Flashing strips for roofing

Flashing strips are specialized shaped elements of the roofing system designed to provide a watertight and durable connection between the roofing covering and the vertical and inclined surfaces of a building. These elements are critically important components since most leaks in a roof structure occur precisely at the junctions where the roofing material meets various architectural elements. The main function of flashing strips is to create a continuous waterproofing barrier that effectively diverts atmospheric moisture from the roof surface, preventing its penetration into the under-roof space. Without correctly installed flashing strips, water will inevitably flow into cracks and gaps, causing destruction of the truss system, insulation, interior finishes of premises, and contributing to the development of mold and fungus.

Structurally, flashing strips serve to connect the roofing material with elements such as building walls, parapets, chimney and ventilation pipes, dormer windows, fire breaks, and other engineering structures exiting onto the roof. They perform not only a protective but also a decorative function, giving the junction points a neat and finished look. Modern flashing strips are designed considering the specifics of roofing materials and the shape of the mating surfaces, ensuring their optimal efficiency. Their operating principle is based on organizing an overlapping layer that directs water flows from top to bottom, following the natural drainage path, and intercepts moisture trying to penetrate under the main roofing covering.

The importance of the correct selection and installation of flashing strips cannot be overestimated, as they operate under conditions of constant exposure to aggressive external factors. These include ultraviolet radiation, temperature fluctuations from extremely low winter to high summer values, freeze-thaw cycles of water, wind loads capable of causing vibration and detachment of poorly secured elements, as well as the chemical effects of atmospheric precipitation. A high-quality flashing strip must not only match the roofing material but also have sufficient mechanical strength, corrosion resistance, and durability comparable to the service life of the entire roofing system. Historically, improvised materials were used for sealing junctions—clay, cement mortars, lead or galvanized sheets bent on-site. The modern approach involves the use of industrially manufactured standard and accessory elements, ensuring high reliability and reproducibility of results.

Within this article, a comprehensive analysis of flashing strips will be conducted: from their classification and materials used to a detailed description of installation technologies for various types of roofs. Special attention will be paid to junctions with walls and pipes, as the most complex and critical areas. Common installation errors and methods for preventing them will also be considered, which will allow both professionals and homeowners to better understand the principles of creating an absolutely watertight and durable roof. Understanding the role and rules for working with flashing strips is the key to successful construction or repair, eliminating costly alterations and restoration work in the future.

Classification of Flashing Strips by Type and Installation Location

Flashing strips can be systematized according to several key features, the main one being the type of surface to which the flashing is made and the design features of the element itself. The most general division involves separation into flashing strips for vertical surfaces and flashing strips for horizontal or inclined surfaces. Flashing strips for vertical surfaces, such as walls, parapets, gables, are the most common type. They are designed to seal the joint between the roof slope and a vertical wall. Structurally, such a strip is usually an angle with flanges of different widths: one, wider flange (base) is attached to the roof, and the second, typically narrower, is brought onto the vertical surface. To improve sealing, the vertical flange often has a hem (folded edge), which is embedded into a groove (chase) in the wall or covered with an overlay strip.

The second major class is flashing strips for pipes (chimney, ventilation) and other penetrations. These strips have a more complex shape, often cylindrical or conical, and are designed to clamp around the pipe. They can be split (consisting of two or more parts for installation on already installed pipes) or non-split (installed on the pipe before its placement on the roof). A modern solution for pipes are elastic penetration seals (roof jacks), made of EPDM rubber or silicone, which can be considered a special type of flashing strip. They have a flange for attachment to the roof and a corrugated part that tightly clamps the pipe, compensating for thermal deformations and vibrations. Flashing strips for valleys (internal corners) also occupy a special place. Although they are often distinguished as a separate category of accessory elements, in essence they perform the same function—sealing the joint of two planes, but in this case not the roof with a wall, but two adjacent roof slopes.

Flashing strips also differ in profile and cross-sectional shape. The simplest form is a right angle (L-shaped profile). More advanced are strips with a special drip edge on the edge of the horizontal flange, which diverts water further from the wall and prevents it from seeping under the strip. There are also strips of complex profile with additional stiffening ribs, which increase resistance to wind loads and prevent rattling. A special group comprises combined or composite flashing strips, consisting of several elements. For example, a system may include a lower strip integrated into the roofing underlayment and an upper decorative-protective cover. Such a solution provides double protection and is often used on critical objects. Strips are also classified by installation method: some are mounted on top of the roofing covering (overlay method), others are integrated into the roofing underlayment before laying the finish coating (underlayment method).

An equally important classification criterion is the type of roofing material for which the strip is intended. Universal strips suitable for many materials are rare. More often, roofing material manufacturers offer their own systems of accessory elements, perfectly compatible in geometry, color, and characteristics. Thus, for metal tiles and corrugated sheets, metal strips with a polymer coating are used, the shape of which corresponds to the wave profile. For flexible bituminous shingles, either metal strips or roll underlayment materials with subsequent sealing with mastic and overlay of regular shingles are used. For natural tiles, accessory elements made of the same material (ceramic or cement-sand) or special lead or aluminum flexible tapes are used predominantly. For standing seam roofs, junctions are often formed directly with seam connections, and strips are used as auxiliary elements. Thus, the classification of flashing strips reflects the diversity of roof structures and materials and emphasizes the need for an individual approach to designing each junction.

Materials for the Manufacture of Flashing Strips and Their Comparative Analysis

The choice of material for flashing strips is a strategic decision that determines the service life of the junction, its reliability, and aesthetic integration with the roof and facade. The modern market offers a wide range of materials, each possessing a unique set of physical, mechanical, and operational properties. The traditional and most common material is galvanized steel with a polymer coating. This is the standard solution for roofs made of metal tiles, corrugated sheets, and composite tiles. The strips are made of cold-rolled steel with a thickness of 0.4-0.6 mm with a zinc coating mass of 100-275 g/m², providing basic corrosion protection. A layer of primer and a decorative-protective polymer coating are applied over the zinc: polyester (PE, matt polyester MPE), plastisol (PVC), pural (Pural, PUR), PVDF. The polymer layer gives the strip color, resistant to fading under ultraviolet light, and increases resistance to mechanical damage. Advantages of such strips are affordable cost, a wide choice of colors according to RAL and RR catalogs, and good rigidity. Disadvantages include the risk of coating damage during installation and limited plasticity, complicating work on complex curved surfaces.

Aluminum flashing strips are characterized by exceptional lightness and high corrosion resistance. Aluminum is naturally protected by an oxide film, and to impart color, it can be anodized or powder-coated. This material does not rust, which is especially important in aggressive environments (coastal areas, industrial zones). Aluminum is ductile, allowing strips of complex configuration to be formed on-site. However, its mechanical strength is lower than that of steel, and its cost is higher. Copper flashing strips belong to the premium segment and are used on elite objects, often together with copper roofing. Copper has outstanding durability (service life measured in hundreds of years), bacteriostatic properties, and unique aesthetics that change over time due to patina formation. Copper strips are very malleable and can be soldered, allowing for absolutely watertight joints. The main and often insurmountable disadvantage is the extremely high cost of material and labor.

Zinc-titanium alloys (e.g., D-锌钛, VMZINC) are modern high-tech materials. They combine durability (80-100 years), the ability to “self-heal” minor scratches, and a noble matte gray appearance that also changes over time. Strips made of zinc-titanium are used in high-class architecture. Disadvantages include high price and the need to follow special installation rules (insulation from contact with other metals, specific soldering conditions). Plastic flashing strips, made of PVC (polyvinyl chloride), ABS plastic, or polypropylene, have found their application mainly with roofs made of PVC and TPO membranes, as well as ondulin. They are not subject to corrosion, chemically inert, lightweight, and easy to install. Some models have an adhesive layer to simplify installation. However, plastic has limited strength and can become brittle in cold weather or lose shape under high temperatures.

For traditional roofs (natural tiles, slate), strips made of lead tape are often used. Lead is a very soft, ductile, and durable material that is easily formed on-site and does not corrode. It is ideal for sealing complex junctions around pipes and in valleys. However, due to the toxicity of lead, its use is limited today, and it is being replaced by safer alternatives. Flexible tapes based on aluminum foil with a bitumen-polymer adhesive layer (e.g., Wakaflex, Onduflesh) represent an innovative solution. They combine flexibility, allowing them to bypass any irregularities, and high adhesion to most building materials. Such tapes do not require mechanical fastening and are often used for repairs or on complex nodes in addition to the main strips.

For clarity, let’s compare the key characteristics of the main materials in a table:

The choice of a specific material should be based on a combination of factors: the type of main roofing material, climatic conditions, architectural requirements, project budget, and expected service life. The most important rule is to avoid contact between dissimilar metals without an insulating gasket to prevent electrochemical corrosion.

Installation Technology for Wall Flashing Strips

The installation of a flashing strip to a vertical wall is one of the most critical stages of roof construction. The quality of this node directly determines whether water will run down the slope into the drainage system or find its way under the roofing covering. The technology may vary depending on the type of roof, but the general principles remain unchanged. Work should be carried out in dry weather, having first ensured the readiness and proper preparation of both mating surfaces: the roofing covering should be laid up to the junction line, and the wall should be plastered, painted, or clad so that the strip adheres to a level base.

The first stage is wall preparation. In brick, concrete, or plastered walls, at a level slightly above the top point of the strip junction (usually 150-200 mm), it is recommended to make a groove (horizontal chase) with a depth of 20-30 mm. The groove is necessary so that the top edge of the strip can be inserted into it and sealed. If making a groove is impossible (wooden wall, frame structure), then an alternative method using a wooden batten of triangular cross-section is used. This batten is nailed or screwed to the wall along the junction line. Its function is to create a smooth transition (rounding) and provide a reliable base for attaching the strip. The wall surface in the junction area must be cleaned of dust, dirt, mortar residues, and primed to improve the adhesion of sealants.

The second stage is preparation of the roofing side. Depending on the type of roof, an additional layer of waterproofing or underlayment may need to be laid under the flashing strip. For example, for metal tiles, a special sealing tape matching the profile is rolled out and fastened along the wall on top of the main covering. For a soft roof, a valley underlayment or underlayment is brought onto the junction with an overlap onto the wall. Next, proceed directly to installing the strip. It is positioned so that the lower horizontal flange lies on the roofing covering, and the upper vertical flange rests against the wall (or is inserted into the groove). Installation should start from the lowest point so that the upper elements overlap the lower ones, ensuring water flow from top to bottom.

The strip is fastened in two stages. First, the lower flange is attached to the roof. For metal roofs, this is done using roofing screws with EPDM washers, which are screwed into the crest of the wave (if the strip has a corresponding profile) with a pitch of 200-300 mm. It is important not to overtighten the screws to avoid deforming the strip. For soft roofs, the lower edge of the strip is often placed under a row of roofing shingles, and the strip itself is additionally coated with bitumen mastic and nailed. Then the upper part of the strip is attached to the wall. If there is a groove, the top edge is inserted into it, after which the groove is filled with a highly elastic silicone, thiokol, or polyurethane sealant. The sealant must completely fill the space and form a convex bead to divert water from the joint. If there is no groove and a batten is used, the upper flange of the strip is attached to the wall over this batten using dowels (for solid walls) or screws (for wood). The attachment point and the joint between the strip and the wall are also carefully sealed.

The final stage is additional sealing of the joint between the strip and the roofing material itself. Even with profiled seals, micro-cracks may form over time. Therefore, it is recommended to go along the entire length of the junction with a high-quality roofing sealant compatible with the strip and roof material. The sealant color is usually matched to the tone. For some types of roofs (e.g., flexible shingles), a row of shingles is laid over the metal flashing strip, which is brought onto the vertical flange of the strip by 50-70 mm and glued with mastic. This creates double protection and a more aesthetic appearance. After installation, a visual inspection and, ideally, a leak test by spraying water from a hose should be conducted. A correctly installed wall flashing strip should form a rigid, impermeable water barrier, smoothly directing runoff from the roof and reliably protecting the internal structure throughout the entire service life of the roof.

Construction of Roof Junctions to Pipes and Ventilation Outlets

Junctions to pipes (chimney, ventilation) and other round or rectangular penetrations are rightly considered the most complex in roofing practice. This is due to the need to ensure tightness at the intersection of two dissimilar materials (roofing covering and pipe material), which have different coefficients of thermal expansion and are subject to vibration. The node design must not only prevent leaks but also consider fire safety requirements (especially for chimneys), ensure ventilation, and allow for thermal movements. The classic solution for pipes on pitched roofs is the use of so-called “flashing” or “collar,” consisting of lower and upper flashing strips.

The installation technology begins with preparation. The pipe must be erected before roofing work begins, and a fire break (gap) must be made in the roofing assembly around it according to standards. This gap is filled with non-combustible material (most often stone wool). The base around the pipe on the roof must be solid and level. For metal tiles and corrugated sheets, the lower apron (lower flashing strip) is installed first. It is placed under the roofing covering from the ridge side. The apron consists of four parts (for a rectangular pipe) or is a flexible tape (for a round one). The side parts are placed under the roofing covering, and the lower part is placed on top of it so that water from the pipe flows onto the slope, not under the roof. All joints of the apron parts are carefully soldered or sealed with sealant.

Next, the main roofing material is laid, which is brought close to the pipe. After this, the upper apron (upper flashing strip) is installed. It is placed on top of the roofing covering from the ridge side. The upper strip is usually inserted into a pre-made groove on the pipe to a depth of 10-15 mm. After installation, the groove is filled with heat-resistant sealant. The side and lower parts of the upper apron should fit tightly to the roof profile. Special rubber or silicone gaskets are used to improve the fit. For round pipes on metal and soft roofs, elastic penetration seals (roof jacks) are increasingly used. They consist of an element with a metal (aluminum, lead) or plastic flange that matches the roof slope angle and an elastic corrugated skirt made of EPDM rubber or silicone.

The process of installing a roof jack is simple: the skirt is placed over the pipe, the flange is laid on the roof, its shape is bent according to the profile of the roofing material, after which the flange is fastened with screws and sealed around the perimeter. The elastic skirt tightly clamps the pipe and is fixed at the top with a metal clamp. This solution perfectly seals the node, compensates for vibrations and minor displacements, and is easily removable if pipe maintenance is needed. For thick brick chimneys, the method of a “saddle” is sometimes used—installing a special metal hood over the pipe, which diverts the main mass of water even before it approaches the junction with the roof. The flashing strips are then placed under this hood. Regardless of the chosen method, it is critically important to ensure the organization of a “tie”—a directing gutter made of roll waterproofing (for example, from a valley underlayment), which is laid under the lower flashing strip and runs down the slope, diverting water that has gotten behind the strip onto the main roof surface. This is the last line of defense that prevents leakage even if the main apron’s seal is compromised.

Features of Installing Flashing Strips for Various Types of Roofing Coverings

Each type of roofing material imposes its own specific requirements on the design and installation of flashing strips, determined by its physical properties, laying method, and shape. For metal tiles and corrugated sheets, as already noted, metal strips with a polymer coating are used, the color and profile of which match the main sheet. The key feature is the need to use special seals (universal or profiled), which are placed under the strip and fill the gap between its flat surface and the relief profile of the roof. When installing a wall flashing strip, it is important that screws are fastened not in arbitrary places but strictly into the crest of the wave; otherwise, waterproofing can be compromised. For pipes on metal tiles, ready-made “penetration” kits are often used, including lower and upper strips, seals, and fasteners designed for a specific profile.

For roofs made of flexible (bituminous) shingles, the approach is different. Here, flashing strips can be either metal (painted or made of galvanized steel) or formed from the material itself. The standard technology for wall junctions includes several steps. First, a triangular wooden batten is applied to the wall and roof in the joint area. Then, an underlayment or special roll material for junctions is laid on it with an overlap onto the wall of at least 300 mm. A metal strip is installed over this underlayment, which is attached to the wall with dowels. The top edge of the strip is sealed, and the bottom edge is placed under a row of shingles that will be laid higher. Or, more correctly, a finishing row of shingles is laid over the strip, which is brought onto the vertical surface by 50-70 mm and glued with bitumen mastic. For pipes on soft roofs, either prefabricated metal aprons with subsequent sealing of the junction area with valley underlayment and overlay of shingle pieces are used, or elastic penetration seals. Special attention is paid to coating all layers with bitumen mastic.

Natural tiles (ceramic, cement-sand) require the most labor-intensive approach. Wall junctions are often finished using special shaped elements (e.g., half-tiles with a flat edge) or using lead or aluminum flexible tape. A tape 200-300 mm wide is inserted with one edge under the tile adjacent to the wall, and the other edge into a groove in the wall, where it is fixed and sealed. A more modern method is the use of special plastic or aluminum profiles with a groove into which the edge of the tile is inserted, and the other side of the profile is attached to the wall. For pipes, special tile elements—”saddles” or “collars”—made of the same material as the tile are used. They are laid in the general row and sealed with mortar. Often this method is combined with the use of lead tape for additional waterproofing.

A standing seam roof, due to its technology, allows creating junctions almost without additional strips. The edges of the roofing sheets at the wall junction are simply raised vertically by 150-200 mm, and then this vertical edge is connected to a special clamping strip, which, in turn, is attached to the wall. The connection between the strip and the standing seam batten can be made using a snapping clip or hemming. This creates an aesthetic and very reliable node. For pipes on standing seam roofs, either standard elastic penetration seals are used, or custom elements (pans) are made from the same metal as the roof, using soldering or welding. Thus, the variety of technologies emphasizes the need for a deep understanding of the characteristics of each roofing material and following the manufacturer’s recommendations to achieve impeccable results.

Errors in Flashing Strip Installation and Their Consequences

Incorrect installation of flashing strips is the most common cause of roof leaks, and the consequences of these errors may not appear immediately, but after several months or even years, when warranty periods have already expired. One of the most serious and common mistakes is attaching the flashing strip only to the roofing covering without reliable fixation to the wall or pipe. In this case, even a small gap between the strip and the vertical surface becomes a capillary channel through which water is drawn in under the influence of wind and surface tension. It is especially dangerous when the top edge of the strip is simply glued to the wall with sealant without mechanical fastening. Over time, the adhesion of the sealant weakens under the influence of UV rays and thermal deformations, and the strip peels off, opening a path for water.

The second typical mistake is ignoring the need to create a groove or use a triangular batten. Installers, wanting to save time, simply press the strip against a flat wall and fill the joint with sealant. Such a seam works in tension and inevitably cracks. The third mistake is related to incorrect installation sequence when flashing pipes. Often, the upper strip is installed before laying the roofing covering around the pipe, or the lower strip is placed on top of the roofing material. In both cases, the basic principle of “lower elements under water” is violated: water flowing from above gets under the overlaps. The fourth group of errors concerns the use of inappropriate materials. Using cheap acrylic or silicone sealants not intended for exterior roofing work leads to their rapid degradation. Using a steel strip on a copper roof without an insulating gasket causes intense electrochemical corrosion of the steel. Installing a rigid standard strip on a curved surface leads to the formation of gaps.

Errors when working with seals are also very common. Installers either forget to lay them altogether or use a universal seal where a profiled one is needed, resulting in voids where snow can be blown in. Often, the seal blocks ventilation gaps in the ridge area, disrupting air circulation. Another problem is saving on fasteners. Screws are screwed in with a pitch of 500-600 mm instead of the recommended 200-300 mm, which does not ensure tight adhesion of the strip along its entire length, especially under wind uplift. Incorrect screwing of screws (at an angle, overtightened or undertightened) damages the protective coating of the strip and compromises the seal of the washer.

The consequences of these errors are always the same—leaks. Water penetrating under the strip gradually destroys the wooden elements of the sheathing and truss system, wets the insulation, sharply reducing its insulating properties and increasing heating costs. Wet mineral wool shrinks over time, creating cold bridges. A damp environment promotes the growth of mold and fungus, the spores of which are dangerous to the health of residents. In winter, water freezes, ice expands, and further destroys the junction node, tearing the strip away from the base. A leak may not go straight down but spread along rafters or sheathing, appearing on the ceiling in a completely unexpected place, making diagnosis very difficult. Correcting such errors requires complete opening of the junction node, which involves significant costs, often comparable to redoing the entire roof. Therefore, the only correct path is strict adherence to technology at the installation stage and the involvement of qualified specialists.

Modern Trends and Innovations in Junction Systems

The roofing industry is not standing still, and junction systems are constantly being improved, becoming more reliable, durable, and easier to install. One of the key modern trends is the shift from rigid, hard-to-fit metal strips on-site to systems based on highly elastic and self-adhesive materials. These are butyl rubber and bitumen-polymer tapes with an aluminum or copper base. Such materials (for example, product lines from leading manufacturers) allow creating watertight connections on surfaces of any complexity, including curved and textured ones. They do not require mechanical fastening (although it can be used for backup), have excellent adhesion to most building materials, and maintain elasticity over a wide temperature range from -40°C to +90°C.

The second noticeable direction is the development of integrated junction systems that are part of the roofing “assembly” from the very beginning. For example, for inverted and green roofs, special edge elements with built-in drip edges and mounting flanges are used, which are installed before laying the insulation and ballast. For membrane roofs, adhesive junctions have become widespread, where the membrane is connected to a vertical surface using special adhesive or hot air welding, and then this connection is protected by a mechanical clamping strip. This creates a monolithic, continuous waterproofing contour. Innovations also include systems for hidden fastening of flashing strips, where fastening elements (screws, dowels) are hidden under decorative covers or covered by subsequent rows of roofing material. This not only improves aesthetics but also eliminates potential leak points through fastener holes.

For pipe junctions, a revolutionary solution has been universal elastic penetration seals (roof jacks) made of EPDM rubber or silicone with an aluminum or stainless steel flange. Their main advantage is the ability to install on pipes of different diameters (thanks to the corrugated part) and on roofs with any slope. Even “smart” penetration seals with leak sensors that can transmit a signal to the owner’s smartphone if tightness is compromised have appeared. Another trend is the prefabrication of complex nodes. Factories manufacture ready-made corner junction elements, elements for valleys, tees for multiple pipes, which require only minimal adjustment on-site. This sharply improves quality and installation speed.

Eco-friendliness is also becoming an important factor. Manufacturers are moving away from lead in favor of safe alloys, developing strips from recycled materials (e.g., aluminum) with the possibility of their subsequent recycling. Decorative aspects are also evolving: flashing strips are offered not only in standard colors but also with the possibility of painting in any shade according to the customer’s catalog, with textures imitating wood, stone, or patinated copper. In highly critical projects (hospitals, data centers, museums), double and even triple junction systems with intermediate drainage channels and humidity sensors are used, providing absolute protection even in case of local damage to one of the contours. Thus, modern solutions for junctions are moving towards increased reliability through elasticity and adhesion, simplified installation, improved aesthetics, and integration into smart building monitoring systems.