– Presentation of the flat roof

 – The different types of flat roofs

 – Supporting materials

 – Calculating the proper slope for a flat roof

 – Insulation of a flat roof

 – Possible openings

 – Waterproofing of the flat roof: the final protection

The flat roof, and its vegetal variant, are part of the 3 types of roof, with the sloping roof and the rounded roof. 

 Presentation of the flat roof

 The flat roof differs from the sloping roof in that it is made up of a single slope and is presented as a terrace. The structure also differs: it is not a framework but support (load-bearing element).

 A flat roof consists of several elements:

 The waterproofing: placed on the surface prevents water from penetrating the flat roof structure and the house.

 – A vapour barrier that protects the insulation from moisture.

 – Thermal insulation to insulate your flat roof from both heat and cold.

 – An independence screen, possibly, to separate chemical elements that would be incompatible.

 – Openings to let in light and air.

 The different types of flat roofs

 The flat roof is very slightly sloping. Therefore, it can be used as an outdoor living space, accessible from the upper floor: it is then called a flat roof. However, this is not always so, as the flat roof is not always accessible.

 There are 2 main types of flat roofs:

 – Accessible flat roof: covered with wood, concrete, stone, tiles or about 30 cm of earth.

 – Flat roof “inaccessible”: accessible only occasionally for the maintenance of the roof or the installations present on the roof (mechanical ventilation, smoke evacuation conduit…), and the waterproofing.

 Supporting materials

 The flat roof has support that fulfills the same role as the framework (for a sloping roof, for example).

 The 3 primary materials for the support of this roof are:

 – Masonry. It is then mainly a solid slab:

 ◦ concrete;

 ◦ in composite material;

 ◦ with steel reinforcement: the concrete slab is poured on-site; this solution is not very suitable for individuals, mainly because of its weight, which can be difficult to bear by a single-family house ;

 ◦ or a reinforced cellular concrete slab, which comes in the form of blocks assembled on site.

 – Sheet steel:

 ◦ in a single-family home, this technique is combined with a reinforced concrete slab;

 ◦ an ideal solution in metal frame houses.

 – Wood, or wood derivative. The support, carried by a rigid and solid structure of wood or masonry, can be in:

 ◦ solid wood;

 ◦ plywood designed for exterior use;

 ◦ water-resistant treated particleboard.

 Correctly calculating the slope of a flat roof

 A flat roof does not mean a completely flat roof. Indeed, if a slight slope is not provided, rainwater :

 – is not evacuated;

 – stagnate on the surface of the flat roof.

 You should therefore apply a slight slope of between 1 and 5% depending on the surface area. Again, methods vary depending on the type and size of the roof:

 – Flat roofs “inaccessible” have a slope of about 1%.

 – Flat accessible roofs or flat roofs have a slope of 1.5% to 2%.

 – Small flat roofs have a slightly curved shape. The center is the highest point.

 – Large flat roofs have a slight slope. The slope never exceeds 5%.

 Good to know: for flat roofs with masonry support, a slope can be created by pouring a layer of concrete or mortar with a slight inclination.

 In all cases, for rainwater to be evacuated, drainage holes are installed at the bottom of this slope, and downspouts carry the water to the bottom of the house.

 Insulating a flat roof

 Insulating a flat roof is essential to:

 – ensure indoor room temperature;

 – prevent energy losses;

 – protect the structure of the house against significant temperature changes.

 Good to know: a poorly insulated house can result in 30% of energy losses.

Insulation materials for flat roofs

A flat roof can be insulated with expanded polystyrene, mineral wool, expanded perlite, cellular glass, rigid polyurethane foam, and expanded clay.

Choosing the proper flat roof insulation

The choice of insulation depends on the roof’s characteristics, for example:

– the type of flat roof: accessible or inaccessible.

– the type of support:

– masonry;

◦ steel sheet;

◦ wood.

– of the protection that will be applied last (the roofing).

Another constraint must be taken into account when choosing insulation for a flat roof: the compressive strength, ranked from A to D, respectively from the least resistant to the most resistant.

Unlike sloped and rounded roofs, flat roofs cannot be insulated from the inside. Typically, it must be insulated from the outside. The insulation is therefore located above the vapour barrier.

Possible openings

The flat roof, because of its particular shape, can only accommodate certain types of openings.

3 openings for the flat roof

Essential to let in the sun and sometimes to ventilate, there are three types of flat roof openings:

– The glass pavement:

◦ glass pavers are small glass cubes installed in the screed when it is poured;

◦ they let light in but not ventilate the room or allow you to see out.

– Skylight:

◦ the skylight is a domed window, often opaque, sometimes openable with a cable, and usually placed on the roof to take advantage of zenithal lighting;

◦ solution mainly used in industrial buildings.

– The roof window:

◦ it is the same roof windows as for sloping and rounded roofs; the only difference is that the minimum slope is compensated by sloping support so that water does not stagnate on the window;

◦ the roof window, therefore, protrudes from the flat roof.

Waterproofing of the roof terrace: the final protection

You must protect the waterproofing layer and other elements of the flat roof:

– the effects of heat, cold;

– the passage of people for maintenance.

To do this, a final layer called protection is applied.

Different types of protection

The protection can be of various kinds:

– The soft protection: composed of aggregates, for example, gravel.

– Hard protection: a poured screed, laying a paving stone with a floor covering (interlocking paving stones or on blocks).

Guardrail for roof terrace

The protection of the roof terrace requires the installation of a guardrail.

Here are some recommendations to implement for the installation of a guardrail around your roof terrace:

– The guardrail must be 1 m high to prevent children from climbing.

– If you opt for a guardrail with bars:

◦ choose vertical bars: horizontal bars would make it easier for children to climb;

◦ the space between the vertical bars must be 11 cm maximum (current regulations) to prevent the passage of a child’s head.

Attention: even the so-called inaccessible roof terraces require a guardrail.

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