There are many ways water can enter your home. Most homes today have a wooden frame with pieces of sheet-rock forming the main barrier between the inside and outside. Sheetrock is quite permeable and can easily degrade, and even begin falling apart when wet.
Thankfully, the outside is typically lined with some form of weatherproof siding at the very least. During a flood, water can easily enter your home both above and below ground, with multiple routes for the water to take.
Above ground, water can enter through doorways and windows, even closed ones! The pressure of even just a few inches of water can be enormous, and can easily force water through the smallest cracks or keyholes!
If you have ventilation openings or pipes and cables that pass through walls (such as telephone and cable lines) water can easily degrade and break through the sealant, and then begin entering your home. Even if your home has brickwork or other forms of masonry instead of sheetrock, if the mortar has become weathered and permeable then water can rush through the spaces between stones.
Below ground, water can move unnoticeably from the surface. If the property surrounding your home becomes saturated, you may not notice until water begins rising up through your floorboards or through the walls of your basement. It may also inundate your pipes and overflow your toilets, sinks, baths, even dishwashers and washing machines!
Water is a force to be reckoned with, and even the most advanced flood prevention measures are not guaranteed. It is always important to maintain structural integrity, and having firsthand knowledge of each portion of your home will help you to assess occurring damage. Be sure to have a plan in case of a flood, and always be safe!