Bought from the list, paid for on the roof

The skylight is the building purchase with the sharpest double life: in the shop it is a catalogue product, with a clear price and photos of the light coming in; on the roof it becomes a worksite — the slope to open, the covering to connect, the flashings, the internal finishes, and the scaffolding underneath it all. The result surprises whoever compares price lists: the installed window typically costs double its price. This calculator breaks the bill into its three forces — windows, installation, site — and sums it up in two honest numbers: the effective cost per skylight and the index ι, how many times the total exceeds the windows-only bill.

The formula: windows, installation and site

The model is the decomposition of work at height. The windows bill is n·u: the number times the unit cost of the window complete with its kit — frame, flashing, blinds. On that bill acts the surcharge r for installation and connections: on a roof it rarely drops below 40%, and rises past 100% with delicate coverings and important internal finishes. Outside the product stays the site fixed cost F: the fixed cost disguised as an accessory, sixth appearance of the geometry — and this time it is the scaffolding, which costs the same for one window and for eight. The index ι = C/(n·u) is dimensionless and works in any currency.

The scale of the number: the lone skylight pays the scaffolding alone

The table grows the series with everything else held equal (900 per window, installation 60%, site 1,000):

On the lone window the site is worth four tenths of the price and the effective piece nearly triples the list; on the series of eight it dilutes until it disappears. It is the roof's honest paradox: the extra window, while the crew is up there, is the cheapest of the series — because the scaffolding does not know how many windows will pass over it.

A complete numerical example

Typical job: 2 skylights at 900 each — the loft with the double light — installation and connections at 60%, a 1,000 site cost across scaffolding and protections. The windows bill is 2 × 900 = 1,800; the installation brings it to 2,880; the site closes at 3,880. The index is 3,880 / 1,800 = 2.16: each skylight in place costs 1,940 — not the list's 900 — and the site weighs 25.8%. Whoever compares two quotes «at 850 and 950 per window» without reading installation and site is comparing the wrong item.

How to read the quote index

The four bands tell four roofs. Below 1.8 you buy windows, not worksite: numerous series, accessible roof — the bill rewards product choice. Between 1.8 and 2.3 the quote is within the roof's physiology: the installed piece costing double the list is the trade's norm, not an anomaly. Between 2.3 and 2.9 the roof makes you pay: demanding installation, custom flashings — survey territory. Above 2.9 the window is the excuse and the worksite is the bill: difficult work at height, important finishes. No band is «wrong»: the index says where to point the lens before signing.

The unit cost: what it really contains

The variable u is as wide as the catalogue: from the basic centre-pivot skylight to the motorised model with triple glazing, rain sensor and integrated blinds. Inside u live the window and its flashing kit — the piece that marries the frame to the roof's specific covering: tile, pantile, sheet metal. Outside stays the rule catalogues do not write: two «900 per window» with different glazing, openings or kits are not the same product — and on a roof the quality of the glass is paid once, while the draught is paid every winter.

The installation: opening a roof is the opposite of drilling it

The variable r is the trade turned into a number. Opening the slope means cutting battens and membranes, rebuilding the perimeter, connecting the water tightness above the window and the vapour tightness below, fitting flashings that must age as long as the covering, finishing the internal reveal that carries the light into the room. The 40-60% is the physiology of the ordinary roof; it climbs with delicate coverings (slate, old pantiles), steep slopes, custom reveals. It is the item where the saving shows immediately and is paid later: the dripping skylight is almost always a saved-on flashing, not a faulty window.

International perspective: curb, deck and the same decomposition

The skylight is universal, roofs are not. North America often mounts on a curb (the perimeter riser) over shingle coverings, Europe sets flush with the slope on tile coverings with dedicated kits; cold climates ask for triple glazing and attention to condensation, hot ones for external blinds against overheating. Everywhere the bill has this model's three items — windows, installation, site — and the index ι compares quotes from different markets on the same scale, because a ratio needs neither exchange rate nor climate.

Limits of the model and correct use

The model photographs installation on an existing roof: it does not cover dormers (which are small masonry works, not roof windows), structural changes to the rafters when the window is wider than their spacing, nor permits where required. The number is the design's true one: light is sized on the room, not on the list. And the skylight lives in the roof system: the attic ventilation calculator checks that the roof breathes around the new window, and the heat load calculator accounts for the light that becomes heat in summer — because the window on the roof is wonderful exactly as long as the rest of the roof supports it.

The scale of the installation: from easy roof to master covering

The table raises only the installation-and-flashing surcharge, windows and site held still (2 skylights at 900, site 1,000):

From one end of the scale to the other the total grows 57% — the same pair of windows, five different roofs. And the first row hides the table's most instructive surprise: it is the only one lit by the heavy-site signal. Removing installation does not remove scaffolding — at 20% the fixed part is worth 31.6% of the bill, more than in any other row.

Reading the inversion: the installation dilutes the site

It is the scale's counter-intuitive detail: while the installation gets dearer, the site share falls — from 31.6% to 20.2%. Not because scaffolding costs less on difficult roofs, but because the total grows and the fixed part dilutes. The operational reading counts double: on the easy job — the replacement from inside, the low roof — the quote must be examined from the site side, because that is where the big slice hides; on the difficult job, from the installation side, item by item. The same total can be honest in one case and inflated in the other: the decomposition, not the figure, tells which of the two you are looking at.

The scale of the site: from tower to full scaffolding

The table raises only the site fixed cost, windows and installation held still (2 skylights at 900, installation 60%):

Between 500 and 4,000 lies a whole world: the tower on the low slope on one side; the full street-front scaffolding, with land occupation and protections, on the other. The windows are the same, the total doubles — and over half the price never touches a window.

The three heavy rows: when the scaffolding commands

From 1,750 up the 30% signal stays lit: the site is worth 37.8%, 46.5% and finally 58.1% of the total. It is the mirror of the number scale: there it was the lone window making the fixed part weigh by geometry, here it is the expensive site doing it by context. And the right answer is the same in both cases: load the scaffolding with work. The extra window, the covering maintenance postponed for years, the ventilated ridge, the lifeline — every item that climbs onto the same scaffolding dilutes its cost. The expensive site is not negotiated: it is filled.

From the quote to the roof: the sequence that protects the budget

The right order is a defence of the wallet. First, the design of the light: number and position are sized on the room — the 10%-of-floor rule — not on the list. Second, the survey: covering, slope and access decide installation and site more than any negotiation. Third, the specification in three blocks: windows, installation, site — the calculator's same three — because two close totals with different blocks are two different jobs. Last, the calendar: if the roof has other works in sight, the skylight's right date is the one when the scaffolding is already paid for by something else.

Typical mistakes and how to avoid them

Four recurring traps. Comparing window price lists: the product is half the bill — compare the index ι, not the catalogue. Saving on the flashing: the dripping skylight is almost always an economised flashing; tightness is paid once, the draught every winter. Giving up the second window: with the crew at height it is the cheapest of the series — the scaffolding gives no discount to those who use half of it. Forgetting summer: zenithal light is zenithal heat — the external blind decided in the specification costs a third of the one added later, and the room under the roof will ask for it at the first heatwave.

Frequently asked questions

How much does installing a skylight cost?

With the decomposition C = n·u·(1+r/100) + F: the number times the cost of the window with its kit, surcharged by the installation, plus the site fixed cost. With 2 windows at 900, installation at 60% and a 1,000 site cost, the total is 3,880 — 1,940 per skylight in place.

Why does an installed skylight cost double the list price?

Because the list is half the trade: opening the slope, connecting the covering, fitting the flashings, finishing the reveal and setting up the site are worth as much as the window. The calculator's index ι measures exactly this: below 1.8 the windows dominate, above 2.3 the roof does.

What is the skylight quote index?

It is the ratio ι = C/(n·u) between the total and the windows-only bill. Between 1.8 and 2.3 it is the roof's physiology (installed costs double the list); between 2.3 and 2.9 the roof makes you pay; above 2.9 you are buying above all work at height.

Why does a single skylight cost so much?

Because scaffolding, access and protections cost the same for one window and for eight: on the single job the site is worth 41% of the price and the effective piece nearly triples the list. It is the fixed cost disguised as an accessory — and why the extra window, with the crew already up, is the cheapest.

What does the window unit cost include?

The complete window and the flashing kit for the specific covering (tile, pantile, sheet metal), plus any blinds. Two «900 per window» with different glazing or openings are not the same product: the specification is read item by item, just like the site.

How much does installation affect the skylight price?

It is the model's surcharge r: 40-60% on the ordinary roof, above 100% with delicate coverings, steep slopes or custom reveals. Opening a roof is the opposite of drilling it: water tightness above, vapour tightness below, flashings that must last as long as the covering.

Manual, electric or solar: does it change the bill?

It changes u, not the structure of the count: motorisation and the rain sensor move the window's list price, not the installation nor the site. The practical rule: on skylights out of arm's reach, motorisation is not a luxury — a window that never opens is half a window.

When is scaffolding needed for skylights?

Almost always, to work safely from outside: it is the big item in the fixed cost F. The alternative — working from inside with a tower — holds for simple replacements on low slopes. If the roof has other works in sight, sharing the scaffolding is the biggest saving in the whole quote.

Does the calculator work for replacing an existing skylight?

Yes, with a lower r: the opening is already there and the work is often done from inside — the typical replacement travels at 20-40% with a reduced fixed cost. Beware though of the non-standard sizes of old frames: custom work pushes the bill back up.

How much light does an extra skylight bring?

Far more than a vertical window of equal size: zenithal light yields about double. The common design rule asks for glazed area around 10% of the room's floor; the number n is sized from there — on the room, not on the list.

Does a skylight overheat in summer or condense in winter?

They are the two risks handled in the specification: external blinds and selective glazing for summer, triple glazing and a well-wrapped reveal for winter. Condensation on new glass tells of the room's humidity more than of a defect: measure first, blame the window later.

What is NOT included in the computed cost?

Dormers (small masonry works, not roof windows), structural changes to the rafters for openings wider than their spacing, permits where required and the room's special finishes. The model photographs the standard installation: windows, installation and site — the rest is added item by item.

Updated July 2026