You are having your roof replaced — new tiles, new membrane, new battens, scaffolding up, the whole job. And there in the middle of it all sit your existing skylights, maybe 15 or 20 years old, still working. The question every homeowner asks at this point: should I replace them while the roof is off, or leave them alone?

Our answer, most of the time: replace them. Not because we want to sell you windows, but because the economics and the practicalities overwhelmingly favour doing it now. Here is the full reasoning — including the cases where keeping them makes sense.

The cost argument: now vs later

This is the heart of it. Replacing a skylight as a standalone job later means:

Scaffolding or tower access hired again (£400–£900 in London for a typical setup)
A roofer cutting into your brand-new roof covering, disturbing tiles and membrane that were just laid
Making good the internal reveal and decoration afterwards
A day rate for labour that is disproportionate to the size of the job

Replacing the same skylight during a re-roof means:

The scaffolding is already up — no additional access cost
The roof is already stripped — the window goes into the new build-up cleanly, with correct flashing, insulation collar and membrane laps
The labour is a marginal addition to a job already happening

In practice, adding a skylight replacement to a re-roof typically adds £400–£800 per window (supply and fit, depending on size and specification). Doing it as a standalone job later typically costs £800–£1,500 per window once access is factored in. On a roof with three skylights, doing them during the re-roof can save £1,000–£2,000 overall. Our new roof cost guide for London puts re-roofing budgets in context.

The technical argument: old windows in new roofs

A re-roof is a 40- to 60-year investment. Bolting a 20-year-old skylight into it creates a weak point with a much shorter remaining life than everything around it. Consider:

Flashing compatibility. The flashing kit seals the window to the roof covering. Reusing an old flashing kit with new tiles is asking for trouble — and fitting a new flashing kit to an old window is possible but not always straightforward, depending on the window’s age and model.

Seal condition. If the window is 15+ years old, its gaskets are already partway through their life. They will fail years before the new roof does, and when they do, someone will be cutting into your new roof to fix them. See our guide to Velux window leaking causes and fixes for what seal failure looks like.

Thermal performance. Roof window technology has improved enormously. A 1990s or 2000s unit might have a U-value around 2.8–3.0 W/m²K; a modern triple-glazed unit can achieve 1.0 or below. In a new, well-insulated roof, old skylights become the thermal weak point — the place where heat escapes and condensation forms.

Guarantee alignment. Your new roof comes with a workmanship guarantee. Your old skylights do not — and if one leaks in five years, expect an argument about whether it is a roof problem or a window problem. New windows installed with the new roof are covered as part of the job.

When it makes sense to keep the existing skylights

Replacing is the default advice, but not a universal rule. Keep the existing units if:

They are under 10 years old and in good condition. A relatively new window has decades of life left — no reason to replace it.
They are high-specification units already — recent triple-glazed windows with good U-values do not need upgrading.
The budget genuinely will not stretch. A re-roof is expensive enough. If replacing the skylights means cutting corners on the roof itself, keep the windows — the roof covering matters more.
They are non-standard sizes that would require structural alteration to replace. (Though this is rare — most sizes are still manufactured.)

In these cases, make sure the roofer fits new flashing kits and insulation collars around the retained windows as part of the re-roof. The window stays; everything sealing it to the new roof should be new.

What to choose if you do replace

If you are replacing, the re-roof is the moment to upgrade properly:

Go for current-generation glazing. Double glazing as a minimum; triple glazing if the budget allows, especially for bedrooms where the acoustic benefit is noticeable.
Match the sizes. Replacing like-for-like sizes avoids structural work. If you want bigger windows, decide before the roof is stripped — enlarging openings means altering rafters, which needs to be designed in.
Consider the operation. Top-hung (opening outward from the top) suits lower installations where you want an uninterrupted view; centre-pivot suits higher positions. Electric or solar-powered units are worth considering for out-of-reach windows.
Check the guarantee. Quality manufacturers offer 10-year product guarantees. Your installer should guarantee the installation work separately.

For cost detail, expect roughly £400–£800 per window supply-and-fit as part of a re-roof, depending on size and specification.

Timing and planning

Raise the skylight question with your roofer at the quotation stage, not after the scaffolding is up. The windows need to be ordered (lead times of 1–3 weeks are common for non-stock sizes), the openings need to be in the roofer’s plan, and the price should be in the written quote — not added as a variation halfway through the job.

If your roofer dismisses the question or seems reluctant to discuss the windows, that is worth noting. A good roofer will have a clear opinion and a clear price — going in with a prepared list of questions helps you have that conversation productively.

The bottom line

If your skylights are over 10–15 years old and you are re-roofing, replace them now. It is cheaper, technically cleaner, thermally better, and it aligns the guarantee on the windows with the guarantee on the roof. The only good reasons not to are young windows, tight budgets, or genuinely high-spec existing units.

Victoria Roofing Expert Ltd handles re-roofs and skylight replacement together across Camden and all of London — our roof installation and skylight installation teams work as one, so the windows go into the new roof exactly as they should. Family-run since 1982, 5.0 from 338 Google reviews, open 24 hours, fully guaranteed workmanship.

Get a free quote — we will assess your existing skylights honestly and price both options so you can decide with the real numbers in front of you.