Flat roof house in Batumi: design and construction

A flat roof house in Batumi differs from an ordinary one above all in its top slab. It is designed as a floor that people walk on and that carries planters, furniture and outdoor units, not as a roof that a roofer climbs onto once a year. The key decision is made at the sketch stage: whether the roof will be a usable one, where the access to it will be and what will stand up there. This determines the reinforcement of the slab, the height of the parapet and how many outlets and risers will run through the house.

A slab instead of a roof: what we decide before the calculations

A cast-in-place concrete frame suits a usable roof best: the top slab is tied to the columns and beams as a single whole and becomes the base for the roof straight away. How this system works and what the walls are filled with is set out on the page about concrete frame houses. A roof terrace is also possible over a light steel or timber frame, but it needs a rigid slab or decking designed by calculation, and the estimate shows this at once.

Before the calculations, we give the structural engineer a list of what will be up there: a seating area, planters with trees, solar collectors, water heaters, outdoor air conditioning units. If a year later you want a hot tub or a pergola on the roof, the slab will have to be checked again. Strengthening a finished slab is harder and more expensive than building a margin into the reinforcement from the start.

Falls: how the water finds the outlet

A flat roof isn't actually flat. Water runs to the outlets along a fall, which we usually set at no less than 2%. The fall is formed with a lightweight screed over the slab or with tapered insulation boards. Tapered boards are lighter and more accurate; screed is cheaper, but it adds weight to the slab and takes a long time to dry in damp weather.

A single fall towards the edge isn't enough: along the edge, between two outlets, there is a strip where water stands. To deal with it, the design includes a falls plan with levels and shallow valleys that steer the water around the roof access, the equipment supports and the stair enclosure. Before the waterproofing goes on, the site foreman checks the levels against this plan with a surveyor's level: a dip of a couple of centimetres becomes a puddle after a downpour, and it sits on the membrane until the next rain.

Drainage inside the house and through the parapet

There are two ways to take water off a flat roof. Internal outlets with risers can't be seen on the facade, but the riser passes through the rooms. We put it in a service duct with an access point for cleaning and wrap it in sound insulation, otherwise in a downpour it is noisy behind the bedroom wall. We also insulate it: in winter cold rainwater chills the pipe, and condensation forms on it. Parapet outlets that discharge into external downpipes are easier to maintain and cheaper, but the pipes can be seen from outside, so we decide where they go together with the facade design.

The number and diameter of the outlets are calculated by the engineer from the roof area and the intensity of the rain. Every separate section of roof gets at least two outlets from us, plus an emergency overflow in the parapet in case both are blocked by leaves. Why a flat roof in Batumi is at risk without an overflow, and how it is flood-tested before the surfacing goes on, is covered in the comparison of a flat roof and a pitched roof. The risers discharge into the plot's stormwater drains; we don't dump the water onto the paved apron next to the foundations.

The parapet: guarding, seismic load and thermal bridging

The parapet holds the edge of the roof build-up, and on a usable roof it also acts as part of the guarding. The height of the guarding is set by the design. If the parapet is lower, we add a stainless steel or glass railing on top and fix its posts into the side face of the parapet: a post driven through the membrane on the floor becomes the site of a future leak.

An unreinforced block parapet can collapse in an earthquake. It stands at the edge of the house with nothing holding it down from above, and it sways more than the walls. We build it in cast-in-place concrete or reinforce it by calculation, tied into the slab and the frame. How seismic load changes other decisions in the house is covered on the page about earthquake-resistant construction.

A concrete parapet is also a thermal bridge. If only the roof and walls are insulated, the corner of the top-floor ceiling next to the parapet stays cold in winter, and in damp air mould appears there. We take the insulation up the parapet on both the inside and the outside, and cap it with a coping that falls towards the roof and has a drip, so that dirty water doesn't run down the facade in streaks.

Roof access and equipment on the roof

A usable roof is reached by a staircase through a stair enclosure or through a hatch. An enclosure is more convenient: it has an ordinary door, and you can go up in any weather. But it adds height to the house, so at the sketch stage we check with the architect whether the zoning rules count it in the height of the building. The threshold of the roof door is raised above the surfacing, and the waterproofing is taken under the door frame.

Outdoor air conditioning units, solar collectors and water heaters go on ballasted frames or on concrete plinths that are poured together with the slab and then wrapped with waterproofing. Pipes and cables pass through the roof only through sleeves cast into the concrete before the pour, with collars that seal to the membrane. If there is going to be a full terrace up there, with zones, planters and a canopy, it makes sense to plan it with a designer as part of terrace and rooftop design.

Heat and vapour under the top slab

In summer the concrete slab overhead soaks up heat during the day and releases it into the bedrooms in the evening. The insulation on our roofs is usually thicker than on the walls, and we choose a light-coloured surface on top: tiles, gravel or decking on pedestals, which keeps the membrane beneath in the shade. For roof terraces we more often choose an inverted roof, where the insulation lies above the membrane and also shields it from the sun.

In winter the problem is the reverse: warm, moist air from the house rises to the ceiling and looks for a way into the roof. In a conventional build-up, where the membrane is on top, we lay a vapour barrier with taped seams under the insulation. Extract ducts from the top-floor bathrooms are taken out above the surfacing through a separate penetration with a collar; we never let them end inside the insulation.

Mistakes that later have to be fixed by opening up the roof

• a single outlet in the low corner of the roof, with no second one and no overflow;
• falls formed from loose expanded clay, which soaks up water and releases it into the slab for months;
• a membrane laid on fresh screed that hasn't had time to dry;
• waterproofing not taken under the threshold of the roof door;
• tiles glued straight onto the membrane, with no drainage layer beneath them;
• galvanised flashings and fixings on a house by the sea.

The cost of a flat roof house in Batumi

A mid-range turnkey house with a usable flat roof costs from 1,890 to 2,160 ₾ per m² of total floor area. The price covers the cast-in-place top slab, insulation, waterproofing, the walking surface, the parapets and the drainage inside the building. An ordinary mid-range turnkey house starts from 1,680 ₾ per m²: the difference comes from the slab designed as a floor, the roof build-up and the internal risers. The house design is priced separately.

Taking the roof on its own, a turnkey usable roof with falls, insulation, waterproofing and surfacing costs from 480 to 625 ₾ per m² of roof, and a flat roof without a walking surface, with falls, insulation and a membrane, from 245 ₾ per m². The price goes up with a terrace over the whole roof, a stair enclosure for access, heavy planters and a canopy that call for a stronger slab, and risers running through several floors. A roof that people go up to only for maintenance, and external downpipes through the parapet, come out cheaper. The final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.