Building a treehouse which will have a flat roof which will double as a deck, so I'm looking for a way to seal it but have it be aesthetically pleasing (looking at flat roof sealers it seems like they tend to be pretty industrial in appearance.
Does anyone have recommendations on how to approach waterproofing this (either by sealer or choice of materials or some other approach other than slanting it)?
Btw, there are no concerns about snow load due to climate.
This is an answer for solid waterproofing (and not necessarily for a DIY project).
If you are looking for a truly waterproof and long lasting roof/ deck surface I think it needs to be a paintable urethane product. These are made specifically for waterproofing decks and often used under tile when it is applied on a deck that is I haveover living space.
There are several manufacturers to choose from (Pacific Polymers is one that comes to mind which I have used before-I have no affiliation). This is usually a 2 or 3 coat process- the first coat being a very thick base coat which is painted on (it has no UV protection). The second (finish) coat (thinner viscosity and has UV protection)is rolled on after first coat is dry (24 hrs). While this coat is still wet, graded and screened sand is broadcast into the wet surface. The excess is swept or blown off after this coat has dried. This is enough if putting tile over it. The sand aides adhesion for the tile thinset or mortar. For a finish deck with no tile another coat is applied over the "sand coat". This provides a very nice sealed non slip surface.
Properly done this will last for a very long time- probably longer than the wood itself. It is pretty bullet proof but, this also involves proper metal flashings at all edges and using mesh tape with urethane caulking at all joints in your plywood (or whatever your base frame material is). It is the way to go but also a whole lot of work and cost- in both materials and time. It is also quite messy to work with and having a pre-set system for it's application is critical.
Butyl rubber sheeting.
Finish (knot free) plywood with marine varnish. sprinkle sand on the final coat to reduce slipping.
"chequer plate" - but probably too industrial.
Various paint-on fibre-reinforced sealing products.
Duckboards over an industrial roofing system.
I've just seen the size figures. fibreglass over plywood might be the way to go.the wood grain will show though so "not industrial", and the glass fibre fabric will seal the joins between sheets of plywood. On the down side you're dealing with smelly polyester resin (which smell probably causes cancer in Californians), but at-least you're doing it outside.
You want 1:100 slope or steeper to effectively shed rain water.
