Turning a flat roof into a roof terrace: the structural questions

Can my flat roof be used as a roof terrace?

Possibly, but rarely as it stands. Most domestic flat roofs were designed to carry their own weight, snow and the occasional person doing maintenance. A terrace has to carry groups of people, furniture, planters and paving, and its edges need guarding that resists people leaning on it. That means the roof structure normally needs checking by calculation and, quite often, strengthening. Planning permission is usually needed as well.

The good news is that most roofs can be adapted. The engineer's job is to find out what is there, work out what it can carry, and design the most practical way to make up any shortfall.

Why a roof is not the same as a floor

The Eurocodes set minimum design loads according to how a space is used. A roof without access is designed for a relatively light imposed load. A floor, balcony or terrace where people gather is designed for considerably more, and terraces are treated at least as onerously as the rooms they serve. Building Control will confirm what applies to your project, but in practice the gap between a maintenance-only roof and a terrace is large.

Strength is only half the story. Joists that are strong enough may still be bouncy, which feels unpleasant underfoot and can crack the ceiling below. Terrace finishes add permanent weight too: paving on pedestals, decking frames, planters full of wet soil and screens. Very heavy items such as hot tubs or large water features need a specific design of their own and should never be assumed to be acceptable.

Snow can also gather more deeply on a terrace than on an open roof, because balustrades, screens and planters create sheltered corners where drifts form. The engineer allows for this alongside the loads from people and finishes.

Checking the existing roof

The engineer needs to know the joist size, spacing, span and direction, what the joists bear onto, and their condition. Flat roofs have a long history of leaks, and timber that has been damp for years can be decayed at the bearings even when the ceiling below looks fine. Some inspection from inside, through a ceiling opening or at the roof edge, is normally needed before the design can be finalised.

Concrete roofs on later blocks of flats and mixed-use buildings need a different check, including what reinforcement is present and how the slab spans. In a building from the 1950s to the 1990s, confirm the roof is not RAAC before planning a terrace; our guide on RAAC explains why.

The walls and beams below the roof also need checking, because the extra load travels down through them to the foundations. In an older building with altered walls on the floors below, that load path is not always obvious.

Strengthening options

Where the existing roof falls short, the common solutions are:

The choice depends on the roof's condition, the headroom in the room below, the threshold level at the access door and how the waterproofing will be detailed. If the roof covering is due for replacement anyway, renewing the structure at the same time is often the most economical answer. Our roof strengthening design service covers the existing roof, while balcony or terrace structural design covers the terrace structure, its connections to the building and balustrade loading, from £750 + VAT.

• Adding new joists alongside or between the existing ones
• Inserting steel beams to shorten the span of the existing joists
• Building an independent steel or timber deck frame that bears onto the walls, leaving the original roof to keep the weather out
• Replacing the roof structure entirely with one designed as a terrace from the start

Balustrades and parapets

Guarding must resist a horizontal load at handrail height from people leaning or crowding against it. The Building Regulations also set a minimum guarding height for roofs and balconies, which Building Control will confirm for your design. Fixings are often the weak point: posts bolted into the top of a thin parapet, or into a timber edge joist, can loosen over time and pull out.

Existing parapet walls are rarely strong enough to act as guarding on their own, particularly half-brick parapets on older buildings with weathered mortar. They may need rebuilding, strengthening or supplementing with a separate balustrade fixed back to the main structure. Glass balustrades need their own checks on the glass, the channel and the fixings into the structure.

If a parapet or balustrade sits on or against a party wall, the Party Wall etc. Act 1996 may apply. Our guide to the Party Wall Act explains when notice is needed.

Waterproofing, falls and access

Every fixing through the waterproofing is a potential leak. Good details keep the terrace finish floating on pedestals or on a separate frame, and take balustrade posts through properly sealed upstands. The terrace must still drain, so falls, outlets and overflows need to stay clear and accessible. In a warm roof, the insulation must be firm enough not to crush under pedestal feet.

Access is often the hardest part. A new door may mean forming an opening in an external wall or raising a section of roof, while a hatch with a stair means trimming joists around a new opening. Both are structural alterations that need designing. The door threshold also has to sit high enough above the finished terrace to keep water out, which can drive the levels of the whole build-up.

Planning and Building Control

Roof terraces normally need planning permission because of overlooking and privacy, and many councils have specific policies on them, particularly in conservation areas such as large parts of Camden. Privacy screens are a common condition, and they add wind load to the balustrade and its fixings.

Building Control approval is needed for the structural changes, the guarding and any new access. All our designs and calculations go through a three-step quality control process, with final sign-off by our Head of Engineering, and we send them to Building Control and liaise with them on any queries.