Garage roof and podium drainage

One layer doing several jobs

On a podium deck the layers above the slab have to drain, insulate, and separate. A double-sided board does those from a single layer, because its supports hold the slab and the structural layers apart. That gap is the drainage path, and it is also the insulation.

Using one layer for several jobs is what makes the build-up fit the levels. Each separate layer costs height, and height above a buried slab is usually the constraint that decides the design.

Draining a surface nobody sees

The deck is covered, so nothing about its drainage is visible once the job is finished. That changes what matters. A surface drain that blocks can be cleared. A buried drainage layer that closes cannot, so it has to be protected when it is installed rather than maintained later.

Two things protect it. A separation layer above the board keeps fines out of the void. Panels overlapped down the slope keep the path continuous, so seepage reaches the outlet rather than collecting in a low spot between two panels.

What to settle before ordering

Work out the build-up height first, because that decides how much room the drainage layer has. Then decide what the surface above it will carry, since that sets the load the board has to hold. Finally decide where the water leaves and how the panels overlap to get it there. Those three answers are the specification.