Mold is one of the problems that can ruin a house, permanently. First, here are some exceptions to the “run away” rule for mold. If the mold is in a finite area or in an unfinished area that can be cleaned, there is hope for the house. So, figuring out the worst-case pricetag can help you make an informed decision about whether to buy a house with mold or not.
1. Attic insulation is a common place for this. The air we exhale is damp. In the winter that damp air rises into the attic. If the attic is not ventilated, mold can form where the damp exhalation condenses on the insulation or roof decking. In mild cases, it can be cleaned. In major cases, the roof decking has to be replaced. (This is big ticket, but could be worth it.)
2. If a window is leaking and the sill has gotten moldy, it is finite and repairable (as long as the water is not leaking into the wall.)
3. If water is leaking under a sink, the wood beneath can be replaced and the mold cleaned up.
4. If a toilet is leaking around the bottom, the wood on the subfloor can get damaged and moldy. This can create a worst-case situation where the whole floor has to be rebuilt from the subfloor up. It can grow to “run away” status, but does not most of the time.
5. Exterior wood can get moldy. Outside, the mold isn’t going to harm you. Rotted wood becomes a big-ticket item if there is a lot of it, if it is on a structural piece of wood, if the wood is neglected enough to cause a leak inside the wall, or if wood-boring bugs have moved in.
Houses with significant interior water damage are very hard to repair back to habitable. Wood holds water for a long time. So does plaster and wallboard. All of these soggy surfaces are perfect places for mold and other microscopic life to move in. The only cure is to remove anything that has mold on it. Sometimes, that’s the whole house.
This was the fate of many houses in flood zones. Hurricane winds don’t knock them down. The flood water got them soggy enough to make them mold incubation centers. If a basement isn’t pumped out promptly, the house is done for. If roofing was blown off and a lot of rain got into the walls and floors, the wet stuff must be removed, fast.
The same thing will happen in a less dramatic fashion with long-neglected houses.
1. Finished basements can slowly become mold centers. If someone puts paneling or wallboard in a basement, there should be heat, too. Otherwise, the walls can start growing mold or mildew. Modern basements have the wallboard installed a little bit above the floor to help keep it from absorbing the dampness. Basements that get seepage and are not ventilated can get moldy; but unfinished surfaces can be cleaned much more easily.
2. If rain gets in frequently enough to soak the plaster and get into the wall and flooring, mold moves in. This can be from a roof leak, or windows that are open all summer.