Up to this point, we have not dealt with fire and its effects in the rules. For fun, however, I quickly and without very much effort into them, made a few "the roof on fire" markers using thin black card - from product packaging - and dyed wool probably designed for burning tank wrecks, and creasing the card in half, these can be placed on the roofs and look OK I think.
When showing these to the boys, immediately the discussion moved to how the rules could be changed/ added to employ these new markers. Again I kinda put the cart before the horse. However none of the proposed ideas grabbed me and so I will not complicate the rules with the addition of fire to the combat. Might still place these on the buildings for fun!