Post the details of what your architect wants and what your contractor wants, please.

Thank you. Our architect prefers deck mounted (she thinks it looks better and can be done right), our contractor says we're asking for trouble with water intrusion and wants to do fascia mounted. We are considering using Ipe tiles over the membrane, so I don't know if that will dictate which direction we need to go.

We're doing a cable railing.

Are you speaking of surface mounting the railing posts? Not the way to go in my opinion. They need to go thru the deck surface and be mounted to the framing for strength. Especially with a cable rail system, when they tighten up the cables you will pull the posts right over if they are surface mounted.

You may want to check with local codes. Some codes only want the posts to be blocked and bolted inside the framing and not fascia mounted.

I checked in with a couple of waterproofing specialist subcontractors before finalizing a typical detail for our practice. Our default proposal for guardrail balustrade attachment is fascia mounted. This is also consistent with my recollection of best practices taught in architecture school. While there are certainly special conditions that can modify this advice, the logic is pretty straightforward in most waterproof deck circumstances.

1. A fascia mounted guardrail typically has a larger distance between the bolts that resist overturning. For this reason, the forces on these bolts typically undergo much smaller forces than bolts associated with a floor-mounted installation.

2. Because waterproof decks often have a "drip" at their edge condition; the mounting plate can be substantially sheltered over a long period of time from weathering.

When we compare this logic to the floor or "deck"-mounted installation, two things become apparent.

1. The bolts on a floor-mounted installation typically need to be close to the vertical post to avoid being a tripping hazard, which increases the dynamic forces on these attachment points.

2. The upward orientation of a guardrail post could not be worse from an exposure standpoint. The installations are frequently accompanied by standing water where the "wiggling" forces imposed by their use invite water leakage.

Both waterproofing contractors I spoke with sounded like this type of poor solution was ruefully a good source of their repair business.

Certainly, if posts support a roof at the perimeter of a waterproof deck, surface-mounted guardrails can make sense. The roof's protection aids the waterproof deck, and the posts for the roof can effectively become part of the guardrail assembly and help with its stability.

It is worth noting that many homeowners, and even commercial clients, get preliminary pricing for a simple deck and only come by the guardrail cost when required by building codes or provide a builder with more detailed documentation. In my experience,

For these reasons, it is a work scope that is often "shoe-horned" into a project budget. It is also the case that, like "options" when buying a car, the options are what cost you. Decking companies and guardrail companies often charge a premium for the guardrail amenity and will provide the deck-mounted option as an "affordable" choice. The fact the guardrail can be mounted to the surface of the area under scrutiny (and not some peripheral fascia board that might not even be in the current scope of work) is often the reason why people jump at this option. It is the most convenient, which is probably the biggest reason I see people do it.

I hope this helps.