We having a garden office built. Poured concrete slab, DPM then MDF base. We are also having a patio built around office. The patio tiles come up higher than the concrete base under the office. The patio slopes away from the office to a soak away. Do we also need an ACO drain or soak away BETWEEN the patio and the office to prevent standing water going under the office and causing the base to rot? Any advice very welcome.