"I remain in constant prayer for those affected and for the rescuers who have been so brave and determined to do so much to help," the star said.
She added that she was working with her charity and her pastor "to implement a plan to help".
Houston has been hit by unprecedented levels of rain, with more set to come.
Officials estimate that more than 30,000 residents need shelter. At least nine have died in the Houston area.
Other stars including Kim Kardashian West and Kevin Hart have also vowed to help.