Retrofit and the Existing Building - How to Specify a Roof Terrace on a Flat-Roofed Building That Was Never Designed to Have One

Most guidance on roof terrace specification is written for new build. The structural loading is known from the design stage. The membrane is newly installed. The drainage design is current. The Building Control submission covers the roof terrace as part of the overall scheme.

The retrofit scenario is different in almost every respect. The structural loading history of the existing roof is often unknown or incompletely documented. The membrane may be original, repaired, overlaid or of uncertain specification. The drainage design belongs to a building that was never intended to have habitable roof space. The planning position is more complex. And the professional liability associated with specification decisions on an existing building is, if anything, more acute than on a new build.

The structural assessment

The first question in any retrofit roof terrace specification is whether the existing structure can safely carry the additional loading.

A flat roof designed for maintenance access typically carries a superimposed load of 1.5kN/m² plus roofing dead load. A roof terrace accessible to occupants requires a minimum imposed load of 1.5kN/m² for a private terrace or 4.0kN/m² for a communal terrace under BS EN 1991-1-1.

Many existing residential buildings - particularly those constructed between the 1960s and 1990s - can accommodate the additional loading, but only after a full structural assessment. That assessment should establish the existing dead loads, structural form, condition where accessible, and the load path through the building.

This step cannot be omitted. Specifying a roof terrace without understanding the capacity of the existing structure creates unnecessary professional risk, particularly under the Building Safety Act where long-term structural safety is now central to the design process.

The membrane question

On an existing building, the condition of the waterproofing membrane cannot be assumed.

It may be the original installation, it may have been repaired or overlaid, or it may have been replaced relatively recently. Before any roof terrace system is specified, its condition should be assessed through visual inspection, review of available maintenance records and, where appropriate, further investigation.

Where a non-penetrative* roof terrace system is proposed, the membrane should simply be capable of continuing to perform for its anticipated service life while supporting the distributed bearing loads imposed by the system. If it is approaching the end of its life, replacement should be considered before the terrace is installed rather than afterwards.

Planning considerations

Adding a roof terrace to an existing flat-roofed building will usually require planning permission.

The principal considerations are typically the change from non-habitable roof space to external amenity space, the visual impact of balustrades and associated works, and the effect on neighbouring properties through overlooking or loss of privacy.

A non-penetrative* system does not require structural alteration of the existing roof waterproofing, meaning the installation can often be considered more reversible than a permanently fixed alternative. Whether that distinction carries weight will depend on the local planning authority and the individual project, but it is a worthwhile consideration at design stage.

Why a non-penetrative system matters in retrofit

Retrofit presents one challenge that simply doesn't exist on a new-build project: the waterproofing is already complete.

Once a roof is in service, its warranty position has already been established. Introducing penetrations through that membrane creates an additional interface that may affect future maintenance, responsibility and warranty arrangements.

A non-penetrative* roof terrace system avoids disturbing the existing waterproofing. Where the membrane remains in suitable condition, it can continue to perform exactly as intended while the roof terrace is installed above it.

For retrofit projects, that can provide a clearer separation between the waterproofing system and the terrace installation, helping designers, contractors and building owners retain clearer documentation throughout the life of the building.

What do we mean by "non-penetrative"?

A non-penetrative roof terrace system is one that does not drill, bolt or mechanically fix through the waterproof membrane or roof build-up. Instead, the system is engineered to achieve the required structural performance while leaving the waterproofing layer intact. BalcoDeck® is an example of a patented non-penetrative roof terrace system that uses a specially engineered substructure to support decking and balustrades without penetrating the membrane.