It is obvious that following a fire, there is some degree of damage. But what can the extent of the damage be? Apart from a fully burned down building, looks can often be deceiving. Not only can there be damage to the structure, which will depend on the temperature that the fire reached, there can also be secondary damage caused by ash and soot.

If you have experienced a fire in your property, whether it’s your home or a commercial building, it is imperative that you do not reenter the building until you have been advised that it is safe to do so by professionals. Depending on the heat that was reached during the fire, the structure of the building can be compromised, including the walls, flooring and fixtures and fittings.

Structural Damage and Damage to the Building Materials Caused by Heat

The average temperature of a house fire burns at 600oC and many building materials are affected at temperatures in this range. The temperature of the fire can cause the affected parts of the building to significantly expand – more than the amount for which they were designed to do. This kind of movement and expansion is likely to result in cracking or structural damage.

Certain materials are more resistant to fire than others, with concrete being particularly resistant. However, its strength can still be compromised once subjected to certain temperatures.

Once it reaches about 300oC, concrete starts to lose its compressive strength. This means that in a typical fire, there will be at least some loss of strength to the concrete elements. Although a much rarer occurrence, it is worth noting that all strength is effectively lost at around 1,000oC – almost double the temperature that an average house fire burns at.

No matter which building materials are used, there could be permanent loss of strength to the structure that was directly affected by the fire, or in areas not directly affected – for example: as a result of thermal expansion. All building materials – except timber – are likely to show a significant loss of strength when subjected to temperatures above 250oC. This strength loss can be permanent, it may not recover after cooling. Generally speaking, all wood that has not charred should be considered to have full strength.

A good indication of whether concrete or mortar containing natural sands or aggregates has reached temperatures in excess of 250oC, is to look out for signs of discolouration. When they contain iron oxide, these materials, along with some types of natural stone, start to show signs of red or pink tones which is often irreparable.

While concrete is particularly resistant to fire, the resistance of solid bricks is directly related to their thickness. Perforated bricks and hollow clay bricks are more sensitive to thermal shock. There can be cracking of the connecting webs and the wythes can have a tendency to separate.

Natural stone is perhaps the most vulnerable to fire and can be seriously affected once certain temperatures are reached. At temperatures above 600oC, the strength of most natural stone is significantly compromised and if thermal shock occurs, the stone can begin to disintegrate.

Health Risks of Ash and Soot

Soot and ash are the tiny residual particles which are carried throughout a building by smoke. While ash is heavier and therefore most often found around the source of a fire, soot is extremely lightweight and can travel far from the original source. It can cover floors, walls, ceilings and it can even get into the HVAC equipment and ducts.

Soot includes fine black particles, mostly composed of carbon, produced by the incomplete combustion of coal, oil, wood and other fuel sources. It can also contain many other substances such as acid, chemicals, dust, plastic, grease and heavy metals which will deposit around the building with the soot. These particles can react with or stain the surfaces within your property, often causing irreversible damage, especially when not dealt with quickly enough.

The nature of the soot and ash will be determined by the materials burned, and also by the heat levels of the fire. For example, wood will produce a black-grey powdery ash, whereas grease will produce a sticky residue that’s incredibly difficult to remove.

Ash and soot are acidic and need to be removed quickly before more damage is done. The only effective way to do this is with a professional clean. Normal household vacuums are inefficient, they merely redeposit the particles around the building, often contaminating previously unaffected areas and rooms.

If left untreated, the acidic soot and ash will begin to deteriorate materials it is in contact with, meaning that the overall complexity and cost of repairing the damage increases. Soot and ash are the primary causes of secondary damage following a fire.

Within hours of a fire, wooden furniture can be damaged to the point of needing refinishing, paint and fiberglass begin to yellow, metal starts to corrode, pit and rust, and clothing can start to stain which often results in permanent damage to the materials.