Squirrels can gain access to a structure in various ways; the most common being broken attic windows however loose shingles and roof tiles with water damage could allow them just enough space to enter undetected. Since squirrels prefer entry points that are at the tree line, those areas should be checked each season and patched or repaired accordingly. These areas include gutters, downspouts, drains and hall windows that are left open for ventilation.
As a last resort, in winter months squirrels will try a basement entry. Uneven doors, vent styled windows and loose dryer exhaust hose ports are just a few ways that they can gain entry on the ground or basement level. Screened doors and screened windows with loose corners are a common entry point for ground squirrels. They are smaller in size than the red squirrel allowing them to squeeze in to spaces that other animals would find impossible.
Dog doors are an excellent way for indoor pets to access the yard when their human companions are not home but it makes for an easy access point for all types of squirrels. Even with raised dog doors that have been reinforced with a heavy rubber flap, squirrels will be able to enter and exit the door with little effort. Leaving doors propped open and unattended for any amount of time is an open invitation to a squirrel.
Cords of firewood stored near a house or in a vestibule are excellent hiding places for adult and baby squirrels. This is a common nesting area and wood should be checked prior to brining it into the home for both live and dead squirrels. Chimney caps and grates should be checked before opening the flue or starting the first fire of the season. A qualified chimney sweep will be able to detect squirrel activity and suggest appropriate measures.