Patio Roofing Question – Transparent corrugated pvc covering

A

If you choose B you will dimple the steel, ruining the look while creating a penetration point for water because expansion and contraction of the metal due to heating and cooling will create an open access point for water to enter, and a big opening behind it to receive it.

A, however, will give you a tight seal against the wood, wood-steel-screw in a nice tight sandwich - resisting expansion/contraction gaps and thus protecting much more effectively against water penetration while preserving the look of the corrugated steel

edit: After reading your updated question, you will be fine installing the smaller drain. I have a large driveway drain that I installed a few months ago, and it works great to drain the huge puddle that used to form. I used a drain box similar to the 9x9 that you have, and 4" pipes. Occasionally, during a very heavy rain, it will get behind and not completely drain until the rain stops, but that is because the pipe itself is only 4"!

Great question! I am not an expert, but I would highly recommend the larger drain. The reason is, on a small drain, every time the drain gets stepped on (or driven over with a lawn mower), 100% of that weight gets put on your pipe underneath. A 4" pipe isn't designed to carry that weight, and could crack.

Even though the 9x9 drain won't necessarily drain any faster, it will have more surface area under it to support being stepped on, that way the soil under the drain, instead of the pipe supports the weight.