roofing

Functions

The roof covering is the exterior part of the roof and does not contribute to the building's stability. It is designed to endure all weather conditions such as rain, snow, hail, and wind, as well as external environmental factors like marine environments and the weight of maintenance personnel. From the ridge to the drainage system, the roof covering directs rainwater by gravity and contributes to waterproofing.

As a visible element from the outside, the roof covering contributes to the heritage and architectural value of the building.

Composition

A roof covering is composed of various elements including:

Roof support (beams, boards, rafters, battens, etc.)

Roof underlayment (waterproof membrane, thermal insulation, etc.)

Ventilation elements for the underlayment (moisture and vapor evacuation)

Roof covering, visible exterior coating (tiles, slates, shingles, etc.)

Elements ensuring rainproofing and proper drainage of the roof (ridge caps, flashing, edge waterproofing elements, etc.)

A water collection system (eaves) and rainwater drainage (gutters)

Roof windows or skylights.

Roof underlayment

The roof underlayment is used to prevent accidental penetration of rainwater or powdery snow, to prevent convective exchanges with thermal insulation, and to control the migration of water vapor. It is an element of the building's thermal performance.

The roof underlayment is placed between the frame and the roof support. Two types of underlayments are distinguished: rigid underlayments, usually made of wood such as panels and boards, and flexible underlayments made of bituminous material or synthetic material, reinforced or not. Flexible underlayments can have High Vapor Permeability (HVP), which affects water vapor migration and also impacts the installation of thermal insulation.

Roof support

The roof support, attached to the frame, serves as a fixing support for the roof elements. It is usually a lathing or boarding. Lathing is a network of horizontal wooden slats, square or rectangular in section, called battens. In the presence of a flexible underlayment, counter-battens are placed under the lathing. Boarding consists of a decking of boards, which are wooden planks. Alternative industrial solutions, such as fiber cement, exist. Some roofing elements, such as steel roofing or slate, do not require roof support.

Ventilation

Roof ventilation ensures the proper preservation of the timber in the attic and regulates the humidity level by preventing condensation. Two ventilation systems are distinguished:

The ridge vent system consists of punctual openings (generally with a 1 dm2 section) arranged to facilitate air circulation. These are placed at the bottom of the windward slopes (in areas of positive pressure) and at the top of the leeward slopes (in areas of negative pressure).

The linear system allows air to pass at the base of the roof (at the eaves), along the slopes, and at the ridge. It is mandatory in mountainous areas and has the advantage, especially with underlayment, of homogenizing ventilation.

Roofing elements

Technologically, two types of installations are distinguished: roofing with small elements and roofing with large elements. Roofing with small elements includes slates, tiles, and shingles. The principle of waterproofing guiding their installation is overlapping. Roofing with large elements includes sheet metals, profiled metal or plastic trays, and corrugated fiber-cement sheets. Their waterproofing principles may involve overlapping, stapling, the application of elastomer seals, etc.

Some traditional roofing materials, such as thatch or green roofs, do not fit into these two categories.

Tiles are rigid plates manufactured by molding or pressing. They come in various shapes depending on regional specifics or their location on the roof: flat, corrugated, curved, and saddleback. The material is often terracotta, but it can also be concrete, glass, or metal (zinc, steel). They can be installed on lathing or boarding, or even on specific tile supports.

Terracotta tiles represent the primary roofing material in France and many other countries. These elements are made of clay fired at high temperatures. The obtained colors depend on the clay used and the surface treatment applied during finishing.

Several types of tiles, each with their relative installation specifics based on their shape, exist, such as the canal tile, flat tile (with regional variations like the glazed tile from Burgundy or the Alsatian tile), Flemish tile, or interlocking tile.

Natural slates are elements made of very fine schist stone. They are manufactured from slate schist, cut and sawn to the desired dimension. Slate shapes include rectangular, rounded, pointed, or diamond-shaped. A slate is waterproof, non-porous, frost-resistant, and resistant to the most aggressive atmospheric agents.

Fiber-cement slates are prefabricated elements made of cement to resemble natural slates. They can be pigmented throughout or surface-colored. The early fiber-cement slates contained asbestos.

Construction technique

Two construction lines, however, are common to all installation techniques:

The level line;

The line of greatest slope (the path followed by water by gravity on a slope).

They are perpendicular. The elements of a roof will always be arranged according to these lines, which will also serve as the basis for all implementation drawings.

Edges

Edges are the lines that determine the geometric limits of a slope. They can be integrated into the slope (chimney passage, roof window, ventilation), at the junction of two slopes, or at the boundary of a building. They are classified and treated differently depending on their orientation relative to the line of greatest slope.

Edges that water follows or straight edges parallel to the line of greatest slope.

Edges that water escapes (ridge, hips) forming an acute angle with the level line.

Edges that receive water (gutters, valleys, beveled edges) forming an obtuse angle with the level line.

Inherent problems with waterproofing, durability, and resistance of roofing

One of the major problems to be solved in establishing installation rules is capillarity (water rising) between elements. It is decisive in the choice of joint type or the value of overlap. Phenomena due to wind action, overpressure, and depression, static loads (snow, ice) influence supports and fixings. Condensation, electrochemical incompatibility between metals or between metals and materials (specific wood species or concrete) compromise the durability of structures.

Roofer

One of the major challenges in the roofing profession lies in how to carry out these works depending on whether they are located at the junction of slopes or not.

The work of the roofer therefore consists of:

Choosing a material

Choosing an installation technique

Marking the location of each element

Shaping the materials

Installing them while respecting waterproofing, fixing, and possibly expansion rules.

Criteria for choosing

The choice of material is made based on multiple criteria. In most cases, local authorities impose types of roofs based on architectural or environmental constraints.

The choice of material and/or the implementation of an installation technique will depend on:

The slope of the slope in % or °

The ground projection of the slope considered in meters

Its geographical location (climate zone defined by maps taking into account the combination of rain/wind or mountainous area, etc.)

Its local geographical location (climatic site)

Snow and wind mechanical constraints (rules and NV 65 map)

Local environmental constraints (aesthetic, architectural, etc.)

Zone I consists of the entire interior of the country, the Mediterranean coast, and altitudes below 200 m.

Zone II includes the Atlantic coast within 20 km deep and altitudes between 200 and 500 m.

Zone III includes the Atlantic, Channel, and North Sea coasts within 20 km deep and altitudes above 500 m.

The building's location relative to the environment overlaps with the climatic zone.

A sheltered site corresponds to a construction in the hollow of a basin surrounded by hills on all sides and thus protected from the wind.

Roofing at high altitudes

Mountain buildings (above 900 m) require a "double roof" composed of several layers. The large temperature differences between outside/inside and night/day cause phenomena such as dew point (Condensation) and freeze/thaw harmful to building preservation.

Interior condensation, often invisible, damages coatings and causes mold. It results from the low temperature inside the walls combined with a high humidity level.

Freeze/thaw causes the formation of extremely heavy icicles at the edge of the slopes, dangerous for pedestrians and destructive for materials. The phenomenon is simple: heated from below, the snow melts, flows between the slope and the snow cover, then freezes passing over the overhangs of the roof exposed to the cold air. Tons of ice can accumulate.

The "double roof" is the most effective way to counteract these drawbacks. The complex consists of:

A vapor barrier is placed at the ceiling of the rooms in the sloping ceiling. Acting as a barrier to vapor, it prevents it from entering the insulation and condensing inside it.

Above, the thermal insulation prevents the temperature of the wall from dropping, reducing the risk of interior condensation.

The insulation is then coated with waterproofing to protect it from condensate from the roof. – An air gap circulates between the waterproofing and the roof elements. It must be at the outside temperature to prevent thaw/refreeze. In some cases, linear ventilation "ridge gutter" will be implemented. – Finally, the actual roofing is carried out.

In order:

Rafters

Thin insulation

Counter batten

27 mm deck board

Rules and techniques of implementation

The design and implementation of roofs are subject to the rules of the trade, standards, and technical opinions of official bodies as well as the installation advice from manufacturers.

In france

The design and implementation of roofs are subject to DTU (Document Technique Unifié) regulations in the 40 series. In the absence of official standards, Technical Assessments (ATec) are taken into account.

DTU 40.11 Slate Roofing

DTU 40.13 Fiber Cement Slate Roofing

DTU 40.21 Interlocking or Sliding Clay Tile Roofs with Relief

DTU 40.211 Clay Tile Roofs with Flat Gauge

DTU 40.22 Canal Clay Tile Roofing

DTU 40.23 Flat Clay Tile Roofs

DTU 40.24 Concrete Tile Roofing with Sliding and Longitudinal Interlocking

DTU 40.241 Concrete Flat Tile Roofing with Sliding and Longitudinal Interlocking

DTU 40.25 Concrete Flat Tile Roofing

DTU 40.35 Ribbed Sheet Roofing from Coated Steel Sheets

DTU 40.36 Pre-painted or Non-pre-painted Aluminum Sheet Roofing

DTU 40.41 Roofing with Metal Elements in Zinc Sheets and Long Sheets

DTU 40.44 Roofing with Metal Elements in Stainless Steel Sheets and Long Sheets

DTU 40.45 Roofing with Metal Elements in Copper Sheets and Long Sheets

DTU 40.46 Lead Roofing on Continuous Support

Roofer's vocabulary

In addition to the technical terms used by roofers, there are names for tiles used for finishing, decoration, and waterproofing of roofs. Here are the main terms to better understand the language of architects, builders, or roofers.

Hip: protruding line formed by the intersection of two roof slopes.

Brisé: the lower part of a Mansard roof.

Cabrons: over-rafter wooden profiles that create waves under flexible covering.

Chanlatte: beveled wooden lath, nailed on the rafters at the edge of the roof to compensate for the missing tile thickness in the first row (tilting). It can be replaced by a double batten.

Pet door: Metal or clay elements designed for roof and attic ventilation.

Coffin: (or cofine) tile or slate curved in the width direction.

Double tile: double row of tiles or slate, laid on the chanlatte, which makes up the roof edge. Also called a battlement.

Eaves: lower edge of a slope often equipped with a gutter.

Ridge tile: half-round or angular tile that covers the horizontal beam, called "ridge," placed at the junction of two slopes of a roof.

Standing seam: roofing and facade covering technique using waterproof metal.

Gambrel tile: tile curved inward in the width direction.

Left-handed tile: tile curved in length on its left edge, called "left to left," or right, called "left to right."