Architecture
Understanding Flat Roof Construction
Considering a flat roof for your next building project? Here are the types available and what you need to think about first
Although there are certainly many times and places throughout the history of architecture where flat roofs have been used, the flat roof has been intrinsically tied with modern architecture since it emerged from the Modernist style in the 1920s.
Influential architects working in the early 1900s such as Walter Gropius and Le Corbusier saw ornamentation and traditional styles of architecture as not being true to the emerging influence of industrial production, and began developing an approach to design that reflected the long straight lines that were easily achieved through materials manufactured by machine. They used modern materials such as steel, glass and concrete to rationalise their buildings, and searched for design ideas that represented this rationale through the form of the building.
Flat roofs became popular amongst early 1900s Modernist architects because they removed the often complicated pitched roofs of traditional construction and replaced them with perfect straight lines. These lines gleamed against the sky in the same way as other products of industrialisation such as trains and ocean liners, and so represented a new way of creating buildings.
Influential architects working in the early 1900s such as Walter Gropius and Le Corbusier saw ornamentation and traditional styles of architecture as not being true to the emerging influence of industrial production, and began developing an approach to design that reflected the long straight lines that were easily achieved through materials manufactured by machine. They used modern materials such as steel, glass and concrete to rationalise their buildings, and searched for design ideas that represented this rationale through the form of the building.
Flat roofs became popular amongst early 1900s Modernist architects because they removed the often complicated pitched roofs of traditional construction and replaced them with perfect straight lines. These lines gleamed against the sky in the same way as other products of industrialisation such as trains and ocean liners, and so represented a new way of creating buildings.
THE PARAPET ROOF
By far the most common type of flat roof isn’t actually flat at all. A parapet roof involves a typical sloped roof that is concealed behind a ‘parapet’ wall so that it gives the impression of being flat.
By far the most common type of flat roof isn’t actually flat at all. A parapet roof involves a typical sloped roof that is concealed behind a ‘parapet’ wall so that it gives the impression of being flat.
Roof pitch
Because parapet roofs are encircled by walls, they are more prone to leaking than traditional pitched roofs. Flat roofs of this type are usually finished in steel roof sheet and fall towards a special type of gutter called a ‘box gutter’.
Although the construction of the actual roof component of a parapet roof is almost identical to a typical pitched roof, it is wise to use a greater minimum roof pitch than is recommended by the roof sheet manufacturer to ensure that no back flow of water occurs at the junction with the parapet wall.
Because parapet roofs are encircled by walls, they are more prone to leaking than traditional pitched roofs. Flat roofs of this type are usually finished in steel roof sheet and fall towards a special type of gutter called a ‘box gutter’.
Although the construction of the actual roof component of a parapet roof is almost identical to a typical pitched roof, it is wise to use a greater minimum roof pitch than is recommended by the roof sheet manufacturer to ensure that no back flow of water occurs at the junction with the parapet wall.
Box gutters
A traditional roof drains water to gutters on the outer edge of the eaves. If the downpipes get blocked or the gutters are overwhelmed, the water will overflow the gutters and sheet off the roof into the garden below. Unfortunately the gutters on a parapet roof are contained within the parapet walls, and if they become overwhelmed, water will flow back into the building and damage the interior of the home.
Box gutters are wide and deep and can handle increased rainfall without being overwhelmed. Box gutters are usually drained by a simple down pipe, but in the case of the down pipe being blocked by leaves or other debris, an overflow should be included to ensure that the gutter is not overfilled. Overflows might consist of a second pipe set slightly above the normal down pipe, or cut-outs at the end of the box gutter that allow water to overflow through the parapet wall.
The outlets of overflows should be in a readily visible location so that it is immediately obvious to the occupants that the primary drainage system has failed. Water coming out of the overflow should act as a signal that the main down pipe needs to be cleared.
Box gutters are usually wide and flat and are an attractive place to walk when working on the roof. Box gutters should be designed to allow people to walk along them, and so they must be built strong enough to sustain the weight of a person.
A traditional roof drains water to gutters on the outer edge of the eaves. If the downpipes get blocked or the gutters are overwhelmed, the water will overflow the gutters and sheet off the roof into the garden below. Unfortunately the gutters on a parapet roof are contained within the parapet walls, and if they become overwhelmed, water will flow back into the building and damage the interior of the home.
Box gutters are wide and deep and can handle increased rainfall without being overwhelmed. Box gutters are usually drained by a simple down pipe, but in the case of the down pipe being blocked by leaves or other debris, an overflow should be included to ensure that the gutter is not overfilled. Overflows might consist of a second pipe set slightly above the normal down pipe, or cut-outs at the end of the box gutter that allow water to overflow through the parapet wall.
The outlets of overflows should be in a readily visible location so that it is immediately obvious to the occupants that the primary drainage system has failed. Water coming out of the overflow should act as a signal that the main down pipe needs to be cleared.
Box gutters are usually wide and flat and are an attractive place to walk when working on the roof. Box gutters should be designed to allow people to walk along them, and so they must be built strong enough to sustain the weight of a person.
THE ROOF TERRACE
Roof terraces are one of the dreams of Modernist architecture, but have not seen the popularity of parapet roofs due to their complexity and the difficulties associated with overlooking provisions written into residential design codes.
Because roof terraces are typically located over internal spaces of the house, ensuring the terrace is waterproof is the biggest challenge for designers.
Roof terraces are one of the dreams of Modernist architecture, but have not seen the popularity of parapet roofs due to their complexity and the difficulties associated with overlooking provisions written into residential design codes.
Because roof terraces are typically located over internal spaces of the house, ensuring the terrace is waterproof is the biggest challenge for designers.
Concrete construction
The simplest way to construct a roof terrace is by utilising a concrete slab. Because typical concrete is porous, it needs to be treated with an additive such as Xypex, which makes the concrete waterproof.
Even though a treated concrete slab is waterproof, standing water on a roof terrace should be avoided, and so all roof terrace slabs should be built with a small slope to ensure that all water is drained.
Houses also usually require numerous penetrations through the roof to accommodate things like sewage vent pipes, extractor fans and other services. These vents should be kept as far from the usable part of the roof terrace as possible. Where they do come through the terrace slab, they should be built into raised hobs to ensure that water build up on the terrace does not find its way down the roof penetration.
The simplest way to construct a roof terrace is by utilising a concrete slab. Because typical concrete is porous, it needs to be treated with an additive such as Xypex, which makes the concrete waterproof.
Even though a treated concrete slab is waterproof, standing water on a roof terrace should be avoided, and so all roof terrace slabs should be built with a small slope to ensure that all water is drained.
Houses also usually require numerous penetrations through the roof to accommodate things like sewage vent pipes, extractor fans and other services. These vents should be kept as far from the usable part of the roof terrace as possible. Where they do come through the terrace slab, they should be built into raised hobs to ensure that water build up on the terrace does not find its way down the roof penetration.
Framed construction
Framed construction is much lighter than a concrete slab, and requires much less structural support, but it is significantly more difficult to waterproof. Framed roof terraces usually achieve waterproofing by using a thin membrane that should not be penetrated by any fixings.
The framed terrace requires a sub-floor that is installed on a slope to ensure that drainage occurs. Waterproofing is achieved with either a sheet membrane that is heat welded to make it monolithic, or a liquid membrane applied to the sub-floor. The membranes are thin and vulnerable to damage and need to be protected by a durable floor surface.
A decking material is usually installed over the sub-floor. It is often advisable to raise the deck off the waterproof sub-floor to create a drainage layer that will move water away quickly so that standing water is avoided and the risk of leaks is minimised.
Fixing the decking material down to the sub-floor is a challenging exercise as any penetration will increase the risk of leaks no matter how well it is sealed. Adhesives may be useful in bonding the framing of the deck to the sub-floor without creating holes. Some decking materials have high thermal expansion and contraction, which should be considered when investigating fixing methods.
Framed construction is much lighter than a concrete slab, and requires much less structural support, but it is significantly more difficult to waterproof. Framed roof terraces usually achieve waterproofing by using a thin membrane that should not be penetrated by any fixings.
The framed terrace requires a sub-floor that is installed on a slope to ensure that drainage occurs. Waterproofing is achieved with either a sheet membrane that is heat welded to make it monolithic, or a liquid membrane applied to the sub-floor. The membranes are thin and vulnerable to damage and need to be protected by a durable floor surface.
A decking material is usually installed over the sub-floor. It is often advisable to raise the deck off the waterproof sub-floor to create a drainage layer that will move water away quickly so that standing water is avoided and the risk of leaks is minimised.
Fixing the decking material down to the sub-floor is a challenging exercise as any penetration will increase the risk of leaks no matter how well it is sealed. Adhesives may be useful in bonding the framing of the deck to the sub-floor without creating holes. Some decking materials have high thermal expansion and contraction, which should be considered when investigating fixing methods.
THE GREEN ROOF
Green roofs allow vegetation to be planted on a flat roof as if it were an extension of the garden below. The soil that is held on the green roof to support the planting also acts as an insulation blanket that will improve the thermal performance of the home.
Green roofs are a wonderful addition to a home, but they do require additional structure to hold up the weight of the soil, and they can be susceptible to leaks.
Although there are many ways to build a green roof, and each method will be suited to the particular type of construction used throughout the home, a number of key components will be present in almost all examples.
Green roofs allow vegetation to be planted on a flat roof as if it were an extension of the garden below. The soil that is held on the green roof to support the planting also acts as an insulation blanket that will improve the thermal performance of the home.
Green roofs are a wonderful addition to a home, but they do require additional structure to hold up the weight of the soil, and they can be susceptible to leaks.
Although there are many ways to build a green roof, and each method will be suited to the particular type of construction used throughout the home, a number of key components will be present in almost all examples.
The ability of water to evaporate from below the soil in a green roof is significantly restricted.
Generally, standing or pooling water is the enemy of any roof, and the waterproofing of the green roof is critical to prevent leaks. A solid waterproof membrane should be installed with a fall toward a roof drain to ensure that as little water as possible is left in the roof system. The fall to the drain can be created by casting a concrete slab with the appropriate slopes or using tapered rigid insulation.
The waterproofing can be achieved with a liquid applied membrane, but is often accomplished via a sheet membrane that can be loose laid and held down by the mass of the dirt.
Generally, standing or pooling water is the enemy of any roof, and the waterproofing of the green roof is critical to prevent leaks. A solid waterproof membrane should be installed with a fall toward a roof drain to ensure that as little water as possible is left in the roof system. The fall to the drain can be created by casting a concrete slab with the appropriate slopes or using tapered rigid insulation.
The waterproofing can be achieved with a liquid applied membrane, but is often accomplished via a sheet membrane that can be loose laid and held down by the mass of the dirt.
To ensure that water flows to the drain, a drainage mat is installed over the membrane. A drainage mat creates a thin airspace between the soil and the membrane. The mat has a geo textile fabric that holds out sand and soil, but allows water to pass through, ensuring that any water flowing through the soil is quickly discharged. The geo textile fabric also prevents any roots from damaging the waterproof membrane.
The soil depth required for a green roof depends on the types of vegetation that are intended to be grown on the roof. The additional weight of this layer of dirt will have a significant influence on the amount of structural support that will be required to hold the roof up.
The soil depth required for a green roof depends on the types of vegetation that are intended to be grown on the roof. The additional weight of this layer of dirt will have a significant influence on the amount of structural support that will be required to hold the roof up.
The flat roof is an architectural device that is loved by designers for its ability to create simplistic linear forms. Flat roofs can be used to create box shapes, cantilevered into daring overhangs or can be planted to create usable vegetated gardens.
Flat roofs come in many shapes and sizes, which include parapet roofs, roof decks, and green roofs, to name just a few of the options available.
Ultimately a flat roof should play a part in a total architectural composition, and has the ability to enhance the aesthetic and practical outcomes of a home.
MORE
Building a Home: 10 Decisions You Will Never Regret
Attention All Renovators: 11 Things to Expect
Dreaming of a New Ceiling? Here’s What You Never Knew Was Possible
Flat roofs come in many shapes and sizes, which include parapet roofs, roof decks, and green roofs, to name just a few of the options available.
Ultimately a flat roof should play a part in a total architectural composition, and has the ability to enhance the aesthetic and practical outcomes of a home.
MORE
Building a Home: 10 Decisions You Will Never Regret
Attention All Renovators: 11 Things to Expect
Dreaming of a New Ceiling? Here’s What You Never Knew Was Possible
Improvements in materials also allow contemporary designers to explore new ways of building flat roofs that could not have been achieved in the past.
While there are any number of ways to build flat roofs, this article looks at the three common ways they are built.