Lesson Notes By Weeks and Term v4 - SHS 3
≈SUBSTRUCTURE AND SUPERSTRUCTURE
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≈SUBSTRUCTURE AND SUPERSTRUCTURE
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Applied Technology
Class: SHS 3
Term: 1st Term
Week: 19
Grade code: 3.3.2.LI.4
Strand code: 3
Sub-strand code: 3
Content standard code: 3.3.2.CS.2
Indicator code: 3.3.2.LI.4
Theme: BUILDING CONSTRUCTION TECHNOLOGY
Subtheme: ≈SUBSTRUCTURE AND SUPERSTRUCTURE
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This lesson focuses on the final and most visible part of a roof: the finish. The roof is a critical component of any building's superstructure, protecting it from the sun, rain, and wind. The "roof carcass" is the skeleton or frame, usually made of wood or steel, that gives the roof its shape. The "roof finish" is the covering or skin applied on top of this frame. In Ghana, the choice of roof finish is influenced by our climate, culture, economy, and the function of the building. From the common corrugated iron sheets on our homes to the traditional thatch on a summer hut, each material tells a story and serves a purpose.
A. Fundamental Definitions Roof Carcass: This is the structural framework of the roof. It is designed to support the weight of the roof finish and withstand external forces like wind. In Ghana, it is commonly constructed from timber (rafters, purlins, king posts, etc.) or sometimes light-gauge steel trusses. Roof Finish (or Roof Covering/Cladding): This is the outermost layer of the roof that is exposed to the weather. Its primary function is to make the building waterproof and protect the interior and the roof structure itself. It also contributes significantly to the building's aesthetic appearance. B. Common Types of Roof Finishes in Ghana
Here, we will discuss the materials mentioned in the NaCCA exemplars, which are widely used across the country. Corrugated Aluminium or Galvanized Iron (GI) Sheets These are the most common roofing materials in Ghana, often referred to locally as "roofing sheets" or "aluzinc." Description: Thin sheets of metal (iron coated with zinc, or aluminium) that have been shaped into a wavy or "corrugated" pattern to increase their strength and rigidity. Advantages: Cost-effective: Relatively cheap to purchase and install. Lightweight: They do not require a very heavy or expensive roof carcass. Easy Installation: They can be installed quickly with basic tools. Waterproof: When installed correctly with proper overlaps, they are highly effective at keeping out rain. Rainwater Harvesting: They provide a clean surface ideal for collecting rainwater. Disadvantages: Poor Thermal Insulation: They transfer heat easily, making rooms underneath very hot during the day unless a ceiling with insulation is installed. Noisy: They are very noisy during heavy rainfall. Corrosion: Galvanized iron sheets can rust over time, especially in coastal areas with salty air. Aluminium is more resistant to rust but can still corrode. Can be damaged: Can be dented by heavy objects or bent by strong winds if not properly secured. Tools & Accessories: Claw hammer, roofing nails with rubber or plastic washers, measuring tape, tin snips (for cutting), chalk line, ladder. Micro Concrete Roofing (MCR) Tiles These are small, flat or curved tiles made from a mixture of cement, sand, water, and sometimes fibre. Description: They are much smaller and lighter than traditional concrete tiles. They are interlocked when laid on the roof. Advantages: Durable: They have a long lifespan and are resistant to rust and decay. Good Thermal Insulation: They keep interiors cooler than metal sheets. Aesthetically Pleasing: They provide a more premium and attractive look to a building. Fire Resistant: They do not burn. Good for Coastal Areas: They perform very well in salty environments where metal sheets would corrode quickly. Disadvantages: Heavy: They are heavier than metal sheets and require a stronger, more expensive roof carcass. Higher Initial Cost: The tiles themselves and the installation are more expensive than corrugated sheets. Brittle: Can crack or break if walked on carelessly or if a heavy object falls on them. Slower Installation: Installation is more labour-intensive. Tools & Accessories: Trowel, mortar (for ridges and hips), tile cutters, nails, battens (thin strips of wood for the tiles to sit on). Wooden Shingles These are thin, tapered pieces of wood used to cover a roof. They are laid in an overlapping pattern. Description: Often made from durable timber like cedar, teak, or other tropical hardwoods. They provide a very natural and rustic look. Advantages: Excellent Aesthetics: Highly valued for their natural beauty. Often used on high-end residential homes, hotels, and eco-lodges. Good Insulator: Wood is a natural insulator, helping to keep the building cool. Durable (if treated): When made from high-quality, treated wood, they can last for a long time. Disadvantages: Very Expensive: Both the material and the skilled labour required for installation are costly. Fire Hazard: Unless treated with fire retardants, they are a significant fire risk. Maintenance: Require regular maintenance (treatment, sealing) to prevent rot, insect attacks, and warping. Not common: Less common in Ghana due to cost and fire risk, but seen in specific architectural designs. Tools & Accessories: Hammer, shingle nails (rust-resistant), utility knife, chalk line, measuring tape. Thatch This is a traditional roofing material made from dry vegetation such as straw, reeds, or palm leaves, layered to shed water. Description: A classic roofing method used for centuries in Ghana, especially for summer huts, traditional roundhouses, and tourist facilities aiming for a cultural feel. Advantages: Excellent Thermal Insulation: Thatch is one of the best natural insulators, keeping interiors remarkably cool in our hot climate. Low Cost/Locally Sourced: The materials can often be sourced locally for free or at a very low cost. Environmentally Friendly: It is a renewable and biodegradable material. Cultural Significance: Represents traditional Ghanaian architecture. Disadvantages: Short Lifespan: It decays over time and typically needs to be replaced every 5-10 years, depending on the material and climate. Fire Hazard: It is highly flammable. Pests: Can attract insects, rodents, and birds. Requires Skilled Labour: Proper thatching is a specialised skill. Tools & Accessories: A long needle (thatching needle), twine or rope, a knife or cutlass for trimming, a legget (a tool for dressing the thatch). Translucent Roofing Sheets These are sheets made from plastic materials like polycarbonate or fibreglass that allow light to pass through. Description: They often come in the same corrugated profiles as metal sheets, so they can be easily integrated into a metal roof. Advantages: Natural Light: Their main purpose is to allow daylight into a space, reducing the need for artificial lighting. Ideal for verandas, carports, workshops, and greenhouses. Lightweight: Very easy to handle and install. UV Protection: Many modern sheets are treated to block harmful UV rays while letting light through. Disadvantages: Heat Buildup: Can create a "greenhouse effect," making the space underneath very hot. Less Durable: Not as strong or long-lasting as metal or tiles. They can become brittle and crack over time due to sun exposure. Can Discolour: May turn yellow or cloudy with age. Tools & Accessories: Similar to corrugated metal sheets, but often require special screws with larger washers to avoid cracking the sheet. A fine-toothed saw may be needed for cutting. C. Factors Influencing the Choice of Roof Finish
Choosing a roof finish is not just about looks. It is a technical decision that involves balancing several factors: Geographical Location & Climate: Coastal Areas (e.g., Accra, Cape Coast, Takoradi): The salty sea breeze accelerates rust. Aluminium sheets or MCR tiles are far better choices than galvanized iron sheets. Hot, Dry Areas (e.g., Tamale, Bolgatanga): Heat is a major concern. A light-coloured metal roof will reflect more sun than a dark one. Thatch provides excellent cooling. MCR tiles are also good insulators. Rainforest/Heavy Rainfall Areas (e.g., Axim, parts of Ashanti Region): The primary need is excellent water-shedding. Corrugated sheets with steep pitches are effective. Moss and algae can grow on tiles or shingles if not maintained. Function of the Building: Residential House: A balance of cost, durability, and aesthetics is needed. Corrugated sheets are common, while MCR tiles are a popular upgrade. Workshop or Warehouse: Function is key. Corrugated metal sheets are ideal for cost and coverage. Translucent sheets can be added to provide natural light for working. Tourist Resort/Lodge: Aesthetics are paramount. Thatch or wooden shingles might be chosen to create a natural, luxurious, or cultural feel. Cost & Budget: This is often the most significant factor for many Ghanaians. Order of Cost (Lowest to Highest, generally): Thatch (if material is local) -> Corrugated Galvanized Iron -> Corrugated Aluminium -> MCR Tiles -> Wooden Shingles. Durability and Maintenance: How long should the roof last? MCR tiles and high-quality aluminium have a very long lifespan with minimal maintenance. Thatch has a short lifespan and requires replacement. Aesthetics (Appearance): What look does the building owner want to achieve? The colour and shape of the roof finish contribute greatly to the overall appearance of the building. Weight of the Material: Heavier materials like MCR tiles require a stronger and more expensive roof carcass. Lighter materials like corrugated sheets can be used on a less robust frame.
Guided Practice (With Solutions)