dark oak wood and plank catching on fire randomly

two houses, made of dark oak wood and dark oak plank, randomly started burning during the day. We first noticed my friends house caught on fire, and the house started burning down. None of the birch wood plank ,caught on fire. Most of the dark oak burnt until we put it out. After house was put out, we started up the recent file save to reverse the effects of the fire. We watched the house to see if it would start burning again, and to see what the cause was. Surely enough, one dark wood caught on fire again, and the fire spread. There were no possible causes for this fire, because there was no fireplace in the house.

Secondly my house, also made of dark oak and dark oak plank caught on fire, shortly after. No possible causes found. This is obviously devastating as these houses were not finished, please fix.