Colette A. O'Brien writes

I am planning to build a brick base on which to bolt the greenhouse. I
am more worried about water just from watering the plants,

I don't have that problem since my plants are all on trays. But unless
you need to water a lot in winter, any water spills should evaporate
pretty soon.

cleaning the greenhouse,

Depends on when you do that. Since my greenhouse is used for
overwintering a lot of plants, any cleaning has to happen in summer.

getting in from the roof etc.

That shouldn't happen, and you should do your utmost to avoid leaks,
since leaking roofs tend to mean drips on to plants and hence rot.


--
Kay