Summary

This comparison comes up constantly on garages, single-storey rear extensions, dormer cheeks, porches, and bay window roofs — anywhere a flat felt roof has reached the end of its life and the customer asks "should I just re-felt it, or is it worth doing properly?" It's a different question from choosing between two coverings on a like-for-like roof shape (see felt vs fibreglass flat roof comparison for that comparison); this is a decision about whether to keep the roof flat at all, or convert it to a pitched structure and cover it in slate.

Flat roofs have a structurally simple appeal — no new rafters, no ridge, no change to the building's roofline — but they carry a well-documented lifetime maintenance burden: felt on a flat roof typically needs replacing every 15-25 years, and flat roofs are disproportionately represented in leak call-outs relative to their share of total roof area. A pitched slate roof, by contrast, sheds water actively rather than relying on falls and outlets, has essentially no flat surfaces for water to pond on, and — if properly specified in natural slate — can outlast the person commissioning it by several decades.

The honest framing for a customer facing this decision: felt is the lower-cost, lower-disruption option that buys another 15-25 years and defers the decision; converting to a pitched slate roof is a bigger structural project with materially higher upfront cost, but removes the flat-roof maintenance cycle altogether and adds genuine long-term value to the property. Neither is universally "correct" — the right answer depends on structure, budget, planning position, and how long the client intends to keep the property.

Key Facts

Quick Reference Table

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Factor Re-felt (stay flat) Convert to pitched + slate
Upfront cost Low (£40-110/m²) High (slating £120-280/m² + structural conversion cost on top)
Lifespan 15-25 years 80-150 years
Structural change None Significant — new rafters, ridge, wall plate
Planning risk Low (like-for-like re-covering is rarely notifiable) Real risk — roofline height change may exceed Permitted Development limits
Building Control Not usually notifiable for like-for-like re-covering Notifiable — new structure and Part L compliance required
Disruption Low, 1-3 days High, multi-week project
Maintenance burden Recurring — ponding, blistering, lap failure risk over life Minimal — pitched roof sheds water actively
Best for Budget-limited jobs, short ownership horizon, structures where pitching isn't feasible Long-term ownership, structures where roofline change is acceptable and budget supports it

Detailed Guidance

When re-felting is the right call

Re-covering in felt is the sensible, defensible recommendation in the majority of real-world cases:

When converting to a pitched slate roof is worth it

A structural conversion earns its cost in specific circumstances:

The planning trap — check before quoting either option

The most common mistake in this comparison is treating it as a purely technical/cost decision without checking the planning position first. A flat-to-pitched conversion changes the building's height and profile, and Permitted Development rights for extensions and outbuildings include specific height and eaves limits (varying by whether it's a rear extension, side extension, or detached outbuilding, and by proximity to a boundary). Exceeding these limits — even by a modest amount to achieve the minimum slate pitch — can require a full planning application, with no guarantee of approval, particularly in a Conservation Area or where the roofline would be visible from a public highway or affect a neighbour's amenity. Always establish the planning position — ideally with a pre-application enquiry to the LPA where the project is marginal — before committing a client to a slate conversion quote.

Structural assessment — the cost that's easy to miss

Converting a flat roof to pitched is not simply "add a ridge and rafters" — it requires the supporting walls (and, where relevant, the floor structure below on a first-floor dormer conversion) to be assessed for the additional dead load of a full pitched structure plus slate covering. On garages and single-storey extensions with modest foundations and wall specifications, this assessment can reveal the need for wall strengthening or additional support, adding further cost. A structural engineer's input is advisable on anything beyond a very small, straightforward conversion, and this cost should be built into the quote from the outset rather than discovered mid-project. See pitched roof structure for the structural principles involved.

The middle-ground options

Where the full slate conversion is out of reach on cost or planning grounds but the client wants better longevity than felt, two intermediate options are worth raising:

Frequently Asked Questions

Is it always cheaper in the long run to convert to a pitched slate roof?

Not always, and it depends heavily on how many re-felting cycles the flat roof would otherwise need and how long the client keeps the property. Over a very long horizon (60-80 years), repeated felt replacement can approach the whole-life cost of a single slate conversion, but the upfront cash outlay for conversion is many times higher, and most clients weight upfront cost and disruption heavily. Run the numbers for the specific project rather than assuming either answer.

Can I put natural slate on a garage roof without converting it to pitched?

No — natural slate requires a minimum roof pitch of roughly 20-25° to shed water correctly; it cannot be laid flat or near-flat. Any garage or outbuilding wanting a slate finish must first have a pitched structure built to the correct minimum pitch, which is the structural conversion cost referred to throughout this guide.

Does re-felting a flat roof need Building Control approval?

Generally no, for a genuine like-for-like re-covering of an existing flat roof — this is treated as repair/maintenance rather than notifiable building work in most cases. However, if the re-covering involves renewing more than 25% of the roof's thermal elements, a Part L thermal upgrade to current insulation standards may still be triggered even without a shape change. A structural conversion to pitched is different — that is new construction and is notifiable.

Will a pitched slate conversion always need planning permission?

Not always — a modest conversion that stays within Permitted Development height and eaves limits for the relevant extension or outbuilding type may not need a planning application. But because pitching a previously flat roof inherently increases the building's height, it's much more likely to breach those limits than a like-for-like re-covering, and checking the planning position early is essential before quoting.

What's the realistic lifespan difference in practice?

Felt: 15-25 years depending on system (2-layer vs 3-layer) and installation quality. Natural slate on a correctly built and BS 5534-compliant pitched roof: 80-150 years, with Welsh slate at the top end. This is the core of the whole-life value argument for conversion, but it only pays off if the client keeps the property, or values the improvement's contribution to resale, over a long enough horizon to capture it.

Regulations & Standards