While searching for Sita in Lanka, Sri Hanuma saw many arches/windows/doors made of gold, some houses decorated with gold, floors studded with many precious stones.
When Sri Hanuma started setting the Lanka to fire, he moved from roof-top of one house to another with his burning tail. Sage Valmiki describes this episode in detail. He also described that after catching fire, those houses burnt down completely and quickly.
न अग्निः तृप्यति काष्ठानाम् तृणानाम् च यथा तथा | हनूमान् राक्षस इन्द्राणाम् वधे किंचिन् न तृप्यति || ५-५४-२८ न हनूमद्विशस्तानाम् राक्षसानाम् वसुन्धरा |
As the fire does not get satisfied with any amount of firewood and straw fed to it, Hanuma was not wearied in killing any number of demons. The earth was not then wearied in receiving the number of demons killed by Hanuma.
युगान्तकालानलतुल्यवेगः | समारुतोऽग्निर्ववृधे दिवस्पृक् | विधूमरश्मिर्भवनेषु सक्तो | रक्षः शरीराज्यसमर्पितार्चिः || ५-५४-३२
That fire diffused on those buildings, together with the wind picked up a speed equal to that of a fire at the time of dissolution of the world. It grew taller, touching the sky with a smokeless splendour. Those flames of fire shot up, as inflamed by ghee.
Till the invention of cement, concrete of Lime was mostly used for construction of houses. Now a question arises, if the same methodology was used even in Lanka also, how can a concrete structure catch fire so quickly?
After thinking for so many days, the following explanation flashed in mind.
The architects of Ravana's period might have used wood in most of the areas in construction of houses, as we see in Western societies now. The houses will not catch fire immediately, if they were made of lime concrete.
And, some type of varnish might have been used to protect the wood to withstand from changing seasons.
In my opinion, the wood, coated with some type of varnish, in the houses caught fire quickly, and spread rapidly.
Am I correct?