Balcony and roof terrace waterproofing

Guide price: £1,900–£3,250
Survey: Free for new and replacement roofs
Quote: Fixed, in writing
Coverage: England, Wales, Scotland, NI

What does waterproofing a balcony or terrace involve?

Waterproofing a balcony or terrace means lifting any paving or decking, stripping the failed membrane, checking the deck and falls, and laying a new system detailed around doors, outlets, walls and balustrade fixings. The surface finish then goes back on top, often raised on pedestals.

Lift the finish: paving, decking or tiles are taken up and stored or disposed of.
Strip and inspect: the old membrane comes off and the deck or concrete slab is checked for damp, rot or cracking.
Correct the falls towards outlets or a gutter, aiming for at least 1:80 finished.
Build up the layers: a warm or inverted roof over heated rooms, or a simple membrane on an open balcony.
Detail every junction: door thresholds, wall upstands, outlets and the base of balustrade posts.
Protect and finish with a walkable topcoat, or pedestals and paving set above the membrane.

How much does balcony waterproofing cost?

A 12 m² balcony in a liquid-applied system, with scaffold, is typically £1,900–£3,250 including VAT. Paving, pedestals, decking and balustrade work are priced separately at survey because they vary so much.

System	Guide rate	Where it fits
Liquid-applied	£90–£140 per m²	Small balconies with many details and posts
GRP fibreglass	£75–£140 per m²	Exposed, walkable finish with a coloured topcoat
Single-ply	£90–£145 per m²	Larger terraces under paving or decking
Mastic asphalt	£85–£150 per m²	Concrete terraces and heavy-use areas

Warm-roof insulation over a heated room adds £35–£60 per m², a new outlet £120–£280, and scaffold for an upper-floor balcony £450–£950. These are guide prices; your fixed price follows a free survey. See the cost guide for more.

Which waterproofing system is best for a balcony?

For most small balconies, a liquid-applied membrane is the practical choice, because it is brushed or rolled on and forms one continuous layer around posts, door frames and outlets with no seams to fail. Many systems include a hard-wearing, anti-slip topcoat.

GRP fibreglass gives a rigid, walkable surface and suits balconies left exposed rather than paved. On larger roof terraces under paving or decking, single-ply membranes are common, and mastic asphalt has a long record on concrete terraces and blocks of flats.

The choice also depends on what goes on top. Paving on pedestals protects the membrane, while an exposed balcony needs a system rated for foot traffic in its own right. We specify the build-up at survey.

Do I need planning permission for a balcony or roof terrace?

For a new balcony or roof terrace, generally yes. In England, balconies and raised platforms are not permitted development for houses, so a planning application is normally needed, and the other nations have their own rules.

Re-waterproofing an existing, approved balcony that looks the same afterwards does not normally need planning. Building regulations are a separate question. A terrace over a heated room is a thermal element, so replacing most of it can bring insulation requirements and approval. Turning a flat roof into a terrace also raises structural questions, because people, furniture and paving add load, and it needs guarding, such as a balustrade, that meets the regulations. A structural engineer should confirm that the roof can carry the new use.

Listed buildings and conservation areas add another layer of control. Our building regulations page covers approvals, and our planning permission guide covers the planning side.

How long does it take to waterproof a balcony?

A small balcony is typically waterproofed in two to four working days, plus time for scaffold if it is above ground floor. Liquid and GRP systems need dry weather and cure time between coats, which sets the pace.

The balcony door will be out of use while the membrane is laid and curing, and the room behind may be noisy while old paving or screed is broken out. If the team needs to work through the house, we agree a route and protect floors. Larger terraces with paving or decking take longer, because the finish has to be lifted and reinstated. Resin systems smell while they cure, so nearby windows are best kept closed.

How are thresholds, drainage and paving handled on a terrace?

They are designed together, because most balcony leaks start at the door threshold, a blocked outlet or a fixing driven through the membrane. Getting those three right matters more than the brand of membrane.

Thresholds: upstands are usually 150 mm above the finished surface, but a level or low step-out door cannot always achieve that. Where the upstand has to be reduced, extra protection is needed, such as a drainage channel at the door and a fall away from it, and the membrane must still run up and under the door sill.

Drainage: every terrace needs a reliable way off for water, with outlets sized for the area and an overflow if one outlet could block. Outlets under paving need access for cleaning.

Inverted roofs and pedestals: on an inverted roof the insulation sits above the waterproofing and is held down by paving or ballast, protecting the membrane from sun and foot traffic. Adjustable pedestals lift paving slabs level above the falls, so water drains underneath and the surface stays flat. Balustrade posts are fixed through upstands or side-fixed where possible, with every penetration sealed into the membrane.

What's included when we do the work?

Free survey of the deck, falls, outlets and door thresholds
Lifting and setting aside of existing paving or decking
Strip-out of the failed membrane and deck repairs
New liquid, GRP, single-ply or asphalt waterproofing
Sealed details at doors, outlets, walls and balustrade posts
Written workmanship guarantee, with approvals handled where needed

What do guide prices look like?

Guide prices including VAT at West Midlands rates; regional pages adjust these for your area. The fixed price comes after a free survey for new and replacement roofs.

Job	Guide price inc VAT
Balcony (12 m²), liquid-applied, scaffold	£1,900–£3,250
Single garage (18 m²), EPDM rubber	£1,600–£2,800
Extension (20 m²), GRP, new deck + insulation	£3,000–£5,400

Does anything differ outside England?

Wales: In Wales, permitted development rules differ from England's, so check with the local planning authority before forming a new balcony or terrace. Most re-roofing needs building regulations approval, so check if more than 25% of a terrace over a heated room is being replaced.
Scotland: In Scotland, a new balcony or terrace will usually need both planning permission and a building warrant from the council's building standards service. Re-waterproofing an existing balcony like for like can be exempt from a warrant, but check first.
Northern Ireland: In Northern Ireland, check planning with the council before forming a new balcony or terrace, and contact the district council's building control before re-waterproofing over a heated room.

How does the job run from start to finish?

Survey and test
We inspect the surface, outlets and thresholds, lift a paver where needed and trace where the water is getting in.
Specification and quote
Your fixed written quote sets out the build-up, the threshold and drainage details and what happens to the paving or decking.
Strip, waterproof, detail
The team strips the old system, corrects the falls and lays the new membrane, with every junction sealed before the finish goes back.
Reinstate and sign off
Paving or decking is relaid on pedestals or protection, the outlets are checked and you receive your guarantee paperwork.

Questions about balconies and roof terraces

Why is water coming through the ceiling below my balcony?

The usual causes are a failed door threshold, a blocked or poorly sealed outlet, or a fixing through the membrane. A survey, sometimes lifting paving, finds the source.

Can you waterproof a balcony without lifting the tiles?

Sometimes a liquid system can go over a sound, well-bonded surface, but if water is already getting under the tiles they usually have to come up.

What is an inverted roof terrace?

It is a terrace where the insulation sits above the waterproofing, held down by paving or ballast. The membrane stays protected and at a steadier temperature.

Can I put decking straight onto my flat roof?

Not directly onto the membrane. Decking needs a supporting system that spreads the load and lets water drain, and the roof must be checked for the extra weight.

How long does balcony waterproofing last?

It depends on the system and how well it is protected. As indicative ranges, liquid-applied systems last 10–25+ years by product and mastic asphalt 30–60, with good maintenance.

Do you install balustrades?

Our work is the waterproofing and drainage. We detail the membrane around balustrade fixings so they stay watertight, and we can advise on where posts should be fixed.

Is a roof terrace heavier than a normal flat roof?

Yes. Paving, people, planters and furniture all add load, so a structural engineer should confirm the roof can carry it before a terrace is formed.

Balcony and roof terrace waterproofing

What does waterproofing a balcony or terrace involve?

How much does balcony waterproofing cost?

Which waterproofing system is best for a balcony?

Do I need planning permission for a balcony or roof terrace?

How long does it take to waterproof a balcony?

How are thresholds, drainage and paving handled on a terrace?

What's included when we do the work?

What do guide prices look like?

Does anything differ outside England?

How does the job run from start to finish?

Survey and test

Specification and quote

Strip, waterproof, detail

Reinstate and sign off

Questions about balconies and roof terraces

Related services

Liquid-applied roofing

GRP fibreglass roofing

Single-ply membrane roofing

Flats, HMOs and block roofs

Where do you offer balconies and roof terraces?

Get a fixed price for your flat roof