1. Project Overview

Photovoltaic power generation is an important component of the national new energy development strategy. Actively promoting distributed photovoltaic power generation is a crucial measure to drive the green and low-carbon transformation of energy, accelerate the construction of a new power system, and promote the development of related industries. The photovoltaic roof waterproofing system perfectly combines roof waterproofing with the photovoltaic system, significantly reducing the risk of leaks, addressing a key pain point for photovoltaic companies, and ensuring the smooth operation of photovoltaic power generation projects.

Deyang Dongfang Boiler Plant, located in Jingyang District, Deyang City, Sichuan Province, is a core subsidiary of China Dongfang Electric Group Co., Ltd. The main structure of the plant is entirely steel, and the roof uses profiled steel plates. It was put into operation in 2011 and experienced leaks during its operation. In response to the national dual-carbon policy and to develop green and clean energy, the owner decided to renovate the metal roof with a waterproofing system while simultaneously installing a distributed photovoltaic power generation system.

2. Roof Structure and its Characteristics

2.1 System Structure  The roof panels of the Deyang Dongfang Boiler Plant project use 0.8 mm thick YX71-380-760 type corrugated steel sheets. The roof has a slope of 3% on both sides; the main roof details include skylights, chimneys, parapet walls, gutters, and fans.

This roof renovation project uses a single-layer self-adhesive TPO membrane waterproofing system. First, a special primer is applied to the base surface, and then a 1.5 mm thick butyl self-adhesive TPO waterproof membrane is applied to form a flexible waterproof layer over the entire roof. The overlapping areas of the butyl self-adhesive TPO waterproof membrane are joined by hot-air welding, which allows the membrane to be completely fused together at the overlaps, ensuring that the peel strength of the membrane welds and the overall service life of the system meet the requirements.

2.2 System Characteristics 

1) The 1.5 mm thick butyl self-adhesive TPO waterproof membrane used in this project consists of a TPO sheet, a butyl self-adhesive layer, and an MPET release film. The TPO sheet is 1.1 mm thick, and the butyl self-adhesive layer is 0.4 mm thick. Through application testing, the average peel strength data of the butyl self-adhesive TPO waterproof membrane with the color steel plate is 3.014 N/mm. In addition, the artificial weathering irradiation time of the main TPO membrane material has reached 10,000 hours, which far exceeds the requirement of not less than 2,500 hours for artificial weathering irradiation time of polymer waterproof membranes in JGJ/T 316—2013 "Technical Specification for Single-Layer Waterproof Membrane Roofing Engineering," indicating excellent durability of the membrane. At the same time, the good airtightness and watertightness of the butyl adhesive can also solve the problem of high leakage risk in traditional metal roofs with rigid connections.

2) The overlapping areas of the TPO membrane are completely fused together by hot-air welding, greatly reducing the risk of water leakage.

3) The use of white TPO membrane has a higher solar reflectance, which can effectively resist heat exchange between the interior and exterior, reduce indoor temperature, and save energy and reduce emissions. 

4) The self-adhesive TPO membrane is directly bonded to the substrate, making construction convenient and ensuring safe and reliable treatment of detailed joints.

5) The roof adopts a single-layer waterproofing system, resulting in a simple structure, low load, easy maintenance, and low cost.

3. Construction Process Flow

3.1 Preparation Before Construction and Substrate Treatment  Since this project is a metal roof renovation project, a site survey was first conducted to check for rust, deformation, and cracks in the metal roof panels. The site survey revealed no rust or deformation in the metal roof panels, so no grinding or rust removal was required.

The roof was cleaned of debris, and the substrate was washed with water.  A mop was then used to remove dust and water stains from the surface of the metal roof panels, ensuring the substrate was clean and free of contaminants (Figure 1). After the substrate was clean and dry, a special primer was applied. The special primer is a new type of material composed of polymer emulsion and multi-functional additives. It has good sealing properties, strong penetration, and high interface strength, which increases the adhesion between the self-adhesive membrane and the substrate, allowing the self-adhesive waterproof membrane to adhere better to the substrate. The special primer can be applied by spraying, which significantly improves construction efficiency (Figure 2). The amount of primer used is approximately 0.2~0.4 kg/m2.

                 Figure 1. Base surface cle          Figure 2. Applying base surface treatment agent

3.2 Waterproof Membrane Installation 1) Laying the membrane: Based on the corrugated steel sheet profile (YX51-380-760) at the site, determine the laying position of the first membrane sheet (Figure 3). Since the butyl self-adhesive TPO waterproof membrane has a width of 1000 mm, calculations show that after the first membrane sheet is laid along the trough and crest of the first corrugated steel sheet, the long edge overlap position of the second membrane sheet with the first sheet will also be located on the flat surface of the trough of the second steel sheet. By following this method, the long edge overlap of all membrane sheets can be kept in the trough position of the steel sheets, which improves the utilization rate of the automatic hot-air welding machine, reduces manual welding, and improves construction efficiency.

Figure 3. Steel plate profile and coil overlap.

After determining the position of the first roll of membrane, unroll it and lay it flat on the substrate to release the stress generated during the manufacturing and winding process, thus reducing wrinkles caused by shrinkage after the membrane is adhered. Based on the on-site construction conditions, the laying direction of the membrane should be parallel to the long side of the profiled steel sheet, and the long-side overlap should be kept in the trough of the profiled steel sheet as much as possible. Rewind the membrane along the long side, tear off the release film from one side of the membrane, and adhere the membrane along the peaks and troughs, ensuring that the membrane is fully bonded to the profiled steel sheet, avoiding voids and poor adhesion during the bonding process. The short-edge overlaps of adjacent membrane rolls should be staggered, with a staggering distance of no less than 300 mm. Figure 4 shows the on-site membrane application process.

Figure 4. Applying the rolled material.

2) When compacting and adhering the membrane, use a pressure roller to ensure that the butyl self-adhesive layer is fully bonded to the profiled steel plate; after the main surface of the membrane is adhered and compacted, use a special tool to compact the inner corners on both sides of the corrugations to prevent the membrane from bulging at the corners (Figure 5).

Figure 5. Compaction of rolled material

3) All longitudinal overlaps of the butyl self-adhesive TPO membrane are welded using a hot-air welding machine. The overlap width of the two membrane sheets is 80 mm (Figure 6). Before welding, a test weld is required to adjust the welding temperature and speed. After the test weld, the membrane is cut into small pieces for peel testing.  If cohesive failure occurs between the membrane layers, formal welding can begin. Before welding, the weld seam must be kept clean and free of contaminants. If the seam has been exposed for a long time without welding, it should be cleaned with a clean cloth and a special cleaning solvent (xylene, vinyl acetate). Hot-air welding should only be performed after the solvent has completely evaporated.

Figure 6. Long-edge welding

The short edges of the membrane rolls are joined using homogeneous TPO membrane and a handheld welding machine, along with a 20 mm wide nozzle (Figure 7).

4) Main Detail Node Treatment ① Roof Ridge Node Treatment: First, self-adhesive TPO membrane is applied to the roof ridge area. Then, a 200 mm wide homogeneous TPO membrane is welded to the self-adhesive TPO membrane at the roof ridge.  Cuts are made at the crests of the corrugations, and finally, the homogeneous TPO membrane is used for repair welding (Figure 8).

Figure 8. Roof ridge joint treatment

 ② Skylight node treatment: The entire skylight facade is wrapped with homogeneous TPO membrane. The upper end is fixed with a closing strip and then sealed with silicone weather-resistant sealant; the lower part of the membrane is welded to the main membrane (Figure 9).

Figure 9. Skylight node processing

③ Gutter joint treatment: TPO membrane is applied to the bottom and vertical surfaces of the gutter. The vertical membrane is turned up and welded to the main surface, and the cut-out areas at the wave crests are repaired and welded with homogeneous TPO membrane (Figure 10).

Figure 10. Gutter joint treatment

④ For the drain outlet node treatment, the homogeneous TPO membrane is welded into a cylindrical shape and placed inside the drain outlet. The lower end is sealed with silicone sealant, and the upper part is heat-welded to the waterproof layer on the bottom of the gutter (Figure 11).

Figure 11. Drainage outlet node treatment

5) Before installing the mounting clips for the photovoltaic modules, the installation position of the clips is determined by marking lines.  Thin metal shims are then pre-installed on the roofing membrane. The shims are made of aluminum foil with a thickness of 0.4 mm. Pre-installing the shims significantly reduces wear on the roofing membrane caused by direct contact with the clips, protecting the integrity of the waterproof layer. Based on the marked lines, the two screws on the clip are initially kept loose. After placing the clip on the shim, an electric wrench is used to tighten the clip at the crest of the corrugated sheet (Figure 12). The clips were tested for tensile strength with the roofing membrane and steel sheet, achieving a tensile force of 1.177 kN, which meets the requirements.

Figure 12. Fixture installation

Based on the position of the clamps, install the photovoltaic rails, secure the rails to the clamps with screws, and finally install the solar photovoltaic panels onto the rails (Figure 13).

Figure 13. Track installation

6) Completed roof installation effect: Figure 14 shows the completed roof installation of the 4 MW rooftop distributed photovoltaic power generation system at Deyang Dongfang Boiler Factory.