What is a concrete roof?
A concrete roof basically is a slab of concrete, that is used to make the roof of a house. The concrete roof is made by pouring concrete into a shuttering structure. A shuttering structure is a structured mould, usually made up of wood planks and boards that are used to mould the concrete into a slab shape. The concrete slab then sits in the mould that is supported by temporary and permanent columns beneath it.
The temporary columns are made up of wood planks or steel pipes and are only used to hold the weight of the slab till it sets. The permanent columns are either made up of bricks or reinforced concrete (concrete with steel mesh). Further the civil engineer may also direct the use of beams in between the columns in open areas and spaces such as an open living area to hold the concrete roof.
It will be apparent by now that concrete roofs are quite hard to install and they require a lot of prep. But they are definitely worth all this effort. You see, a concrete roof requires a lot of tedious prep and hassle but the one thing going in its favour is that it is permanent. You may have heard a friend or a relative talk about how expensive it was to change the roof of their house or fix it. Well, with a concrete roof there is no maintenance. Once a concrete roof sets it requires zero maintenance. A concrete roof if done properly doesn’t leak nor does it require repair off and on.
Concrete roofs are not recommended to be used in houses made up of wood. This is because the wood planks are not able to withstand the weight of a concrete slab that in the case of large houses can be several tons on each column. Concrete roofs are only used in large apartment buildings, mega-projects, and houses that are made up of bricks. Even if you construct a house out of bricks you will have to support the concrete roof with support columns and beams.
The conventional rule used when using a concrete roof is that every brick wall should sit atop another brick wall below it. This is mostly used in the construction of a double-storey houses. As the weight of the concrete roof is several tons, the house needs to be built like a cocoon. This is to ensure that the weight is evenly distributed among many support structures beneath the concrete roof. Being built like a cocoon means that each structure adds to the support. Whether the structure is a beam or a column or a pillar or a wall. This makes the structure just like a cocoon i.e. it is very hard to crack it until one of the structural elements of the building breaks down. Once that happens the whole structure is weakened and risks collapse.
Many people are concerned about the fact that a flat concrete roof will not be able to drain water from rainfall. This will lead to stagnant water pools on top of the roof which would then cause seepage and would ruin the roof. This is mostly a false belief. Concrete roofs are not flat. They have a slight degree of slope. This is so that water doesn’t stand and becomes stagnant on top of the concrete roof. The slope allows the water to drain the rainfall towards a corner of the roof. This is usually where the external ducting will be installed to channel water from the roof to the ground. So concrete roofs are not going to get ruined by a little rainfall. Don’t get worried.
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What is the lifespan of a concrete roof? How long do concrete roofs last?
The lifespan of a concrete roof is 50 years to 200 years if it was built with a full rebar frame and poured. The lifespan of a concrete roof is 25 years if it is built using interlocking concrete blocks, shingles and tiles.
The average lifespan of a poured concrete roof is between 50 years to 200 years depending on the maintenance of the building also. Even after 200 years, most concrete roofs will stand if they have not been damaged by water seepage or algae growth. We see many cathedrals and palaces with concrete roofs built as early as the mid-18th century still standing tall. As we invented concrete only in 1824, we are still unsure as to how long concrete will last if properly maintained.
If you are curious about the different grades of concrete and the type of concrete used in roofing you can learn more about it here.
What is the thickness of the concrete roof? How thick should a concrete roof be?
The thickness of a single-storey concrete roof varies depending on the type of slab used (ribbed or flat) but typically ranges from 3 inches (7.6 cm) to 4 inches (10.2 cm) thick for an average-size single-storey family home.
The thickness of a typical concrete roof is usually 7 inches for double-storey houses and commercial apartment buildings. But the thickness can be increased or decreased depending upon the expected load at the point of no support. For e.g., if you are building a concrete roof in a multi-story industrial complex, the concrete roof may have to withstand a load of heavy industrial machinery on top of it. To withstand such loads you would have to increase the thickness as well as change the concrete mix ratios to increase the strength of the concrete roof for such applications.
What is the purpose of a concrete roof?
The main purpose of using a concrete roof is to build a building that will last a very very long time. For e.g, if you are building an apartment complex you want to ensure that the complex lasts for a long time time to keep on providing housing to the residents. You also want the building to last a long time to meet the minimum code requirements. While a typical shingle roof will last for about 7 to 10 years concrete roofs can last more than 50 years on average. The purpose of a concrete roof is also to keep out water leakage, provide shelter from the outdoors, protect occupants from the weather and increase property value.
What are the types of concrete roofs?
Concrete roofs have many different types. They are classified on the basis of whether they have a slope or not. They are also classified on the basis of the type of material used to form a concrete roof. They are as follows:
Classification on the basis of slope
A sloped concrete roof
A sloped concrete roof is usually used at the topmost level of the building. For a single-story house, it is the only type of concrete roof that is going to be used.
A non-sloped concrete roof
This type of concrete roof is used in multi-story buildings to make the roofs between the various levels or stories of the building. They are used as the roof of one apartment and the floor of another. They cannot be placed at the topmost level due to the risk of water seepage on a flatbed by stagnant water.
Classification on the basis of material used
There are four main types of concrete roofs on the basis of material used for building the concrete roof. Each type has its own specific advantages and disadvantages. They are:
Poured concrete roofs are the best and have the longest lifespan. But to get a poured concrete roof you will need to get shuttering and a steel rebar frame to hold the concrete in place. The poured concrete roof will then take 14 to 21 days to fully set depending on the weather. In summers it usually takes 14 days while in winters it will roughly take about 21 days.
A concrete block/beam is a concrete roof that is formed by placing already made concrete blocks/beams. These are made in a factory by pouring concrete into wooden shuttering. Once the concrete sets the blocks/beams are loaded onto a truck and transported to the area of the house being constructed. The beams are laid side to side over opposing walls to cover the open void using a lift. The gaps between the blocks/beams are then filled using mortar. Mortar sets in 24 to 48 hours and then you can continue building on top of these beams. However, this type of roof is not a single structure rather it is blocks/beams held by glue. So in the event of an earthquake, a single beam can break and fall down. There is always this risk.
Concrete shingled and tiled roofs are not actually real concrete roofs. They are basically wooden roofs to which concrete shingles or tiles have been added to make the roof water and heat resistant. They are applied like regular shingles over a tarp and are glued in place using mortar.
Is a concrete roof durable? Are concrete roofs good?
Yes, it most durable out of all the options available. It is not only durable but will last pretty much your entire life. A concrete roof is durable because the material can withstand harsh weather conditions such as heavy rain, hail storms, high winds and snowfall without becoming damaged. But the problem is the upfront cost. Which can be as much as $40,000. That is a hard pill to swallow.
How much does concrete roofing cost?
Concrete roofing can cost as much as $25,000 to $40,000 depending upon the state. But in states where concrete roofing is common such as Newyork, you may be able to hire a workforce for the project at a cheaper rate. In areas where concrete roofing is a rare sight such as Idaho, making a house with a concrete roof can demand a hefty bill. This is based on the supply and demand philosophy.
Learn more by watching the video below on how it’s done in practice.
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If you are unsure about when to start construction of your house read our article on the best months to start construction here.