Construction techniques and methods for pitched roofs

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From the Internet, for educational purposes only: I. Overview of the layout: Section I of Area B, constructed by the company, is the section with the most complex building structure in the entire Area B; it consists of ten villas in an antique style and two supermarkets, with a total construction area of 37,000 square meters. Each villa is built with brick-concrete structures, has three floors, and covers an area of around 3,700 square meters, housing 20 households each. The inner courtyard is divided into four small gardens by walls about 3 meters high. The external decorations of each courtyard, from large elements such as the roof, walls, railings, and doors and windows, to smaller details like eaves, beams, hanging ornaments, and gourds, all reflect the characteristics of ancient western Sichuan architecture ； The interior layout further embodies the essence of modern decoration; every room in each villa fully reflects the blend of tradition and modernity, achieving an atmosphere of a culturally rich dwelling ； Outside, the antique style blends well with the nearby bridges, flowing water, and lawns, complementing each other perfectly. The construction techniques for the foundation and the main structure are not very different from those used in other brick-concrete structure buildings. The key points and difficulties in construction lie in the construction of the sloped roof, which accounts for a significant portion of the overall construction time ； This characteristic is evident in everything from elevation control to leveling and setting out, from the formwork support systems to the assembly of the formwork, from the processing of rebar to its binding, as well as the sequence of concrete pouring. Taking the completed No. 2 courtyard as an example, this section introduces the construction techniques and methods for sloped roofs. II. Characteristics of the roof construction in Courtyard No. 2: This is a residential building in an antique style, with a roof structure that is entirely made of cast-in-place reinforced concrete. The structure is relatively complex, the roof slope is steep, and there are many details to take into account. Special attention must be paid to the handling of beam and slab joints, as well as to controlling the quality of the cast-in-place concrete for the sloped roof. For the pitched roof of this project, the ridge is the highest point, with an elevation of 9.861m ； The eaves represent the lowest point, with an elevation of 6.500m ； The entire sloping roof, from the ridge to the eaves, has four different slope gradients, which range from 65% at the top to 55%, 50%, and 45% as it goes downward ； The thickness of the roof panels also varies depending on the location, ranging from 80┨ to 120┨ ； In many places, the beams also intersect and overlap from different directions; to ensure this, the construction of the roof requires strict control over the elevations of various details such as the roof panels and beams. III. Safety requirements for roof work: Due to the steep slope of the roof structure, safety is given top priority to ensure the safe progress and completion of the construction work. Before construction, the Technical Department and the Quality and Safety Department organize safety training and briefings for all personnel involved in the roofing work, ensuring that all relevant precautions are communicated to every individual. Specific safety details are omitted. IV. Identification of control points during construction: Based on the characteristics of the roof structure design for this project, control was exercised in the following areas: 1. Due to the large slope and numerous transitions in the roof of this project, it was necessary to control the elevation of various sections (wall masonry, the lower parts of beams and slabs, as well as each transition point) during construction ； 2. Control of material transportation and on-site concrete pouring during roof structure construction ； 3. Control of on-site concrete mixing and the sequence of concrete pouring. V. Main construction techniques and methods 1. Construction of the brick wall on the third floor of the main structure: Since the construction of the brick wall on the third floor directly affects the construction of the pitched roof, the elevation of each turning point of the roof’s slope, as well as the elevation and position of the bottom surface of each beam, must be determined based on the brick wall on the third floor. Therefore, it is necessary to control the construction of this brick wall before starting the work on the pitched roof, in order to ensure the accuracy of all dimensions related to the roof. Since many of the detailed dimensions of the sloped roof in Courtyard No. 2 were not clearly specified in the design drawings, it was not possible to carry out direct setting out for the walls and roof. To ensure the accuracy of the construction height at each point on the walls, the technical department made use of modern management tools – computers and computer software – to conduct simulated setting out prior to construction: using the AutoCAD software, the existing dimensions and shapes from the design drawings were entered into the computer, and the positional dimensions and elevations of each structural detail were determined based on the same scale ； Then, on-site lofting is carried out based on the dimensions determined by the computer, in order to determine whether the original dimensions match. Using computer-based simulation for layout not only saves time but also reduces unnecessary rework. After repeated simulations and calculations, the positions and heights are determined on-site by marking out lines based on these dimensions. Marking stakes are set up at each turning point, with the height of each such point indicated. The highest points of each turning point along the same wall are then connected by straight lines, thereby allowing control over the slope of the brick wall being constructed. Since the brick wall is constructed using KP1 porous bricks, there is a reinforced concrete ring beam at the upper part of the wall, following the slope of the wall. To prevent the concrete mixture from leaking out through the pores in the porous bricks during pouring, a layer of solid bricks was laid on top of the porous bricks during construction; this measure effectively prevented the formation of honeycombing, pitting, and other defects in the concrete due to leakage of the mixture. 2. Formwork and support system: The formwork work is one of the key factors in ensuring the quality of concrete construction for sloped roofs and in accelerating the progress of roof construction. Therefore, selecting an appropriate formwork and support system that takes into account the characteristics and scale of the sloped roof in this project is a crucial consideration in the construction of such roofs. The formwork and its support system must possess sufficient strength, stiffness, and stability to reliably bear the weight of the newly poured concrete, the lateral pressures, as well as the loads generated during the construction process. The bottom formwork for this roof panel is made of high-strength bamboo plywood with a thickness of 12 mm, featuring a flat surface free from warping, deformation, cracking, or delamination. The support system uses wooden beams of 100×50 mm that are free from severe corrosion, cracking, or warping, as well as welded steel pipes of Ф48×3.5 mm without serious defects such as rust, bending, flattening, or cracks, along with corresponding fasteners. A full-house scaffold is erected to support the formwork for sloped roofs; the vertical and horizontal spacing between the scaffold poles is 0.8–1.2 meters, while the horizontal spacing between the crossbars is 1.5 meters. A ground bar is installed at a height of 150 mm above the ground, and an additional crossbar is added along the slope direction beneath the formwork of the cast-in-place roof panel, so that the support system forms a grid structure. Before installing the vertical posts of the support frame, shims measuring 50mm×200mm in size were placed along their entire length, in accordance with the construction specifications. Before erecting the full-house scaffold, a row of scaffolds is set up using tension ropes based on the turning points, beam positions, and elevations determined through computer-based simulations. Using these as reference points, the formwork for the roof panels is then erected; once it is confirmed that the slope of each transition is accurate, the full-house scaffold is installed in accordance with the aforementioned requirements. Due to the large slope of the roof structure, to ensure the stability of the bottom formwork, a horizontal bar was installed along the bottom formwork of the sloped roof at the locations where the formwork scaffolding provided support; the top supports of the formwork were secured using wooden wedges for reinforcement. After the scaffolding was erected, various management departments of the project team, along with on-site representatives from the construction party, carefully and thoroughly checked whether the connection between the wooden beams beneath the scaffold planks and the scaffold poles was stable and secure. Based on the specified elevation lines, they adjusted the height of the horizontal bars beneath the wooden beams to level them out. After the base formwork is laid, use a straightedge, feeler gauge, and level to check the flatness and the elevation of the floor slab bottom, and make adjustments as necessary. Only after everything is correct will work proceed to the next stage. 3. Rebar work: The processing of rebar for this project is carried out in a dedicated processing shed, with the rebar being fabricated strictly in accordance with the design drawings and relevant **standard requirements. Since the rebar in the roof panels runs the full length, and the roof needs to bend at certain points, the rebar must be bent at each turning point in a processing shed using a cold bending machine at the angles specified in the design, in order to ensure the correct cross-sectional dimensions of the structure at those turning points. The transportation of rebar is carried out manually to the binding site, depending on the actual conditions of on-site construction. The binding of the rebar meets the design requirements as well as the relevant acceptance standards; in addition, at the location of the ridge beams, a rebar hook that is higher than the ridge is installed every 1.5 meters along the direction of the ridge, so as to provide a means for attaching safety belts during the pouring of the roof concrete and the construction of the roof waterproofing system. 4. Concrete work: The key aspects of constructing the structural concrete for this roof project lie in controlling the mixing of the concrete, its transportation, and the pouring process. The concrete for the roof of this project is mixed mechanically on site, with pump trucks and manual two-wheeled vehicles used for horizontal transportation on the ground. Given the high slope of the roof in this project, the preparation of the concrete was carried out strictly in accordance with the specified mix proportions, with close control over the water-cement ratio, workability, and slump of the concrete. The slump of the concrete mixed on-site should be kept within the lower limit of 3 cm, in order to ensure the quality of concrete pouring for sloped roofs. The horizontal transportation of concrete on the ground is carried out using manual two-wheel carts, which work in conjunction with gantry cranes. For the horizontal transportation of concrete along the inner ring of the roof ridge, a 2.8m wide passage is constructed on the roof following the ridge; manual two-wheeled carts are used to transport the concrete to the pouring site, where it is delivered using chutes. The horizontal transport passage is constructed between the WXL1 beams and XQL3 beams on both sides of the ridge beam (WXL2); the vertical posts of the passage are spaced 1.0–2.0 meters apart, while the horizontal bars are spaced 1.5 meters apart. The base of the vertical posts is fixed by passing it directly through the beam bottom at the location where there is a brick wall, and securing it to the top surface of that brick wall ； Where there is no brick wall under the beam, aircraft brackets are used to fix it vertically to the scaffolding and rebar ； When the formwork is removed, knock out the dry spots and cut away the exposed parts of the aircraft support from the concrete surface, leaving the remaining parts embedded in the concrete. Two gantry cranes are used for the vertical transportation of concrete, located adjacent to axis 01 and axis 027 respectively. An operating platform must be set up at the location where materials are unloaded from the gantry crane basket. This operating platform is connected to the walkways; two diagonal braces are installed to connect it to the external scaffolding, and diagonal bracing is added to the vertical poles on both sides, with planks covering the crossbars entirely. To enable safe unloading and transportation to the pouring site. The external scaffold is erected 1.5 m above the roof eaves and close to them; two horizontal bars are installed in the area above the eaves, with a partition bar placed between them. Safety nets are everywhere around. Throughout the concrete pouring process for the entire roof structure, the preparation of labor and equipment, as well as the determination of the order in which the concrete is poured, play a significant role in the quality of the concrete pouring. Due to the design requirements, which prohibit the presence of cold joints in the pouring of the entire roof concrete, the concrete must be poured all at once. Due to the large volume of cast-in-place concrete required for the roof, along with the steep slope of the roof and limited construction space, it is not possible to carry out large-scale pouring. Based on the calculation of the volume of concrete required for the roof and the setting time of the concrete, the following arrangements have been made for the construction of this roof project: The concrete for the roof will be mixed on-site using two mixers, which are placed next to two gantry cranes respectively to facilitate timely transportation of the concrete. The pouring of the roof concrete is carried out simultaneously on both sides of the roof, using the ridge as a dividing line, and progress toward the ridge to form a continuous layer. The personnel are arranged in two teams for the work; both teams carry out construction simultaneously, with 15 people in each team. A shift system is implemented to ensure continuous construction. The first group poured concrete in a clockwise direction along a ring 50┩ wide, starting from the eaves of the outer perimeter of the ridge; they connected these sections before the freshly poured concrete set. Then, they poured concrete for the second ring, also 50┩ wide, moving gradually closer to the ridge ； While the first team is pouring concrete, the second team pours concrete in a counterclockwise direction along a ring 50┩ wide, starting from the eaves of the inner perimeter of the ridge. They also complete the formation of this ring before the freshly poured concrete sets, after which they pour the second layer along another ring 50┩ wide, moving gradually closer to the ridge; finally, they join forces with the first team at the ridge to complete the enclosure of the area, thus finishing the pouring of concrete for the entire roof. Due to the steep slope of the roof, operators are unable to stand firmly on it. To ensure their safety, they must wear safety belts attached to special steel bar hooks installed at the roof ridge; once one section is completed, the belt is moved to a neighboring steel bar hook. To fully ensure the quality of concrete pouring, 4 insert-type vibrators (with 2 as spares) should be prepared before pouring the concrete ； 2 lightweight flat vibrators ； One standby generator (75KW) ； Materials such as sand, gravel, and cement are fully prepared on site ； All materials required for roof concrete curing and insulation are ready. The project team organized relevant personnel to conduct a final inspection and control, adjusting the dimensions, specifications, quantities, and positions of formwork, rebar, protective layers, and embedded components. Only after the inspection was satisfactory was concrete pouring carried out. When pouring concrete, chutes are used to keep the free fall height of the concrete within 2 meters. To ensure the proper progression of the concrete’s hydration process and to prevent the increase in concrete strength from being hindered by water evaporation, which could lead to surface shrinkage cracks, curing should be carried out within 12 hours after concrete pouring (depending on temperature conditions). The concrete surface is covered with plastic film for curing. Judging from Courtyard No. 2, which has already been completed, the roofs built using the aforementioned construction techniques and methods have achieved satisfactory results in terms of safety, quality, and construction time. This earned the approval of Party A, securing credibility and advantages for our company. Last edited by Lailai on 2009-3-24 19:00]