Flames can spread vertically several stories or horizontally to adjoining occupancies inside concealed spaces. The largest concealed space is the cockloft. This roof space, above a top floor ceiling and below the roof deck, is large and can sometimes extend over several buildings. A fire in a cockloft or roof space extending over a row of three or four houses or stories can destroy the entire row of structures. Fire spreads inside concealed spaces of a type III building by convection.
Convection is the transfer of heat by fluid motion, such as by a liquid or gas.
Heated fire gases and flame in a concealed space can travel upwards several floors and break out in a cockloft. When firefighters search for hidden fire, and must open walls and ceilings to find it and extinguish it, they should remember this fact and try to cut off the rising fire. For example, if you discover fire in a floor, open up the wall above it. If you discover fire in a wall, open up the ceiling. If you discover fire in a ceiling, open up the baseboards on the floor above it.