Flat roof design and maintenance guide

Flat roofs: waterproofing, insulation, drainage, and valuable tips for contemporary design choices

Flat roofs are one of the most common solutions in modern construction. Their versatility and contemporary, clean, and minimalist design make them a popular choice for a wide range of buildings, from apartment buildings to commercial structures.

The design, construction, and maintenance of a flat roof require special attention to detail and in-depth knowledge of industry best practices. BIM methodology and a building design software can be of great support in all phases of the roof’s life, from design choices (shapes, materials, performance, etc.) to its maintenance. Below, we examine in detail each phase of this process and provide a series of useful design tips.

Flat Roof material layers

The material layer of a flat roof is a crucial part of the design and construction process to ensure the roof remains durable and efficient over time and should include:

• load-bearing element or roof support, which can be:
  • in reinforced concrete (cast-in-place or mixed with autoclaved aerated concrete, precast concrete panels, etc.), coated with a lightweight cementitious layer of variable thickness necessary to form the minimum slope required for rainwater drainage on flat roofs;
  • wood;
  • steel, in corrugated sheet;
• vapor barrier, generally consisting of a distilled polymer bitumen membrane, which is present in the material layers if thermal insulation is provided and serves to protect it from moisture from the indoor environment. It is used only in climates where indoor temperatures may be higher than outdoor temperatures and must always be placed on the “warm” side of the insulating layer. Obviously, for uninsulated roofs, a vapor barrier is not required;
• thermal insulation is essential for building thermal control, to reduce heat loss, limit structural expansions, and prevent condensation on cold walls. Common insulation materials include glass wool, rock wool, expanded polyurethane, and extruded polystyrene, agglomerates of perlite and cellulosic fibers, etc.;
• waterproofing layer consisting of a continuous membrane (bituminous, polymeric, etc.) that protects the roof from water and prevents leaks;
• roof covering layer: the first layer, the outermost one, is the covering membrane, which plays a fundamental role in preventing water infiltration. Common materials include bituminous membranes, PVC or TPO membranes (thermoplastic polyolefin), or elastomeric membranes like EPDM (ethylene-propylene-diene monomer). This membrane must be resistant to water, UV rays, and weathering agents;
• protective layer: above the covering membrane, especially if the roof is walkable or used as a terrace, it is advisable to add a protective layer. This can consist of covering slabs, gravel, decorative stones, or tiles. The protective layer reduces the risk of mechanical damage to the membrane and provides an additional waterproofing layer. In case of load problems, if the membrane is visible, it can be painted with protective paints, or the choice of membranes with surface protection in slate chips, called “self-protection,” is advisable;
• accessory works to complete the work, such as perimeter walls, skylights, gutters, roof drains, fascia boards, etc., which are essential for the proper functioning of the roof and must be carefully designed, executed, and maintained.

Flat Roof – Material Layers

The correct installation of each layer, along with periodic maintenance, is essential to ensure that the flat roof remains durable and trouble-free over time. Thermal insulation, drainage management, and the choice of high-quality roofing materials are particularly important for the long-term success of a flat roof.

With BIM architectural design software, you can create a 3D model of the entire building and assign material layers to the flat roof by choosing from the online catalog of stratifications. You can also customize the stratigraphy (thicknesses, materials, characteristics, etc.) according to your design choices.

Assigning flat roof in the project with the BIM software improve the quality and durability in time

The 3D model can also be used to verify the energy characteristics of each individual component (vertical enclosures, floor slabs, flat roofs, etc.), using BIM software for energy analysis.

Minimum slope for Flat Roofs

The slope of a flat roof is a critical aspect to ensure proper drainage of rainwater and prevent water accumulation that could cause structural damage and leaks. Although “flat” roofs may seem entirely level, they must have a minimum slope to facilitate drainage. The recommended slope for a flat roof varies, but it is generally between 1% and 2%. This means the roof should have a 1-2 centimeter incline for every meter of width to allow for adequate drainage. This slope is sufficient to direct water toward drainage points, such as gutters or downspouts.
Here’s how to create the slope on a flat roof:

• Precise design: The slope should be considered during the design phase of the flat roof. Architects and engineers must determine the water flow direction and drainage exit points;
• Use of drainage boards: One of the most common methods to create slope on a flat roof is by using drainage boards. These boards have an integrated slope and are placed above the thermal insulation but beneath the waterproof membrane. Drainage boards can be made from various materials, including expanded polystyrene (XPS), polyurethane, and more. They are designed with a sloped profile to promote water flow towards the drainage systems;
• Self-leveling slope: Another approach involves applying self-leveling mortar with an integrated slope. This material is applied over the thermal insulation and shaped to create the desired slope. Once it hardens, the self-leveling mortar forms a uniform surface with the necessary slope;
• Drainage systems: Gutters and downspouts should be positioned along the perimeter of the flat roof or in other strategic points to collect water and direct it toward the drainage system. Proper sizing of these systems is essential to handle the expected volume of rainfall;
• Quality control: During construction, it’s essential to check the slope and proper installation of all layers, particularly the drainage boards or self-leveling mortar, to ensure the slope is uniform and correct across the entire roof.

Creating adequate slope on a flat roof is a fundamental part of its design and construction. Properly done, it will help prevent drainage issues and keep the roof in good condition over time, extending its service life and reducing the risk of damage.

Roof Slope

Waterproofing for Flat Roofs

Waterproofing a flat roof is another crucial aspect of its construction to prevent water infiltration and protect the underlying structure.
Here are the main types of waterproofing:

• Bituminous membranes (torch-applied or cold-applied): Bituminous membranes are commonly used for waterproofing flat roofs. They can be applied with heat (torch-applied) or cold adhesives. Torch-applied bituminous membranes require a torch or burner for fusion and adhesion to the substrate. Cold-applied membranes come with self-adhesive or are glued with mastic. They consist of multiple layers of bitumen, fiberglass, and mineral granules, offering good durability;
• PVC or TPO membranes: PVC (polyvinyl chloride) or TPO (thermoplastic olefin) membranes are popular choices for waterproofing flat roofs. These membranes are lightweight, weather-resistant, UV-resistant, and can be heat-welded to ensure a durable seal. They are also root-resistant, making them suitable for green roofs;
• EPDM membranes: EPDM (ethylene propylene diene monomer) membranes are made of synthetic elastomeric rubber. They are highly flexible, durable, and weather-resistant. They are typically adhered to the substrate with adhesives or mechanically with screws and washers;
• Liquid membranes: Liquid membranes, such as polyurethane or acrylic, are applied in liquid form and then cured to create a continuous waterproof coating. They are suitable for complex or irregular roofs and can be used as a protective coating over other membranes.

Here are the phases and tips to follow for flat roof waterproofing:

• Surface preparation: The roof surface should be clean, smooth, and free from debris or contaminants that could compromise the adhesion of the waterproofing material;
• Protection layers: To safeguard the waterproof membrane from mechanical damage, it’s advisable to use protection layers like cover boards or gravel over the membrane;
• Thermal insulation layers: Thermal insulation should be installed beneath the waterproof membrane to prevent condensation and enhance energy efficiency;
• Proper membrane installation: Carefully follow the manufacturer’s instructions for membrane installation. Ensure they are well-welded, glued, or securely fastened to the substrate;
• Critical details: Pay special attention to critical details like joints, flashings, and gutters. These areas are more vulnerable to water infiltration and require meticulous sealing;
• Regular maintenance: Conduct regular inspections and schedule preventive maintenance. Membranes may require repairing any damage or replacing compromised areas;
• Drainage system: Ensure the drainage system is functional and free of obstructions to ensure proper rainwater drainage.

Warm Roof vs. Cold Roof

In the context of roofs with a continuous waterproofing layer, the warm roof is widely used, especially in industrial construction, while the cold roof, also known as a ventilated roof, is primarily employed for wooden roofs and residential buildings.
In a warm roof, all layers are compactly structured, adjacent to each other, and resting on one another. In contrast, the cold roof stratigraphy incorporates a ventilation space between layers. This space is positioned between the thermal insulation and the roof deck if the roof is insulated, or beneath the roof deck that supports the waterproofing layer if the roof is not insulated.

In the case of a warm roof, the waterproofing layer is generally applied directly over the thermal insulation, which acts as the support. In the cold roof, the waterproofing layer is placed on the roof deck, which is typically made of wood.

Regarding the thermal insulation in a warm roof, it’s important to consider that insulation materials must be selected based on their thermal insulation capabilities and should meet requirements for dimensional stability, compression resistance, and moisture resistance. It’s also crucial to evaluate compatibility with adjacent layers and the ability to withstand adhesives used during the membrane installation. The insulation should not deform or bulge due to temperature and humidity variations. Additionally, its dimensions should prevent movements of the overlying membrane along the panel joints and ensure good adhesion against wind forces.

The choice of insulation type for an insulated parking terrace will be influenced by the need for compression resistance, while for an exposed roof, this feature might be less critical. Several insulation types are used in roofing, and the specific choice should be based on the project’s requirements.

Here’s a brief video demonstrating how to correctly model a roof using building design software.

Designing a Flat Roof

Let’s briefly recap the stages of designing a flat roof:

• Initial planning: determining the intended use of the building and specific requirements for the flat roof, such as accessibility for maintenance or equipment installation;
• Taking into account local climatic conditions, snow loads, wind, and torrential rains to determine the required structural strength;
• Identifying the minimum slope required to ensure proper drainage of rainwater;

For more in-depth information, we recommend reading the article “Roof design architecture: guidelines and 4 useful tips“.

Flat Roof construction

The construction phase of a flat roof is the most complex. A small oversight can compromise the proper functioning of the roofing system. Here’s how to proceed:

• Substrate preparation: the roof surface must be carefully prepared, ensuring it is clean, smooth, and free of debris or contaminants;
• Thermal insulation: thermal insulation should be installed above the load-bearing structure or above the waterproof membrane, depending on the design. Insulation should be placed without gaps to prevent condensation;
• Waterproof membrane: the waterproof membrane must be carefully installed following the manufacturer’s specifications. This membrane is responsible for keeping the roof watertight and must be securely welded, glued, or fastened;
• Protective layers: protective layers such as cover boards or gravel should be applied over the waterproof membrane to protect it from mechanical damage and UV rays;
• Drainage system: gutters, downspouts, and a well-designed drainage system should be installed to divert water away from the roof;
• Quality control: during construction, it is essential to perform quality checks to ensure that each component is installed correctly.

Architectural model created with Edificius

Flat Roof maintenance

Maintaining the specified technical characteristics during the design phase is essential for the efficiency of a flat roof. Here are the measures to follow to avoid problems over time:

• Regular inspections: schedule regular inspections to identify any damage or signs of deterioration, with a focus on joints and drainage systems;
• Timely repairs: any damage should be repaired promptly to prevent it from spreading and causing more significant damage. Repairs may include replacing portions of the waterproof membrane or adjusting drainage slopes;
• Drainage cleaning: keep drainage systems clean and free from debris to ensure effective drainage;
• Preventive maintenance: schedule preventive maintenance, which may include periodic replacement of the waterproof membrane or updating thermal insulation;
• Monitoring weather conditions: be aware of local weather conditions, especially heavy precipitation or extreme weather conditions that may require preventive actions.

In summary, the design, construction, and maintenance of a flat roof require attention to detail, the use of high-quality materials, and the scheduling of periodic maintenance interventions. A well-designed and well-maintained flat roof can offer a long service life and protect the building from water infiltration and water-related damage.

The Benefits of BIM in designing and maintaining a Flat Roof

The use of Building Information Modeling (BIM) in designing and maintaining a flat roof offers numerous significant advantages. BIM is an advanced approach to data and information management throughout the entire lifecycle of a building, including design, construction, and maintenance. Here’s how BIM can be effectively applied:

• 3D visualization: BIM enables the creation of detailed 3D models of the flat roof. These models allow all professionals involved to visualize the project more accurately, identifying potential design issues or compatibility between different systems before construction;
• Analysis and simulations: with BIM, advanced analyses and simulations can be conducted to assess the performance of the flat roof. For example, it is possible to evaluate thermal insulation, perform load analyses for floor calculations, or predict the roof’s behavior in various weather conditions;
• Accurate documentation: BIM produces detailed documentation, including drawings (sections, detail graphics, etc.), specifications, and material lists. This documentation is essential to ensure that the roof construction complies with the specifications and can also be used for regulatory compliance purposes;
• Preventive maintenance: after construction, BIM can continue to be used for maintenance management. Data on roof conditions, maintenance schedules, and records of past interventions can be integrated into the BIM model, simplifying the planning and execution of preventive maintenance activities;
• Cost and time reduction: BIM contributes to reducing design and construction costs and time by optimizing processes and better resource management;
• Sustainability: BIM can be used to assess the environmental impact of the flat roof, contributing to the design of more energy-efficient and environmentally friendly solutions.

In summary, the use of BIM in designing and maintaining a flat roof allows for more effective project management, reduces the risk of errors or problems during the building’s lifecycle, and provides greater transparency and control over all aspects, from initial design to long-term maintenance.