Particle board and plywood are composed of pieces of wood products that have been pressed and glued together. Although they are popular siding materials for storage units, they are not most people's first choice in regions where it rains a lot. For if water does work its way into these products (a possibility enhanced by their composite nature), then you're lost.
Buying an outdoor storage building with a loft greatly increases storage capability, allowing you to store rarely used supplies overhead. For the exterior, build your own lean-to and attach it to the outdoor storage building to form a sheltered area for firewood.