Q.
(a) Outline four (4) factors that affect the performance of a roof. (10 marks)
(b) Using sketches, explain the construction of reinforced concrete flat roof for high rise buildings. (15 marks)
(25 marks, 2016 Q4)
A.
a) 4 factors which affects the roof are (any of the 7):
- Roof design
- Roof system - material used
- Installation by skilled contractors
- Maintenance over time
- Ponding Water - water accumulation
- Temperature extremities
- Blistering & Shrinkage
Blistering and shrinkage are also factors that impact the performance of your roofing system. When liquid gets trapped between membrane plies, the vapor expands with increased temperature and pushes the membrane out into raised strips.
While this is initially a minor issue with no penetration, neglected blisters can be detrimental to the performance of your roof. Shrinkage occurs from both weathering and improper installation.
Stress cracks and wrinkles are common byproducts of shrinkage, allowing water leaks to impair the integrity of your roofing system.
Regular inspections can help identify blistering and shrinkage issues before they dramatically harm the performance of your roof.
In another write up:
Here are the factors that affect roof performance, which should help you make informed decisions on how to keep it in good condition for a long time:
- Roofing System and Design. This means how the complete roofing system works for your home after installation – roof deck, underlayment, ventilation, shingle types and fasteners. A top roofer should be able to discuss how each stage of the installation contributes to the design – and consequently to the roof’s performance. Our factory-certified installers will be glad to answer any questions you may have prior to installation of your roofing in Belleville, IL.
- Maintenance. Much has been said about how important roof maintenance is. It should be done by professionals. In addition, you shouldn’t wait for a problem to manifest before doing it. Nonetheless, proper maintenance will prolong your roof’s life span. If your contractor can perform scheduled maintenance and emergency roof repair in Belleville IL, so much the better.
- Choice of Material and the Climate. The merits of which roofing material is best – be it tile, steel, membrane or asphalt shingle roofs in Belleville, IL – are all up for debate. Each has its advantages that affect performance. Still, choose the roofing material that’s appropriate to your general climate. You obviously wouldn’t install a membranous roof if the area gets a lot of hail. You would probably go for steel roofing if you’re in an area with extremely heavy rainfall.
Ref:
http://www.fibertite.com/blog/9-factors-that-affect-roof-performance-part-1
http://www.hhiroof.com/roofing/3-factors-that-affect-roof-performance/
b) Reinforced concrete flat roof
The term ‘roof’ is used to refer to an entire assembly, including both the roofing components such as the membrane, insulation, and vapour retarder, as well as the structural deck. The structural deck not only supports the roofing components, but it also supports any intended rooftop equipment (such as heating/air-conditioning units), as well as people and snow loads.
For flat roofs (also referred to as low-sloped roofs, since all roofs require some slope to drain), structural decks are usually constructed of concrete or steel. For the latter, the most common is fluted metal decking on open web steel joists (OWSJs). Roof decks are designed to support a particular load (or weight). Therefore, it is important any changes to the loading conditions be carefully assessed.
Cast-in-place reinforced concrete roof decks are typically seen in high-rise or multi-storey buildings. Concrete decks often have some amount of excess load-carrying capacity. In other words, they can usually support a little more weight than for which they were strictly designed, without structural distress.
Membrane for water proofing
The membrane is what keeps the water out. There are many different types of membrane systems, but they generally fall into three broad categories:
- bituminous membranes, such as modified bitumen (mod-bit) or built-up felt and asphalt systems;
- liquid-applied coatings, such as rubberized asphalt or urethane-based elastomeric traffic deck coatings; and
- single-ply membranes, such as polyvinyl chloride (PVC), thermoplastic olefin (TPO), and ethylene propylene diene monomer (EPDM).
A wide selection of membrane systems is available, each with its own performance, cost, and durability characteristics.
Some membrane systems are more resistant to ultraviolet (UV) exposure from the sun, while others stand up better to punctures, tears, abrasion, or other physical damage. Some membranes are more resistant to thermal fluctuations or severe climates, while others perform better from an energy perspective.
In selecting a membrane, it is important to carefully consider the service conditions. For example, an exposed single-ply membrane would not be advisable for a roof with a large amount of rooftop equipment as it would be less resistant to inadvertent punctures or physical damage caused by service contractors.
Another example of a service condition would be when the roof is readily visible (such as by occupants from overlooking windows). In these cases, it might be a good idea to avoid an exposed light-coloured membrane as it will look dirty, even with regular cleaning.
Roofs are meant to shed, not hold, water. Large amounts of standing water sitting for long periods can increase the risk of leakage, and decrease the membrane’s service life. In certain situations, stagnant water can become a breeding ground for insects like mosquitos, or create a safety concern. For example, water can freeze, causing a slipping hazard for maintenance staff accessing the roof.
Effective drainage relies not only on an adequate number of drains, but also effective slopes to those drains. Drains, even if numerous, are useless if they are located at the roof’s high points.
In conventional roofing assemblies, where the membrane is installed atop the insulation, tapered insulation boards are frequently used to slope the membrane surface toward the drains. In protected membrane assemblies, the waterproofing membrane is installed below the insulation; in these cases, effective drainage relies on the slope of the structural roof deck itself. For some instances, lightweight concrete toppings are used to provide slopes to drains. In other cases, as in poured concrete slabs, the top surface of the slab is simply finished to drain at the time of construction.
Drainage can also be affected by changes to the thickness of the insulation. Increasing the thickness may cause problems with scuppers and overflow scuppers. As well, improperly designed overlays can affect drainage, particularly when installed on a roof that already had problematic drainage.
Wind uplift: go beyond manufacturers’ guidelines
Construction Canada (2013) Critical considerations for replacing a flat roof. Available at,
Ballast
Ballast is usually required in the construction of ‘inverted’ warm roofs to prevent the insulation boards from being lifted by high winds or floating as water drains from the roof. It normally consists of graded pebbles or concrete paving slabs.
