Roof Terraces: Load, Waterproofing and Safety

What to check before turning a flat roof into a usable terrace: structural capacity, the order of roof layers, drainage, guarding and access.

A flat roof is a rare piece of open space in the city: a view to the horizon, quiet away from neighbours, cool evenings. In private houses and some residential buildings, wanting to turn it into a terrace is natural. But a roof terrace is very different from an ordinary balcony. A mistake here can lead to leaks into the floor below, overloading of the structure or the risk of a fall. So the work starts not with design but with checks.

The first question: can the roof carry the load

An ordinary non-trafficable roof is designed mainly for its own weight, snow and people going up for maintenance. A terrace is an entirely different scenario:

• many people at the same time;
• furniture, awnings, a barbecue or outdoor kitchen equipment;
• soil-filled beds and large planters — wet soil is very heavy;
• finish layers: stone slabs, pedestals, decking;
• special loads such as a hot tub or water tank.

Only a structural engineer's calculation can establish whether the existing slab, beams and supports can carry these loads. Sometimes only part of the roof, such as the strips over load-bearing walls and columns, is suitable for heavy items. In an apartment building the roof is common property, and any change must be agreed with other residents and the relevant authorities.

Layers of a trafficable roof

On a roof used as a terrace, the build-up differs from an ordinary roof. The general principle: a solid base and a layer that creates the fall at the bottom, then a vapour control layer, thermal insulation, a waterproofing membrane, a protection layer and, on top, the walking surface.

In some systems the insulation goes above the waterproofing. This shields the membrane from heat cycles and knocks, provided the insulation boards can tolerate being wet. Which arrangement is chosen depends on the building's structure and the technical specifications of the materials.

The weakest points of waterproofing are not the flat areas but the junctions: upstands at the parapet, door thresholds, vent outlets and the points where railing posts are fixed. Every screw that pierces the membrane is a potential leak, so supports are fixed without breaching the membrane where possible, or sealed with special collars.

Where the water goes

A flat roof is not really flat: the surface falls towards roof outlets. The terrace finish must not interfere with that fall. The most common solution is paving or decking on adjustable pedestals: the walking surface stays level, while water drops through the joints onto the sloped membrane below and runs to the outlets.

Keep access to the outlets for maintenance and fit leaf and debris guards. A single blocked outlet can flood the whole terrace. An additional overflow through the parapet is recommended.

Guarding, access and safety

The edge of a terrace at height must be protected by a railing or parapet of the height and strength required by building regulations. Glass balustrades keep the view open but require laminated safety glass and secure fixing. Benches and planters a child could climb should not be placed next to the railing.

Convenient, safe access is also part of the design. A steep ladder and a narrow hatch make daily use of a terrace practically impossible. Options include extending the stairwell to the roof, a covered access pavilion, or a staircase combined with a roof light.

Wind is stronger at height than at ground level. Awnings, umbrellas and light furniture should be secured, and the pedestal system chosen with wind uplift in mind. Open-flame grills and fire features need a separate assessment against the roofing materials and the building's fire safety rules.

Checklist before starting

• a structural engineer's assessment and permission issues are resolved;
• the condition and age of the existing roofing have been checked;
• it is known whether there are signs of leaks in the rooms below;
• outlet positions and the fall scheme are known;
• routes for power and water up to the roof are planned;
• positions of heavy items are coordinated with structural supports.

Zaferoğlu İnşaat carries out construction and architectural design of private houses and other buildings. If you have an existing roof in mind, dig out the building's structural drawings first, and we can check with them whether the extra load is possible.