Remember this when building in the rain

Wednesday April 14 2021

Plan in advance and endeavour to do your roofing nonstop to minimise chances of being caught up in a storm. Soaked roofing materials could lead to unexpected costs. -PHOTO/

By Roland D. Nasasira

Construction during the rainy season is one of the most challenging tasks to undertake. Almost everything that is carried out at the site is done hurriedly in a race against the rain and this may come with effects such as erosion of some of the materials.

Regardless of whether you are building on a flat or sloping area, if you have to excavate your foundation, you do not want rainwater accumulating therein, because then it becomes waterlogged and softens the underlying ground. This may not only cause a structural change within the foundation ground but it also creates more work where you have to pump or scoop out the rain water from the foundation before any construction can go on. 

According to Amon Atuhaire, a site supervisor, to prevent your foundation from getting waterlogged, it is advisable to create channels of approximately two metres to divert water away from the foundation. This will prevent any deterioration of the foundation because of the rain.

“Apart from filling the excavated foundation with running water, rainwater also tends to push the disintegrated soil back into the foundation since it is always deposited at the edge of the foundation. You will have to pump out the water but also re-excavate to obtain the initial foundation depth,” Atuhaire says.

Covering existing works

Rain is as unpredictable as sickness. To  be safe, when the rain comes, it should find you at a certain level of preparation. According to Joseph Oryang, a site engineer with Century Investors Limited, the challenge when it starts raining, for instance when the concrete has just been cast at the foundation or slab level when the concrete is still wet, is that the cement will be washed from the upper layer of the concrete, thereby weakening the structure underneath. 


“When the rain hits the top layer of cast concrete, it is no longer concrete but just a mixture of stones and sand. It is not strong enough for the structure to be erected on top in the long run and you need to remove the top layer if it was not covered and cast a fresh one before you can construct further. When rain mixes with the mortar, it becomes diluted and weakens the cement which is the binding material,” Oryang explains.

Sometimes using simple protective covers such as tarpaulins or cement packs to cover the cast concrete could actually allow certain works to continue so that it is neither diluted nor washed away. If there is no adequate supervision on the site, your builders will run away from the rain to take shelter and leave the concrete exposed and realise the effects when the rain has stopped.   

Protecting materials

Chris Magezi does not recall how many trips of sand he bought when he was building his apartments at Buwaate, a Kampala suburb. It was during one of the rainy seasons in 2016 when, during random site visits, he observed that large chunks of sand were being washed away from the site. He says he almost fired the site supervisor because of carelessness.

William Sserwadda, a builder advises that materials such as sand when deposited at a site, should be surrounded with a dug water channel that leads water away from the sand after carefully identifying the direction of the runoff. You may also use bricks to create a boundary for and prevent the sand from being washed away by rainwater. Alternatively, you may deposit the sand in a place where it is not at risk of flood or rainwater.


Unlike sand, cement is more fragile at a site than any other material and needs to be kept as far away from moisture as possible.

According to Oryang, the moment water comes into contact with cement, it triggers and offsets a chemical reaction that takes place within the cement bag even if it has not yet been opened. The cement tends to solidify or form particles within the bag. Unfortunately, some people think they can crush the particles to become powdery again, and mix it with the cement that did not solidify.

The crushed cement particles, Oryang, cautions, cannot be converted back to the initial cement powder and if you insist on using it, the end product regardless of where it is used, will be much weaker. Most of the local cement starts to set or form particles or solidify within 30 minutes after contact with water. That is why when you mix cement with water and sand and gravel, it should be used within 30 minutes, otherwise, it will harden.

“If you wait and it hardens and you add water thinking you will make it softer, the water breaks the bonds that were already formed and the mortar or concrete will never be as strong as it would have been if it had been cast immediately it was mixed. Because of the nature of the cement, you have to make sure that it does not come into contact with water at the site. If you realise the place where you meant to keep the cement is cold, it is advisable to at least place pieces of wood on the floor before you place the cement. Otherwise, it will solidify,” Oryang advises.

Iron bars and other metals

Many of the metallic materials such as iron bars and wire mesh are often kept outdoors, not only due to their weight but also due to the amount of space they occupy. If it rains on the iron bars but they are not submerged, they normally dry out when hit by the sun.

According to Samuel Katumba, a builder, the challenge is when rainwater accumulates and covers or submerges the iron bars and other metals. When steel gets in contact with water, it rusts and when you use such bars, it means your structure will be weak because the rusty layer of the bars was rendered useless by water.

“The best protective measure is to keep steel bars and any other metallic material on a raised surface to remove them from direct contact and allow rainwater to run off easily to avoid rusting,” Katumba advises.

Roofing stage

Atuhaire and Oryang agree that when it comes to roofing, you need to have planned adequately and in advance,  especially financially. It is advisable to do all the roofing nonstop to minimise any regrets which often come with unexpected costs. Have all the roofing materials such as tiles or iron sheets before you even think of raising the timber. Even though some types of wood such as pine do not easily bend due to changes in weather, still, there is a little bit of warping that you may not notice.

“It is a big mistake to put up a wooden roof structure and leave it hanging for long and exposed to the elements such as rain and sunshine. When the rain wets the timber, it expands or bends and may even rot when left exposed to rain for a long time,” Atuhaire says.

Many use eucalyptus wood for roofing. This type of wood, according to Oryang, is very sensitive to the elements. For instance, if the sun hits it from the top, it bends inwards and if it exposed to the sun from the side, it will bend sideways. If you have a framework of eucalyptus wood and leave it exposed to harsh weather for approximately two months, you may need to replace it because it is already damaged and will give the roof a deformed structure should you insist on using it.

Protecting the brick work

When you build up to a certain level, for instance up to window or ring beam levels and stop work for a while, the brickwork, just like roofing timber,  is exposed to different weather elements, including rain. Freshly laid brickwork should not be easily damaged by rain. If it is,then it is a clear sign that the bricks you used were of poor quality especially if they were made from ordinary soil, and not clay. Even then, it should be covered.

If resources do not permit you to complete the roofing stage, Sserwadda recommends pausing at the ring beam level at least, and casting the ring beam with concrete because the concrete is weather resistant.

“If the worst comes to the worst and you do not reach the ring beam, then find ways of covering all the brickwork every time it rains so that it is not damaged. Covering for long periods is tricky because the materials you use may be blown away by wind,” Sserwadda advises.

The disadvantage with leaving brickwork exposed to rain is that you will have fungal growth and grass all over the wall. 

Endeavour to finish the structure within the shortest time possible to avoid extensive damage.