Guide · Skylight & Rooflight Installation & Replacement Chelmsford
Reading a rooflight quotation properly
Two quotations for the same rooflight can differ by a wide margin and both be honest. The gap is almost never the glass. It is the work around the glass, the things one document names and the other leaves to be discovered on the day, and the only way to see that is to read both documents line by line rather than comparing the figures at the bottom.
This page is about interpretation. It assumes you already have a written document in front of you and want to know what its wording actually commits somebody to. The cost drivers behind the numbers are set out on what a rooflight costs in Chelmsford, and the wider subject sits under the guide to rooflight and skylight installation in Chelmsford.
What separates a quotation from an estimate
The two words are not interchangeable, and the difference is contractual. An estimate is a considered guess at what the work will come to. It can move, and if the final invoice lands higher, the contractor is generally entitled to the extra provided the increase is reasonable and the scope is the same.
A quotation is a fixed offer to do a defined piece of work for a defined figure. Accept it and the figure holds unless the scope changes. Sunspire works in two stages for exactly this reason: a range by email while the job is still being described, then a fixed written figure once a surveyor has been on the roof and in the loft. Check which of the two words is printed on your document, because it changes what you are agreeing to.
The header, and who you would actually be contracting with
Before the specification, read the top of the page. A trading name is not a legal entity. Look for either a company name with a registration number or a named individual or partnership, because that is who the guarantee is enforceable against later.
Also check the date and the validity period. Glass and aluminium prices move, so a quotation carrying an expiry of thirty days is normal rather than pushy. What matters is that the period is stated. A document with no date and no expiry cannot be compared fairly against one that has both, because you do not know how old its assumptions are.
Checking the unit against the maker’s own catalogue
A quotation worth comparing names the unit precisely enough that you can find it yourself: manufacturer, model code, and external size. VELUX, Fakro, Keylite and Roto all publish full size and code tables online, and five minutes spent checking is the single most useful thing a homeowner can do with the document.
The reason is that model codes carry the specification inside them. A code tells you the frame material, the opening action and often the glazing variant. Where a quotation says only “roof window, white, 1140 x 1180”, there is nothing to check and nothing to compare, and the specification is being decided by whoever orders it rather than by you.
Reading the glazing code on the line beneath
Glazing is usually written as a short code plus a build-up. The build-up looks like 4-16-4 or 6-16-4, which is the thickness of the outer pane, the width of the cavity and the thickness of the inner pane in millimetres. An asymmetric build-up, where the two panes differ, is a deliberate choice that breaks up rain drumming.
Three numbers should sit alongside it. The U-value in watts per square metre kelvin, where lower means less heat loss. The g-value, a fraction describing how much solar energy passes through, where lower means less summer gain. And light transmittance, the fraction of visible light that gets in. A quotation naming a U-value and staying silent on the g-value is describing half the behaviour of the glass.
Three different sizes, and which one the price is built on
Rooflights are described by at least three dimensions and they are not the same number. Confusing them is how a unit arrives that will not fit the hole somebody has already formed.
| Dimension | What it measures | Why it matters on a quotation |
|---|---|---|
| Structural opening | The hole formed in the roof structure or the internal face of the kerb | This is what the carpenter and the engineer work to |
| External frame size | The overall size of the unit itself | This is what is ordered, and what the size code refers to |
| Clear view or daylight area | The glass you can actually see through | This is what the room receives, and it is always the smallest |
A document should say which of the three it is quoting against, and for a flat rooflight it should say whether the kerb is being built to suit the unit or the unit ordered to suit an existing kerb. Sizing for the room itself is a separate exercise, covered on sizing a rooflight for the room.
How much of the total is broken out
A single figure against the words “supply and fit rooflight” is not something you can compare, negotiate or check. It is a number waiting for a conversation on the day.
What you want is a breakdown that separates at minimum the unit, the labour, the access and the making good. It does not need to expose margins to be useful. It needs enough structure that when a second quotation arrives you can see whether the difference sits in the glass, the scaffold or the plastering. Where everything is bundled, ask for it split before you compare anything.
Provisional sums, and what they are telling you
A provisional sum is an allowance for work that cannot be priced accurately yet. It will be adjusted up or down when the real extent is known. Seeing one is not a warning sign in itself, and on some jobs it is the honest answer.
What matters is how many there are and what they cover. A provisional sum against a structural engineer’s fee before the calculation exists is reasonable. A provisional sum against the flashing kit, which is determined entirely by a covering somebody has already looked at, suggests the survey did not go far enough. Ask what would move each one, and in which direction.
Assumptions, written and unwritten
Every quotation rests on assumptions. The good ones list them. Typical entries are that the roof structure is as observed from the loft, that the covering is available in a matching profile, that scaffold can stand on the customer’s own ground, and that the ceiling void is clear at the proposed position.
Listed assumptions are a courtesy to you, not a hedge. They tell you exactly which discoveries would change the figure, so nothing arrives as a surprise. A document with no assumptions section has the same assumptions, unwritten, and you will meet them individually while the roof is open.
Putting two documents side by side that describe different jobs
The common trap is comparing two totals that cover different quantities of work. Before comparing, build a short list of everything either document mentions, then mark which of them appear on both.
Typical divergences are the flashing kit named by covering profile on one and generic on the other, plastering and decoration included on one, an insulated shaft lining on one, waste removal on one, and the Building Control notification on one. Once the lists match, the totals become comparable. Very often the cheaper document becomes the dearer one at that point, which is exactly what the exercise is for.
The lines that are quietly absent
Four items go missing more often than any others, and each is expensive to add later.
- Making good internally. The plaster around a new reveal, and whether it is finished ready for decoration or left as bare board.
- The underlay apron. The membrane detail beneath the covering around the opening, which does the real weathering work and rarely appears as a line.
- Access. Scaffold, tower, edge protection and any highway licence, each of which has its own lead time.
- Waste. The covering that comes off, the timber cut out and, on a replacement, the old unit itself.
VAT, and how it is presented
Check whether the figure quoted includes VAT or excludes it, because a document showing net figures and one showing gross figures will look very different at the bottom while describing identical work. A VAT registered business shows its registration number on the quotation.
Some domestic work attracts a reduced or zero rate, most commonly on properties that have been empty for a long period or on certain conversions and adaptations. Where that might apply, it belongs in writing before the invoice rather than being raised afterwards.
Deposits, stage payments and what the money is buying
A deposit on a made to measure unit is normal, because the unit is ordered specifically and cannot be sold to somebody else. What you want is the deposit tied to something identifiable: the order being placed, or materials arriving on site.
Beyond that, look at the shape of the schedule. A balance due on completion, with completion defined as the roof weathered and the internal finish done, is straightforward. A schedule front-loading most of the value before anybody has been on the roof is worth a question. So is a demand for cash outside the business account, which removes whatever protection the paperwork was giving you.
Guarantee wording, and the two things it can mean
Two separate guarantees sit behind any rooflight and a quotation should distinguish them. The manufacturer warrants the unit: the sealed unit against failure of the seal, the frame and the moving parts, on their own terms and for their own period. That warranty comes from the maker and follows the product.
The workmanship guarantee comes from the installer and covers the perimeter detailing: the upstand height, the falls, the flashing laps and where the covering terminates. Those four are invisible once the job is finished, they decide whether the rooflight is dry in a decade, and they are the reason Sunspire’s workmanship carries a ten year guarantee. Look for both periods stated separately. A document offering one figure covering “everything” is usually covering less than it sounds.
Lead time, programme, and the phrase “subject to survey”
Two durations belong on the page. The lead time before work starts, which is dominated by the unit itself, since standard sizes are held in stock while made to measure and specialist glass are manufactured to order. And the duration on site, which for a single roof window in a pitched slope is a day, for a flat rooflight one to two, and for a lantern with a structural opening two to four.
The phrase “subject to survey” is doing real work wherever it appears. On a range issued by email it is honest. On a fixed price issued after a survey it should not be there at all, and if it is, ask which specific item remains unresolved.
The paperwork that should follow the work
The document should say what arrives afterwards, because that is the part a buyer’s solicitor asks for years later during a sale. Expect the Building Control completion certificate, which Sunspire arranges as part of the job, the manufacturer’s warranty registration for the unit, and the written workmanship guarantee naming the same legal entity that signed the quotation.
Where an electric unit is fitted, the operating instructions and any handset pairing information belong in the same envelope. Where a structural calculation was produced, ask for a copy, because it is part of the record of the building.
Questions worth asking before you sign
Five questions settle most remaining ambiguity, and none of them is awkward to ask. Which flashing kit is being used, named by covering profile. Who forms the opening and who makes good the plaster. What condition the roof is left in each evening. Which items are provisional and what would move them. And what happens to the figure if the survey finds a truss where a rafter was assumed.
A contractor who answers those clearly is giving you the same document twice, once in writing and once in conversation, and the two agreeing is the point. If you want to test a document against a real one, ask for a range by email and read the two side by side.
The cheapest quotation is usually the one describing the least work, and the difference turns up later as an invoice.
Keep reading
More on skylight & rooflight installation & replacement chelmsford
Sizing a rooflight for the room
Read the guide →Roof window or rooflight, and the difference
Read the guide →Flat roof against pitched roof rooflights
Read the guide →What happens on installation day
Read the guide →Cleaning and maintaining a rooflight
Read the guide →All about skylight & rooflight installation & replacement chelmsford →
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