Cost to install lean to roof?

Hi, thanks for your question.

For an outbuilding of around 144 sq ft (approximately 13.5 m²), the cost can vary depending on the condition of the existing structure, access, and the materials you choose.

As a rough guide:

Replace the existing felt roof: approximately £900–£2,000, depending on whether it's a simple re-felt or upgraded to a high-performance felt or GRP fibreglass system.
Convert to a new lean-to pitched slate roof: approximately £3,500–£7,000, as this involves constructing a new timber roof structure, installing breathable membrane, battens, slates, leadwork, fascias, guttering, and making good where the new roof joins the existing building.

A pitched slate roof is a more expensive option initially but generally offers a much longer lifespan, improved appearance, and can add value to the property.

If you're considering the conversion, we'd be happy to inspect the existing outbuilding and advise whether the walls and foundations are suitable for supporting a new pitched roof. We can then provide a free, no-obligation quotation for both options so you can compare costs.

Kind regards,

SJC Roofing Ltd

4 to 8k depending on tiles

144 sq ft is about 13.4 m², so this is quite a small outbuilding roof by roofing standards. That matters because small jobs often get hit by minimum labour and scaffolding costs rather than pure square-metre pricing.

For a straightforward attached outbuilding in the UK, you’re roughly looking at:
• Replace existing felt flat roof: about £900–£2,000
• Convert to a pitched/lean-to slate roof: about £3,000–£7,000+

Typical costs (UK / London context)
• Felt roof replacement typically runs £40 – £70 per m² installed. ?
• For 13.4 m² that’s roughly:

£540 – £940 total

This assumes:
• The existing deck is sound (no major joist/OSB replacement)
• No scaffolding or difficult access
• Standard roofing felt materials

If you upgrade to a higher-quality finish (e.g., EPDM/rubber or GRP fibreglass), expect:
• EPDM: £50 – £90/m² ? £670 – £1,200 ?
• GRP: £80 – £130/m² ? £1,070 – £1,740 ?

This is still broadly cheaper than a pitched roof conversion. ?

?

???? 2) Lean-to pitched roof with slates

Unlike a flat roof, a pitched/lean-to slate roof is a bespoke steel/timber structure job and costs significantly more due to:
• New structure (rafters, wall plate, fixings)
• Roof pitch framing
• Felt underlay + battens
• Slate tiles + fasteners
• Leadwork/flashings
• Waste removal + regulatory compliance

Rough cost breakdown (typical UK figures)

Slate roof materials:
• Natural slate tiles tend to be pricier than basic flat roof materials — often £60–£100 /m²+ just for the slate tiles themselves. ?

Labour and framing:
• Adding a pitch means carpentry, battens, underlay, fixings — these costs usually dwarf the material cost.

Indicative total cost
For a small 144 sq ft (~13.4 m²) lean-to conversion, a naïve estimate you’ll hear from trades is:

???? £2,000 – £4,500+ as a ballpark

This takes into account:
• Timber framing
• Slate tiling to a pitched deck
• Fascias/soffits
• Flashing and trim work
• Waste disposal

Slate roofs are typically 3×+ the cost of a simple flat-roof replacement, sometimes more when you include structural work.

Replacing the felt is the cheapest option, roughly £400–£1,400 for 144?sq?ft. Converting to a lean?to slate roof is a major structural job and can easily cost several thousand pounds due to timber, slates, and labour. For a small outbuilding, refelting is far more cost?effective.

Converting a felt and concrete outbuilding roof to a lean-to slatted roof is significantly more expensive than replacing the felt, as it involves a complete roof structure change. For a 144 sq ft outbuilding, a felt roof replacement might cost around £720 to £1,080 (£50-£75 per m²), while a new slate roof, including the structural changes, could cost upwards of £2,500 to £5,000 or more, with prices for materials alone varying significantly based on the type of slate.

eplacing a roof felt on a 144 sq ft outbuilding costs approximately £300 to £1,000, while converting to a pitched, slate roof is significantly more expensive, likely £2,000 to £4,500, but this also requires additional structural changes to create the pitch. The major difference is that a re-felting job is a simple replacement of the existing material, but a lean-to conversion involves adding new timber supports and a complex pitch, making it a much larger construction project

To replace felt with a concrete tile roof would cost about £1650

Compared to replacing a slatted lean to roof the same size that would cost more like £2900 to replace