Balcony and roof terrace structural design
Structural design for a new balcony or roof terrace, including its connection back to the building.
About this service
Balcony and terrace structural design covers everything needed to make a new outdoor space safe: the deck or slab, the supporting beams or brackets, the connection back into the existing building and the loading from balustrades. We produce calculations and structural drawings for Building Control and your builder. Design fees start from £750 + VAT, confirmed in writing before any work starts.
The connection to the house is usually the critical part. A cantilevered balcony puts large forces into the wall or floor behind it, and older London brickwork and timber joists were never designed for this. Many schemes need a steel frame, posts down to new foundations, or a back-span tied into the internal floor. Thermal breaks are often needed where steel passes through the insulated envelope, to avoid cold bridging and condensation.
Turning an existing flat roof into a roof terrace is a common London project, especially over rear extensions and on mansard and loft conversions. Flat roofs designed only for maintenance access are rarely strong enough for people, planters and furniture. All our designs and calculations go through a three-step quality control process, with final sign-off by our Head of Engineering, before they are issued.
When you need it
- You want to add a balcony or roof terrace
- An existing flat roof is being turned into a terrace
- Planning has been granted and you need the structure designed
What's included
- Design of the balcony or terrace structure
- Connections to the existing building
- Balustrade loading
- Calculations and drawings
How it works
- Review the schemeWe look at the architect's drawings or your proposal, the planning consent and any survey information, then confirm scope and a fixed fee in writing.
- Check the existing structureWe establish how the existing roof, floor or wall is built, often from a site visit, and what it can carry, including the wall that will receive the balcony connection.
- Design structure and connectionsWe design the deck, beams, posts and foundations as needed, and detail fixings back into the building, including thermal breaks and drainage falls where relevant.
- Balustrade and finishes loadingWe account for crowd and barrier loads on the balustrade and the weight of paving, planters and screens, so the whole assembly is checked together.
- Issue for approvalYou receive calculations and structural drawings. We send them to Building Control, answer their queries and support your builder with any technical questions once work is on site.
What you receive
Turning a flat roof into a terrace: what usually has to change
Most flat roofs over London extensions use timber joists sized for snow and maintenance access, not for a group of people. Converting one to a terrace usually means strengthening or replacing joists, checking the beams and walls below, and designing the balustrade fixings so they do not puncture the waterproofing badly. Planters filled with wet soil are surprisingly heavy and must be included. Our guide to roof terraces on flat roofs explains the main checks, and roof strengthening design may be part of the solution.
Planning permission is a separate matter, and councils often look closely at overlooking and privacy screens. Building Control will also check guarding height, fire performance of materials and access. Where a balcony connects across a party wall or affects a neighbour's structure, the Party Wall etc. Act 1996 may apply. For existing balconies with concerns, a balcony and walkway safety inspection is the right starting point instead.
Frequently asked questions
Do I need a structural engineer for a roof terrace?
Yes, in almost every case. Converting a flat roof to a terrace increases the load significantly and adds horizontal forces from the balustrade. Building Control will expect calculations showing the roof, its supports and the balustrade fixings are adequate. Most existing flat roofs need some strengthening to be used as a terrace.
Can you add a balcony to an existing brick house?
Often, yes, but the design must deal with how loads reach the existing structure. Older brick walls and timber floors may not resist the forces from a cantilever, so the balcony may need posts to the ground, a steel frame or ties into the internal floor. Planning permission is also usually required.
What loads does a balcony need to be designed for?
A balcony is designed for its own weight, finishes such as paving and planters, people using it, and the horizontal load on the balustrade from people leaning or pushing against it. Values come from the Eurocodes and are higher than for a roof used only for maintenance. The connections must carry all of these safely.
How long does balcony structural design take?
Our typical turnaround is 10 working days from receiving the information we need, confirmed in writing with the fee. If the existing structure needs investigating first, for example to confirm joist sizes or wall construction, we allow for that in the programme and tell you before starting.