Repair of Timber Roofs Affected by Flooding
The biggest challenge the construction and repair industry in India facing today is the repair of buildings damaged due to various natural or man-made disasters, with floods being the most frequent calamity. Countless buildings have suffered due to floods, and some have been destroyed completely beyond repair. Low-lying areas have been completely wiped out, and homes have been disrupted. The main purpose of a home is to provide shelter (a roof over our heads). They form an important structural part of a building, and unfortunately, they can be the worst affected when it comes to heavy rains and flooding. Additionally, old buildings with pitched roofs that are made of wood rafters can become weak, and crumble after being wet from the floods. This blog post talks about the damage of timber roofs by floodwater, heavy rains and timber roofing repairs.
List of Contents
1. Horizontal Cracks in Walls with Sloping Roof
If the timber roof is damaged due to floodwater, it exerts an outward thrust. The top portion of the wall may bulge towards the outside, thus forming a crack. This crack will appear on both sides of the wall but will be more visible on the inner surface. The roof tiles will absorb more water because of immersion in floodwater, thereby increasing the weight of the roof.
When there is an outward movement of the roof above, horizontal cracks begin to appear on the interior surface of the walls below it, near the level of the eaves. Timber roofs also exert an outward thrust when there is an over-loading of the original structural members, because of damage caused by insect or fungal decay.
2. Repair of Pitched Roofs
The pitched roof needs to be repaired or strengthened by adding ties. If the wall is out of plumb (leaning in a direction), the top portion may have to be rebuilt after giving temporary supports to the roof. Depending on the width of the cracks, suitable remedial measures as given in “Repair of Cracks in Flood Affected Buildings” are to be taken.
3. Cross-Bracing of Sheds
Several temporary roofs and sheds are vulnerable to floodwater. If a building becomes weak at its base due to flooding, cross-bracing will help to keep the structure stable. The process of Cross-bracing (the process of reinforcing structures with two diagonal supports that intersect in the shape of an X) may be used to increase the stability and the wind resistance of the structural frame of flood affected buildings.
4. Repair of Timber Rafters And Beams
In case of decayed timber members, the damaged portion should be cut out and replaced with a new piece of timber of the same cross-sectional area. . It is also a good idea to attempt to match the grains as much as possible. One can make use of water-proof glue when joining them together. Timber or steel dowels may also be inserted for increasing the strength of the damaged roof.
5. Weakened Timber Roofs
Timber roofs may be weakened by the collapse of the walls or by poor connections between the members. In cases, where the roof truss has become weak, it is possible to strengthen it by cross-bracing or adding more ties using bolted joints. The timber members used for bracing or ties may be a minimum size of 100X25 mm (4 inches x 1 inch).
6. Repair of Rotten Ends of Timber Beams
The connection between the timber beam and the wall plate may get damaged due to the movement of the roof during the floods. Such movement can hasten a decayed beam from its position. The different methods of roof repairs are illustrated below:
6.1 Repair of Rotten Ends of Timber Beams Using A New Timber Member
In this case, the decayed portion of the timber is removed and replaced with new timber, that is butted (joined) to the old. On either side of the junction, a steel plate is inserted in the centre slot and pushed back at least three times its own depth. A sequence of staggered bolts with recessed heads hold the plate to the timber surface.
6.2 Repair of Rotten Ends of Timber Beams Using Steel Plate
In many cases, we might not be able to repair the damaged timber roofs with new timber segments alone. Some original members which are in sound condition get damaged during the process as mentioned above. In such scenarios, another option of roofing repair would be to remove the degraded portion and fix a steel plate in its place. Also, the use of steel will avoid dismantling, reduce the cost of the repair, and help to maintain the integrity of the building. When the work is completed, the steel used to repair the timber beams must be placed so that it is concealed. The steel plates must be cleaned and applied with an anti-corrosive coating.
7. Weakening of Timber Beams
Following are some ways of repairing timber beams that may have been weakened by flooding:
7.1 Scenario 1
In places with small damage/cracks, steel plates can be fixed on either side and can be bolted horizontally to the timber beam. The side plates must not be fixed against uneven surfaces as the voids behind them form excellent breeding grounds for insects.
7.2 Scenario 2
In this method, the metal plates can be bolted vertically to the bottom and top of the weakened timber beam. The steel plates act as the top and bottom flanges of an I section, and the timber beam acts as the web. The bolting of the plate to the beam must be a rigid joint spread over a wide area, to reduce the splitting of the timber. The bolt holes must be spaced out to avoid the weakening of the beam. To allow for seasonal movement, the bolting of the plate to a joint must be flexible. It can be made through angled holes and should not be filled with glue or grout.
Inevitably, the first thought after a disaster like the Chennai flood or Kerala flood2018 would be to demolish the buildings, rather than repair them. The durability of a building depends on the quality of workmanship and the design details. Many well-designed buildings have survived the floods, while other poorly constructed buildings have been badly damaged. This is the trend that has been going on for all these years.
The success of a government or an individual depends on how they learn from the mistakes of the past, and prepare for the future. Consequently, if we can take a step back; and restore these flood affected buildings, we can save a lot of resources and time. But, people have to understand that, and awareness needs to be spread on good quality work so that people also develop an interest in the repair of building affected by flooding, rather than rebuilding them completely.
Check out the other two parts of this series of blogs on repairing flood affected buildings: