Roofing systems Waterproofing

Waterproofing Of A Flat Roof. Recommended Materials, methods Of Installation, And Prices For Work

Written by David J

Table of Contents

Flat roofing is a very common type of roofing both in commercial buildings, business centers, shopping malls, and in private homes. The flat roof is distinguished by its horizontal arrangement with a relatively small degree of slope. Unlike pitched roofs, flat roofs are more vulnerable to moisture and water, since water can stagnate on the surface for longer. That is why the waterproofing of a flat roof is one of the most important elements of a roofing layers.

In this article we will take a detailed look at all possible waterproofing materials and methods of their installation on a flat surface.

Why it is necessary to do waterproofing of a flat roof?

The flat roof is made in such a way that its slope towards the drain funnels in most cases does not exceed 10-20 degrees. Due to a slight slope, water stagnates on the surface and, in the absence of high-quality waterproofing, it is absorbed into the base plate (or overlap) and destroys it. 

What is the danger of water entering the building through a flat roof:

  • Cracking and destruction – Moisture and water are absorbed into the concrete, freezes in winter, and thaws in spring forming microcracks and pores in the concrete floor. These cracks are getting bigger every year, gradually threatening the healthfulness of the structure. 
  • Flooding of the room – a leak in the roof can flood the room under it, which is fraught with damage to the property located in the building or house and repairs made. 
  • The formation of high humidity, mold and mildew in the house that is dangerous to human health. 

Types of flat roof

A flat roof has several main types, their difference comes from the final purpose of the roof. Depending on the type of roof, the location of different elements of the roof, such as thermal insulation, waterproofing, etc., is set in different places. 

The main types of flat roof:

  1. Traditionally unoperated – It is assumed that such a roof will not be constantly used and recreation areas or other areas where people will constantly be located will not be equipped on it. This design is not intended to withstand constant loads and mechanical impact, its main task is to protect the building from the penetration of water, moisture and other weather elements.
  2. Operated – This type of roof is designed so that terraces or other exploited areas can be equipped on it, where people and various objects (equipment, sofas, tables, etc.) will constantly be located.
  3. Inversion roof – This type of construction is suitable exclusively for flat roofs. With this type of structure, some traditional layers change places, namely thermal insulation and waterproofing. Thermal insulation is placed not under, but on waterproofing. Inversion technology is used to protect the finishing layer from temperature fluctuations.

The place of waterproofing in the roofing layers: How to arrange properly the waterproofing coating depending on the type of flat roof

Different types of flat roofs provide different layers and different locations of the waterproofing layer. The main elements in the layers of a flat roof:

  •  Thermal insulation 
  • Vapor barrier 
  • Primer 
  • Waterproofing 
  • Finishing coating (tiles, decking, gravel) 

Traditionally unoperated

 In the traditional version, waterproofing is located on top of the insulation as the last link. 

Its task is to protect all the elements laid below from the effects of atmospheric precipitation.

Arrangement of layers: 

  1. Floor plate; 
  2. Concrete screed with a slope thickness of 4 mm. The slope can vary from 2 to 15-20°; 
  3. Vapor barrier; 
  4. Thermal insulation with a total thickness of 20cm (2 layers each of 10cm can be used) in 2 rows to eliminate the cold bridge; 
  5. Waterproofing – fits in 1 or 1-3 layers, depending on the selected material.
The layout of the layers of the traditional unused flat roof

Recommendations: 

To enhance the reliability of waterproofing and additional protection of the building from water, it is recommended to make 2 contours of waterproofing protection in addition to the standard layers layers. To do this, you need to perform the following actions: 

  1. The waterproofing device of the base plate – this should be the first waterproofing circuit that will protect your house or building from water penetration, in case the last protective layer passes water through.
  2. If you plan to use coating, sprayed, or rolled materials as waterproofing, then after installing the insulation, it is necessary to perform a screed with a slope of at least 4 mm (it can be performed instead of the lower screed on the floor slab). This should be done, since the above-mentioned materials are applied to a solid base, and thermal insulation is not of this kind (it has strong dynamics).
  3. The device of the second waterproofing circuit – a second protective circuit of reliable waterproofing materials is applied to the made screed. 

Operated roof

A flat-operated roof implies the targeted use of the free space of the roof for personal needs with a constant load. 

Arrangement of layers: 

  1. Floor plate; 
  2. Concrete screed with a slope thickness of 4 mm. The slope can vary from 2 to 15-20°; 
  3. Waterproofing; 
  4. Drainage spiked membrane; 
  5. Thermal insulation with a total thickness of 20sm (you can have 2 rows of 10sm each); 
  6. Separation layer of geotextile; 
  7. Finishing – The device of screed, tile (on the CPS base), or decking. 
Layout of the layers of the operated flat roof

Features of waterproofing of the operated flat roof

When installing waterproofing of the operated flat roof, it is necessary to take into account some of its features: 

  • Constant load on the waterproofing coating; 
  • Constant mechanical pressure resulting from the placement of various structures and/or equipment; 
  • Location of people on the roof. 

Waterproofing of the operated roof

For additional protection of the building from water, it is recommended to make 2 contours of waterproofing protection for operated and non-operated flat surfaces. To do this, you need to perform the following actions:

  1. The waterproofing device of the concrete base plate – this should be the first waterproofing circuit that will protect your house or building from water penetration, in case the last protective barrier passes water through. 
  2. Considering that the roof will be operated, which means it will be subjected to a constant load, then after laying the thermal insulation, it is recommended to make an additional screed with a slope. This will significantly strengthen your design and protect it from mechanical damage during operation. 

Finishing coating of the operated roof

As a finishing coating, it is recommended to use materials resistant to mechanical impact and abrasion. For the device of operated flat surfaces, the following coatings can be used: 

  • Tile – To install tiles, a screed with a slope is applied to the waterproofing base, and then the tile is mounted on a moisture-resistant tile adhesive. If you do not make a screed, then the tile adhesive will not have good adhesion with the installed waterproofing, which will lead to future deformation of the tile.

    If you want to install a tile for waterproofing, you can use polymer-cement compounds, it is with them that the tile adhesive will have 100% adhesion (adhesion). When performing tile grout work, it is recommended to use not a simple but an epoxy grout.
  • Decking – it is installed on a pre-mounted frame or on adjustable supports.
  • Polyurethane waterproofing – As a finish, you can also use polyurethane waterproofing according to the PolyFlex type from the German company HYDRO. If desired, such insulation can be tinted in any color. 

Inversion roof

The main task of a flat inversion roof is to protect the waterproofing from seasonal temperature changes and their impact on the waterproofing coating. Inversion roofing is performed mainly in buildings with a large roof area.

Arrangement of layers: 

  1. Concrete base; 
  2. Ceramic gravel laid with a slope from 2 to 15-20°; 
  3. Reinforced screed with a thickness of 50mm; 
  4. Primer 
  5. Waterproofing; 
  6. Separation layer of geotextile; 
  7. Thermal insulation with a total thickness of 20sm;
  8. Drainage spiked membrane; 
  9. Separation layer of geotextile; 
  10. Ballast made of gravel fractions of 20-40mm – If desired, paths made of paving slabs or other elements that simplify the operation of the surface can be installed on the arranged ballast. 
Layout of inversion flat roof layers

Scheme of various options for the installation of a flat inversion roof

Waterproofing materials and their installation technology

There are many different materials for waterproofing flat roofs. Waterproofing materials differ from each other in their characteristics, reliability, and the way they are installed. 

Surfaced rolled coatings based on bitumen

Surfaced waterproofing is rolled sheets based on modified bitumen. The characteristics of the rolled surfaced insulation allow you to retain water for a long time.

For the installation of the protective coating of the roof, the surfaced membranes should be mounted in 2-3 rows with the approach to the seams of the previous one, while if the top layer is the finishing one, then it should necessarily be with a special sprinkling that protects it from exposure to solar ultraviolet rays.

Bitumen-based rolls are divided among themselves mainly by the service life: 

  • The premium service life of 25-30 years 
  • Standard – service life 10-15 years 
  • Economy – service life up to 10 years 
Arrangement of roll-guided insulation

Mounting method: 

  1. It is necessary to prepare the base; 
  2. Bitumen primer is applied to the surface; 
  3. Roll membranes are deposited on the surface using a burner and fire. The surfaced sheets should be installed on top of each other in order to overlap the existing seams. 
  4. The surfacing of the 2nd and subsequent layers should be performed with the overlap of the seams of the previous one. If the last waterproofing circuit is exposed to sunlight, then it should be made of materials with slate sprinkling. 
  5. It is recommended to fix the material abutments to the vertical surface with a special clamping bar. 
Important: When installing surfaced waterproofing, it is very important to follow strictly the numerous rules and instructions of the manufacturer. One of the many mistakes of installers is overexposure of fire on the bitumen seam layer, which eventually leads to the burning of bitumen and weakening of the docking seam, through which water can subsequently pass into the roofing layers. 

Advantages: 

  • Availability in hardware stores and markets; 
  • The high service life of Premium material; 
  • Relative resistance to mechanical loads; 
  • Resistance to ultraviolet solar radiation when using a special layer with coarse-grained sprinkling on the front side. 

Disadvantages: 

  • The presence of seams – Seams are the weakest points in the surfaced waterproofing coating and water, first of all, begins to pass through the seams; 
  • Lack of adhesion – The surfaced materials have no adhesion to the surface, which means that water can stagnate under the sheets and walk under the coating, appearing from the inside in the weakest place; 
  • Human factor – At the slightest mistake on the part of the manufacturer of the work, the integrity of the seam may be violated, which may eventually lead to leaks; 
  • Fire mounting method. 

Self-adhesive materials

Self-adhesive material is rolled on a durable polyester base with coarse-grained slate sprinkling and without, for waterproofing flat roofs. They are used for the device of a single-layer roofing carpet of garages, household, and country buildings, cabins, etc.

Self-adhesive rolls can be installed on the roof surface without the need to use live fire.

Advantages: 

  • Availability of the material in construction shops and markets; 
  • Possibility of laying without the use of fire and burner; 
  • The laying speed is higher than that of the surfaced analogs. 

Disadvantages: 

  • The presence of seams – Seams are the weakest points in the coating and water, first of all, begins to pass through the seams; 
  • Lack of adhesion to the base – The adhesive materials do not have 100% adhesion to the surface, which means that water can stagnate under the sheets and walk under the coating; 
  • Low reliability.

Polymer and PVC membranes

Membranes are film coatings based on various polymers, for waterproofing various structures, including roofs. Membranes belong to the mounting type of waterproofing materials. 

Membranes are divided into several main types: 

  1. PVC – Made on the basis of elastic polyvinyl chloride (PVC-P) and reinforced with polyester threads. These coatings are quite durable and resistant to water, moisture, and aggressive environment. Widely used as waterproofing of terraces and roofs. 
  2. EPDM is an ethylene-propylene-diene monomer. Membranes of this type are made of synthetic rubber with various polymer additives, in some cases, they can be reinforced with polyester reinforcement. 
  3. TPO is thermoplastic polyolefin membranes, which are made of ethylene propylene rubber and polypropylene. These membranes are highly resistant to hydrostatic pressure. 

Mounting method: 

  1. Preparation of the foundation; 
  2. Laying the substrate; 
  3. Membrane laying – Membranes are attached to vertical surfaces with the help of special pressure strips and slats. To avoid leaks, the membrane sheets are mounted overlapping at least 10 sm. The seams are welded with special glue or hot air using a special hair dryer.

Advantages:

  • Possibility of laying without the use of fire and burner; 
  • High water resistance.

Disadvantages: 

  • The presence of seams – Seams are the weakest points in the coating and water, first of all, begins to pass through the seams; 
  • Complete lack of adhesion to the base – The membranes lie on the surface like a carpet, which means that if a seam breaks somewhere or there is a small hole, then water will penetrate under the membrane and will spread throughout the roof; 
  • The lack of adhesion makes local repairs very difficult since by the nature of leaks from inside the room it is absolutely impossible to identify the entry point. 

Coating materials and mastics

When we talk about coating materials, we mean various mastics that are supplied in liquid form and applied to the surface using rollers, spatulas, and brushes. Today, there are quite a large number of materials on the market for roofing mastics, which differ in their characteristics and components. 

The main types of coating waterproofing: 

  1. Bitumen-based mastics – These compositions are based on bitumen and polymer additives. Some bitumen mastics require heating before application or dilution with various solvents, and some are supplied ready-to-use. 
  2. Polyurethane mastics – Made of highly elastic hydrophobic polyurethane resin with the addition of polymers. Polyurethane waterproofing can be either one-component or two-component. It is a great fit for waterproofing and protecting roofs, balconies, and terraces, as they are resistant to solar radiation and have a high coefficient of elasticity.
  3. Polymer-cement compositions are prepared on the basis of cement and various polymers. These formulations are single-component and two-component and are supplied in the form of a powder mixture, which is diluted with water and, if necessary, a second component is added to it. These are quite durable materials, if desired, you can install tiles on them even without a clamping screed. 

Method of application

  • Preparation of the base — Cleaning the surface from dust, dirt, peeling elements, and sharp corners; 
  • Primer – Applying primer to the surface; 
  • Preparation of waterproofing mastic for work; 
  • Application of coating mastic in 1-3 layers with the drying of the previous one;

Advantages: 

  • Coatings based on polyurethane and liquid rubber have high elasticity and strength – Due to which they do not deform and do not diverge during shrinkage of the building or the dynamics of the structure;
  • No seams – ensures reliability and durability; 
  • 100% adhesion to the base – So that water cannot pass under the waterproofing and walk under it; 
  • The possibility of spot repairs – Thanks to 100% adhesion, you can easily understand where the leak is and perform spot repairs; · Some coating waterproofing resistant to UV light, for example, polyurethane; 
  • Some coating compositions can be used as a decorative finishing layer, as they can be tinted, for example, polyurethane; 
  • Non-firing mode of operation.

Sprayed waterproofing

Sprayed waterproofing is a variety of 1- and 2-component materials that can be applied to the surface by spraying. 

The main types of sprayed waterproofing: 

  1. Liquid rubber is a bitumen-latex emulsion, which, when sprayed, forms a seamless, elastic rubber coating on the surface with a thickness of 2-4 mm. 
  2. Polyurea is the processing of polyuria and polycarbamide. Spraying with polyurea forms a seamless, durable coating on the surface, something in between plastic and hard rubber. 
  3. Polyurethane waterproofing is the main component of this sprayed composition, it is polyurethane. This material is very similar to polyurea, but it is not as expensive and has a higher coefficient of elasticity. The seamless membrane formed after spraying is able to withstand mechanical loads and solar radiation. 

Method of application of sprayed compositions: 

  • Preparation of the base — Cleaning the surface from dust, dirt, peeling elements, and sharp corners; 
  • Primer – Primer Application; 
  • Preparation of the installation for airless spraying; 
  • Spraying of waterproofing composition; 
  • Cleaning of the installation and its preservation – if the installation is not cleaned in time, the remaining clots in it can harden and spoil the equipment.

Advantages: 

  • The speed of work – in 1 day, you can apply from 400-800m2 of waterproofing, by a team of 4 people; 
  • High elasticity; 
  • No seams; 
  • 100% adhesion to the surface: 
  • The possibility of spot repairs; · Some coating waterproofing resistant to UV light, for example, polyurethane; 
  • Some coating compositions can be used as a decorative element, without a subsequent finishing device, since they are amenable to tinting, for example, polyurethane; 
  • Non-firing mode of operation.  

Comparison between materials in the table

 Sprayed compositionsSurfaced bitumen-based rollsMembranes
Mounting methodRecklessFiring with a burnerWith a hair dryer and a metal attachment
Seams availabilityNot availableAvailableAvailable
Adhesion to the base100%40%No adhesion
Coefficient of elasticity600-800%10-20%10-20%
Installation speed in m2/per day400-60020-5030-60
The warranty period for works7 years3-5 years1-2 year

Recommendation: Which material is best to choose and what not to forget

If your task is to make waterproofing once and forever, it is recommended to choose a waterproofing system with the following characteristics: 

  • Without seams – the presence of seams means the presence of weak points in waterproofing; 
  • With 100% adhesion to the surface – The lack of adhesion means that water can penetrate under the waterproofing in one place, and then move freely from one place to another;
  • Elasticity – The more elastic the material, the less it is subject to deformation during shrinkage of the house or dynamic load; 
  • Price—quality ratio. 

All these qualities are possessed by such waterproofing systems as: 

  1. Polyurethane waterproofing
  2. Liquid rubber 
Important: 
Regardless of which waterproofing system you have chosen, when installing waterproofing, do not forget to pay special attention to the following constructs ·
 * Installation of a fillet or a small skirting board during the transition of waterproofing from the horizon to the vertical - this is necessary in order to prevent the fracture of waterproofing at this node; 
* Do not forget to make the drain funnels correctly and seal them well; 
* Try to make a good slope towards the drain funnels so that the water does not stagnate; 
* Pay special attention to sealing the entry points of various communications into the building; 
* Pay special attention to the additional sealing of cold and expansion joints.

Prices for a flat roof waterproofing installation

Below is a table of prices for work related to waterproofing the roof. In the table, you can see the prices and the total cost of waterproofing works, depending on the method of their execution and the material used in this case (type). These prices are preliminary, as they do not take into account the complexity of the work and other possible newness. 

Works descriptionUnit of measurementPrice in $
Works on installation of surfaced waterproofingm25
Works on installation PVC membranem27
Works on installation of polyuream211
Works on the mastic waterproofing installationm23
Works on the installation of liquid rubber sprayingm24
Works on the installation of liquid rubber manuallym25
Works on the installation of cement-mineral 2-component waterproofingm27
Works on the installation of polyurethane waterproofingm27
Applying of primerm22

Calculator of the cost of waterproofing a flat roof

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David J

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