Plenty of Worthing homes have a flat roof that nobody ever walks on: a rear extension, a garage, a flat-roofed bungalow wing or the top of a bay. A terrace or small roof garden up there is easy to like, particularly where the garden below is shaded or tiny. The difficulty is that a flat roof is built to shed water, not to carry people, furniture and planters, and the rules treat a place to stand quite differently from a place that simply keeps the rain out.
So the order of work matters. The first question is whether you may use the roof as a terrace at all, which is a planning matter. The second is whether the structure underneath can carry the load. After that come the details that make it safe and watertight: the edge, the way water leaves the roof, and a waterproof layer that tolerates being walked on for years.
This guide takes those questions in the order we would ask them on a survey. It covers the planning and Building Regulations rules, then the practical design points, and says where a designer or engineer must take over. Our flat roofing page shows the wider range of work.
Why a terrace is a planning question before a roofing one
Roofers like to talk about membranes, but the first hurdle for a terrace is permission. Under the General Permitted Development Order, the rules for adding to a roof (Class B) say that development is not permitted if it includes a verandah, balcony or raised platform. A railing around a flat roof, a platform laid over it or a balcony built out from it all sit outside what Class B allows. In plain terms, turning a roof into a terrace normally means applying for planning permission rather than relying on permitted development.
Class B is also the class used for dormers, and it carries a second restriction that matters locally: it does not apply on article 2(3) land, which includes conservation areas and the South Downs National Park. Worthing has a large number of conservation areas, and parts of the borough fall inside the National Park for planning purposes. If your house is in one of them, the answer is stricter still.
Flats are a separate case. The Order defines a dwellinghouse so that it does not include a building containing flats, or a flat within one, which means flat owners do not have these householder rights in the first place. A lease may also forbid use of the roof.
Before you design anything, ask the Adur and Worthing planning team whether the roof can be used as a terrace, and mention any listing, conservation area or National Park status. Where the South Downs National Park Authority is the planning authority, ask them instead.
Neighbours' privacy is the point planning officers usually weigh first, because a person standing on a roof sees into windows and gardens that a person standing at ground level does not. Our guide to planning permission for roof work explains the wider picture, and a short pre-application conversation with the council costs less than redrawing a scheme that was refused.
Whether the structure can carry people, furniture and soil
Most flat roofs on houses are designed for access to maintain them, not for a crowd on a summer evening. Joists sized for a ceiling and a light covering may not suit a terrace, which brings its own weight (decking or paving on supports), the people on it and anything they bring, from loungers to heavy planters. Wet soil is far heavier than it looks. A roof garden with deep beds can ask a lot more of the structure than a deck with a few pots.
That is a job for a structural engineer or a competent designer, and it should happen before the work is priced, because the answer can change the whole scheme. Where a roof's loading will change, the Planning Portal points owners towards a surveyor or structural engineer before work begins, and a covering that adds 15 per cent or more to the roof's loading is one of its triggers for building control. A terrace is a larger load change than any covering.
What triggers Building Regulations
Even a plain re-cover can bring in Part L when enough of the roof is involved. A flat roof that is re-waterproofed counts as renovation of its thermal element, and past 50% of the surface the entire element is expected to improve. For a roof built new or swapped on a house that already stands, Table 4.2 sets the target at 0.15 W/(m²·K). A terrace build usually renews the whole roof, so expect the upgrade to apply.
That has a knock-on effect on height. A thicker insulation layer raises the finished level, and the raised level needs to be thought about at doors, which need a step down or a threshold detail that keeps water out. Our page on flat roof Building Regulations goes through the thresholds, and building control will tell you what applies to your roof.
Edges, guarding and getting onto the roof
An ordinary flat roof has an edge trim or a small upstand and little else. A terrace needs a barrier at the edge that stops a person falling, and the roof has to keep its waterproofing at the exact points where that barrier is fixed. This is where terraces most often go wrong. A post bolted through the membrane with a dab of sealant around it is a hole that will open up with the first winter.
The better routes avoid puncturing the waterproof layer at all. A parapet wall raised to a safe height does the job and has the benefit of a solid face you can flash into. Where posts are unavoidable, they are mounted on an upstand or a sleeve that the membrane is dressed up and over, so that the water-resistant layer is continuous and the fixing sits above it. Guarding height, the gaps allowed between rails and the loads a barrier must resist are set out in the Building Regulations, and your designer or building control body gives the figures for your house. We do not guess them.
Getting onto the roof is the other half. A window turned into a door, a new stair or a ladder hatch each changes the building and may bring in fire safety, structural and planning questions of their own. If the access is from a first-floor room, the door threshold becomes the weak point for water, because the finished roof surface has to sit below it with enough drop to keep rain from splashing or running inside.
Planters and furniture
Heavy items should stand on spreader pads or a raised deck, not directly on the membrane. Pots left in one place can also hold water against the surface and stain or damage it, so lift them on feet that let water through.
Falls and drainage when the surface is no longer the weather layer
A terrace hides the waterproof layer under a deck or paving, which makes drainage easy to neglect and expensive to fix later. The rules for the waterproofing itself do not change. The 2018 edition of BS 6229 (since revised), as set out by IKO, recommends a minimum fall of 1:80 on a flat roof, and the roof should be designed with a fall of 1:40 so tolerances and sag cannot eat the slope. The terrace has to work with that fall, not hide it.
Paving or decking set on adjustable pedestals does this well. The pedestals sit on the membrane and level the walking surface, while the water runs through the gaps and down the fall below, out of sight. It also means you can lift a slab to clear an outlet. Decking laid on timber battens straight onto the membrane is cheaper, but it traps water and leaves, and the battens rot or the membrane beneath them gets pressed and abraded.
Outlets and gutters need more planning than on a roof nobody uses. A terrace sends more debris down the roof: leaves from planters, compost, grit and the odd dropped sock. A leaf guard that can be reached and cleared matters, and so does a secondary route for overflow, so that a blocked outlet shows itself at an edge rather than building a pond under the deck. Standing water is a topic of its own, and our guide to flat roof ponding explains why it shortens the life of a covering.
Where a terrace has been laid over a roof of unknown age, consider whether the covering below will outlast the deck, because lifting a terrace to renew the roof is a heavy job.
Membranes that cope with foot traffic
Not every flat roof covering likes being walked on. A felt roof with a mineral top surface can be scuffed through by regular traffic, and lifting a loaded terrace to repair it is the expense you want to avoid. The usual choices for a roof with traffic are single-ply EPDM, a GRP fibreglass system or a quality built-up system, each with a surface or protection layer designed for the job. Our guides to EPDM against GRP and to walk-on flat roofs compare how they behave under use.
Lifespan matters more under a terrace, since the covering has to outlast the deck above it. Published ranges give EPDM at 20 to 30 years or more, GRP at 25 to 40 years (Less, March 2026), and torch-on felt at 10 to 20 years, with sources disagreeing on how long a good felt job lasts. Treat them as typical ranges rather than promises; the detailing has to be right for them to apply.
MyBuilder (updated 21/08/2026) gives typical UK ranges per m² for replacing a flat roof covering of £70 to £100 for felt, £90 to £130 for EPDM and £95 to £150 for GRP. A terrace adds the deck, pedestals, guarding, door alterations and engineering on top, so those figures are only the waterproofing part. The price reaches you in writing once the survey is done.
Decking and paving over the top
Whatever sits above the membrane needs a protection layer so that nothing sharp touches the waterproofing. Pedestal supports should have broad bearing plates or rubber pads, and rigid insulation under a deck needs to be rated to take the point loads. These are small details that the manufacturer's data sheet specifies, and we follow that sheet.
Roof gardens, wind and keeping the whole thing in good order
A roof garden asks the most of a roof. Growing media holds water, and a deep, wet layer weighs far more than dry paving. Planting also introduces roots, and a roof without a root-resistant layer can be damaged by them in time. Our page on the sedum green roof covers the lighter option, where a shallow mat of low plants spreads the weight and needs less water and upkeep than a garden with beds.
On this coast, wind is the other design factor. A raised, exposed roof takes the sea air with nothing to slow it down, and light furniture, loose planters and flimsy screens become things that can blow off. Fix screens properly, choose heavy or tethered pots and place any glazed barrier with the same care as the guarding. Salt air collects as a deposit on surfaces that rain does not reach, so look under the deck and behind the screens when you inspect.
Once the terrace exists, maintenance matters more, not less. Clear leaves and debris from outlets, lift a slab or a deck panel at least once a year to look at the membrane beneath, and check the upstands and the door threshold after every winter. Our guide to flat roof maintenance sets out the routine.
If you plan to extend the house upward or outward at the same time, it is worth reading about flat roof extension design so that the terrace and the roof are designed together. And if the covering underneath is already tired, repair or replace is the decision to make before the deck goes on.
