches furthest and a joint is least wanted.
- Support it while it grabs: a couple of screws driven part-way in below the bottom edge, pulled out and filled once the bed has taken.
The bedded cove in section
- Filled mitre and the two finishing lines — the last work on the wall and the only part the decorator sees, measured as bead length round the room rather than as an area Caulk & Sealant Calculator
- The cove — bought as whole lengths against the ceiling perimeter, and the only layer here whose joints can be made to disappear Crown Molding Calculator
- Adhesive bed at both back edges — two fillets doing the entire job of holding a gypsum cove up, and the packing that takes out a ceiling running off level
- Keyed plaster faces and what is behind them — the two bands scraped back to bare plaster, which is what the bond is actually made to rather than to the paint over them Gypsum Plaster Finish Coat Volume Calculator
How much of the order is adhesive and how much is filler
Coving is one of the few interior jobs where the consumables outrun the moulding as a source of estimating error, because the moulding is a length you can count and the adhesive is a bead whose section nobody writes down. Two beads hold the cove on — one under the wall edge, one under the ceiling edge — and the finishing work then runs along both of the same lines, plus every mitre and butt joint. A room of four point two by three point three metres has a fifteen metre perimeter, so the bedding alone is thirty metres of bead.
Bead volume is a square law, which is where estimates go wrong by a factor rather than a percentage. A six millimetre bead is about twenty-eight square millimetres in section; a ten millimetre bead is about seventy-nine, close to three times the material for a bead only a little fatter on the nozzle. A three hundred millilitre cartridge at six millimetres runs a little over ten metres as pure geometry, and less in practice once nozzle waste and the tail in the tube are counted — roughly the gap between that arithmetic and the twenty-five feet per tube the calculator below works to.
The calculator below is written round a door or window rather than round a room, and it still does this job because the shape of its arithmetic is a perimeter: twice height plus width, times the number of openings. Enter half the room length as the height and half the room width as the width, and each opening is half a lap of the ceiling line rather than a whole one — twice height plus width, on halved dimensions, comes to length plus width, which is half the room. So two openings make one lap: four for the two adhesive beds, eight if both edges are being filled with a gun as well. One hard edge: the dimension fields cap at three metres, so a room with a wall over six goes in as a third of itself instead, at three openings to the lap.
Read the answer as bead length first and tubes second. The tube count is honest for a cartridge laid at a quarter-inch bead, which is what the finishing caulk on the ceiling line is. It is not honest for the bedding: gypsum cove adhesive comes as a powder in a bag or mixed in a tub, goes on several times fatter, and is quoted in metres of cove per bag. Take the bead length from the calculator and the coverage from the bag.
Enter half the room length as the opening height and half the room width as the opening width, and two openings then make one lap of the ceiling line — four openings for the two adhesive beds, eight if the finishing lines are being run with a gun as well. A fifteen metre perimeter comes out at about three tubes a lap. Both dimension fields stop at three metres, so a long room is entered as a smaller fraction of itself with the count raised to match, and the tube figure is right for cartridge caulk rather than for bagged cove adhesive.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The height of the door or window opening being sealed.
The width of the door or window opening.
How many identical openings you're caulking.
Extra bead length on top of the openings' perimeter, for the sealant lost at the ends of tubes and in tooling.
Caulk tubes needed
1 tube
- Caulk bead length (with waste)
- 20.9 linear ft
They open the calculator with your figures already in it
Caulk & Sealant Calculator: 1 tube — 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
- The whole estimate is locked to one figure — a 1/4 in (6 mm) bead at 25 linear ft (7.6 m) per 10.1 oz tube — and there is no field for gap width or joint depth, so the answer never moves when the joint does. Widening the bead to 3/8 in (9.5 mm) at the same depth uses about half as much caulk again per unit length, and a gap that is both wider and deeper can take more than double. Size the bead against the actual gap before ordering to this number.
- It measures the opening, not the bead path. The exterior bead runs around the outside of the casing where trim meets siding, which is longer than the opening perimeter by roughly eight times the casing width, and most exterior details also take a second bead on the inner edge where casing meets the frame. The waste allowance on the result, 10% as the page opens, does not stand in for either.
- The formula takes the full perimeter, 2 x (height + width), so it charges caulk along the bottom of every opening. Standard practice on many window and door details is to leave the sill joint and any weep openings deliberately open so water that gets behind the trim can drain out. If you follow that, the real bead is shorter than the figure here.
- Nothing here addresses the sealant itself: substrate compatibility, the movement the joint has to survive, whether a backer rod is needed to set depth and break the third-side bond, primer, or the temperature and surface-moisture limits printed on the cartridge. A correct tube count applied to a joint bonded on three sides still tears through its own middle.
- This is a material take-off, not an air-sealing assessment. It does not identify where the leaks are, and it is not a blower-door result or evidence of compliance with any air-leakage or energy-code requirement — trim caulk is one small part of a wall's air barrier.
The joint you can hide changes what you buy
Everything the trade knows about buying running trim assumes the joint is the enemy: long lengths are worth carrying up a stairwell, and going up one stock length can buy less timber and no joints at all. That argument is worked through for sprung mouldings in the crown guide and it is correct. It is also almost inapplicable to gypsum coving, for the reason this page turns on — a butt joint here is filled, sanded and gone. There is nothing to hide, so nothing to buy your way out of.
What replaces it as the driver is handling. A four point eight metre length of gypsum cove is a fragile object with a long moment arm, it snaps at the mitre if it is picked up wrong, and it cannot be turned on a domestic landing. A two point four or three metre length can be carried by one person up a ladder and offered into a corner without a second pair of hands. So on this job the shorter length is often the better buy, which is the exact reverse of the timber answer, and the waste that results is not really waste — the offcuts go into the short returns round a chimney breast and into the second room.
The waste allowance wants rereading rather than accepting. Fifteen per cent is a compound-mitre allowance, and in a mitre box there are no compound cuts and almost nothing wasted at a mitre. What consumes coving is breakage in transit and on the ladder, plus short tails too near a corner to be worth fitting. The figure lands in much the same place for entirely different reasons — but it means a plain four-cornered room with careful handling comes in under it while a bathroom with a boxed stack and three externals comes in over. That the rectangle knows nothing about that stack is dealt with in the crown guide and applies here word for word.
Twice length plus width at the ceiling, fifteen per cent on top, divided by the length your merchant racks. A four point two by three point three metre room returns seventeen and a quarter metres of cove, or about fifty-seven lineal feet with the page set to imperial, and six three-metre lengths either way. Two things to do with that before it becomes an order: treat the six as a floor rather than a plan, since coving joints are filled and every offcut over about half a metre is genuinely usable; and choose the length for what will get up your stairs intact rather than for the fewest joints, which is the opposite of how the same figure is read for timber.
SettingsSettings for this calculation
Waste is set to 15% by hand. Pick a tier above to replace it, or keep your own figure.
The longer wall, measured at ceiling level.
The shorter wall, measured at ceiling level.
The length crown molding is sold in at your supplier.
Extra run for compound-mitre corners, test cuts and the piece that comes up short.
The angle the back of the molding makes with the wall when it is in place.
The angle the two walls include at the corner, measured in plan.
Crown molding needed
56.35 linear ft
- Ceiling perimeter
- 49 linear ft
- Sticks needed
- 8 x 8 ft sticks
- Mitre, set on the saw's table
- 31.62 °
- Bevel, set on the saw's blade
- 33.86 °
They open the calculator with your figures already in it
Crown Molding Calculator: 56.35 linear 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
- Only a length and a width are collected, so the ceiling line is taken as a plain rectangle at 2 x (length + width) — an alcove, a bay, an L-shaped plan or a ceiling that steps around a stairwell opening is longer than that formula returns. Total such a ceiling as two or more rectangles, run each one separately, and add the stick counts.
- The mitre waste is a single percentage on the whole perimeter rather than a per-corner allowance, and neither the corner count nor the profile size is asked for, so a square room with four inside corners and an L-shaped one with six are padded alike unless the percentage is raised for the second.
- Sticks needed is the padded run divided by one stock length and rounded up, which pools every offcut as though the molding were continuous: in the room each wall is cut to its own length, a wall longer than one stick has to be made up from two pieces joined mid-run, and an offcut already cut at a compound angle on one end suits only a corner of that hand. One stock length also applies to the whole room, so an order that mixes 16 ft pieces on the long walls with 8 ft on the short ones cannot be costed here.
- Metric and imperial stock are two different products in this table rather than one length read twice, so changing the page's measurement system re-reads the same selection as 2.4 m where it had been 8 ft. The metric side can come out a stick higher for a room that has not changed, and the two counts are not comparable against each other on price.
- The two cut angles are for the molding laid FLAT on the saw table, which is the method that needs both settings. Cutting it nested — held against the fence at its spring angle, the way it sits on the wall — is a different method with its own fixed settings and these numbers do not apply to it. Decide which method you are using before you cut, because both produce a plausible-looking joint from the wrong one.
- An inside corner and an outside corner of the same angle take the SAME two settings; what changes is which side of the line is waste and which way the stick is handed. The page cannot tell you which, because it never asks whether the corner turns into the room or out of it, and getting that wrong wastes a stick rather than producing a bad joint.
- Molding length is the whole of what comes back — backing blocks, adhesive, brads and filler all sit outside the figure — and each room dimension stops at 30 m, so a hall or an open-plan ceiling longer than that has to be split into runs and its stick counts added.
Where cornice cracks, and why it is rarely the mitre
The crack along the top of a cornice in the first winter is almost never a bad joint. It is a cove doing what it was asked to do: bridge rigidly between a wall and a ceiling free to move relative to one another. A timber-joisted ceiling deflects under load and moves again as the joists dry to the moisture content of a heated house; the wall below does neither. The codes expect it — the International Residential Code's deflection table allows a ceiling with a flexible gypsum board finish half as much deflection again as it allows one with a brittle plaster finish — span over two hundred and forty against span over three hundred and sixty — which says the finish, not the structure, sets what is tolerable.
That is why the ceiling line is the one to sacrifice, and why the rule against caulking a joint is a joinery rule that does not transfer. Bed and fill the wall edge properly, and treat the ceiling edge as the moving line: a flexible decorator's caulk there absorbs a season of movement a gypsum filler would crack across. It is the only place on this job where caulk is the right answer rather than a cover for a joint that never fitted.
Two other cracks are worth telling apart from that one. One that follows the mitre rather than the length is a joint filled too wide, and it returns until the piece is cut out. One that appears at a single point along a run, usually within months of a new build, is the ceiling shrinking at a board joint underneath — a plasterboard problem, and filling the cornice repeatedly will not fix it. On heavy fibrous cornice, a crack with the run standing away from the wall is no finishing defect at all: it is a fixing that was adhesive when it should have been mechanical, and it wants dealing with before the run comes down.
Pricing a room of coving
The moulding is the easy count and the smallest number on the order. What decides whether the day runs is the consumables, the state of the two bands the cove has to stick to, and how you are getting a three metre length up to a stairwell ceiling.
- Ceiling perimeter, plus every return — Coving runs over doors and windows, so nothing is deducted — but a chimney breast, a boxed stack or a bulkhead each add a run and a pair of corners that a rectangle cannot see.
- Length chosen for the stairs, not the joints — A filled butt joint disappears, so the usual reason to buy long has gone; pick the length that will reach the room intact and count the offcuts as stock for the short returns.
- Corners counted by hand — Internals and externals listed separately, because the externals are the ones cut in pairs against work already bedded and are where the time and the breakages go.
- Two beds and two lines of bead — Adhesive under the wall edge and the ceiling edge, then filler along both after — four laps of the room in bead length before a single mitre is filled.
- Preparing the bands — Stripping paper or scoring paint along the two lines is labour that has to be priced, and skipping it is the reason coving comes off walls.
- Access for the awkward run — A stairwell or a tall room needs a platform rather than a leaning ladder, and a fitter holding three metres of cove overhead needs both hands.
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.