A method for joining shipping containers together to create a modular structure includes providing a first shipping container and a second shipping container; removing a wall of the side of the first shipper container to create a first opening; removing a wall of the side of the second shipping container to create a second opening; positioning the first opening adjacent and opposite to the second opening; aligning the first opening with the second opening; after the aligning, securing the first shipping container to the second shipping container to create the modular structure; and securing an inner flashing around an inner periphery of the first opening.