Should You Re-Roof When Having an Extension Built?

Why an Extension Is the Right Moment to Think About Your Roof

Having an extension built is one of the most disruptive—and expensive—projects a homeowner takes on. Scaffolding goes up, trades come and go, and large sections of your home are exposed to the elements for weeks. That disruption, uncomfortable as it is, creates a practical window to address your existing roof at the same time, often at a lower combined cost than tackling it separately later.

Many homes in Ely are Victorian or Edwardian terraces, inter-war semis, or post-war bungalows, and a good number have roofs that are approaching or past their natural lifespan. If your roof is already 25 to 35 years old, or if you've been patching it regularly, combining the work with your extension build is worth serious consideration.

What to Check Before You Decide

Before committing to a full re-roof, have an independent roofer inspect the existing structure—not just the tiles, but the felt, battens, ridge, and any flashings. In Ely and the surrounding Fenland area, driving easterly winds and sustained damp winters put roofs under particular stress, especially north-facing slopes and any sections overshadowed by trees that retain moisture.

Look for these warning signs that a re-roof makes sense alongside your extension:

• Felt that is brittle or torn — once the underlay fails, water tracks into the timbers rather than running off
• Multiple cracked or slipped tiles — isolated repairs are fine, but widespread movement suggests the battens beneath are deteriorating
• Sagging ridge or rafters — a structural issue that won't be solved by new tiles alone
• Persistent moss and algae — particularly common on low-pitched roofs in Ely's flat, humid landscape, which accelerates tile degradation
• Age over 25–30 years — most concrete interlocking tiles and standard clay tiles have a practical lifespan in this range under UK conditions

If several of these apply, a roof replacement at the same time as your extension is almost certainly the more economical choice.

The Practical and Financial Case for Combining the Work

Scaffolding is the biggest single cost driver in most re-roofing jobs. When your builder has already erected a scaffold for the extension, a roofer can often tie into it or extend it for a fraction of what independent hire would cost. That saving alone can run to several hundred pounds, sometimes more on larger properties.

Labour is also more efficient when trades are already mobilised on site. A roofer scheduling their visit around your build programme avoids multiple site set-ups, which keeps your overall project cost down. The installation of a new roof at this stage also means your builder can integrate the junction between the extension and the main roof properly from the outset, rather than working around an ageing structure.

From a planning perspective, re-roofing your existing house using the same or similar materials typically falls under permitted development in England, so you usually won't need a separate application. However, if your property is in a conservation area—which applies to parts of Ely city centre near the cathedral—it's worth checking with the government's planning guidance before proceeding. Your extension itself may already require planning permission, and it's worth asking your architect or local authority whether the roofing element needs to be specified.

Flat Roofs on Extensions: A Common Decision Point

A large proportion of single-storey extensions in Ely and nearby villages like Littleport and Soham are built with flat or low-pitched roofs. If your extension will have a flat roof, this is an excellent point to consider whether the flat sections elsewhere on your main roof—over a bay window, a rear addition, or a garage—also need attention.

Modern flat roofing systems, such as GRP fibreglass or EPDM rubber, typically outperform the traditional felt that was standard on most properties built before the 1990s. Having a roofer assess and replace tired flat sections while on site for the extension avoids the need to return, re-scaffold, and re-mobilise a separate team in a year or two. Our flat roofing services cover both extension roofs and existing flat sections, and we're happy to survey both at the same visit.

When Re-Roofing Alongside an Extension Isn't Necessary

If your existing roof is relatively young—say, less than 15 years old and in sound condition—there's no compelling reason to replace it simply because you're building an extension. A thorough inspection to confirm the structure is sound, followed by any minor repairs to flashings or ridge mortar, is usually sufficient.

The junction between a new extension roof and your existing roof does need careful attention regardless. Lead flashing and correct weathering at that join are essential, and cutting corners here is one of the most common causes of water ingress in extended properties. Make sure your roofer addresses this properly before any scaffolding comes down.

If you're unsure where your roof stands, the honest answer is to get it inspected before your build starts—not after. By then, your options narrow and costs rise.

We carry out free local roof surveys for homeowners in Ely and across the surrounding area. If you're planning an extension and want to know whether your roof is worth combining the work on, get in touch and we'll arrange a survey at no cost to you.

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