Question from Alexi:
Here in Southern California, some deck contractors specify that they will use diato metal or deck diato for finishing the bottom of the stucco repair. It is a combination plaster stop with L flashing to be used against a deck surface. The plaster stop portion looks a lot like FHA 7 weep screed, but there are no holes.
If you drilled holes, you'd just be letting water in behind the sideways 'V' shape, but it seems like an odd choice to use a plaster stop for this exterior stucco waterproofing application.
(The deck contractor I hired ended up using FHA 7 plus L metal and stapling the two together, then sealing the top of the two metal edges with sealant.) Can you see any application where this 'deck diato' would be appropriate?
My Answer:
Hi Alexi, thanks for your detailed question! The use of "deck diato" metal, as described, does seem like an unconventional choice for a deck-related stucco waterproofing application.
Based on the product's design (a plaster stop without drainage holes) and a small horizontal flange for the deck, I can't see it being a good fit for a waterproof deck system, especially in areas where water management is critical. Here’s why:
Lack of Drainage: Unlike FHA 7 weep screed, which has holes to allow moisture to escape, deck diato metal doesn’t include this critical feature. Drilling holes in it, as you mentioned, would likely compromise the design by allowing water to get behind the flashing and cause damage. Proper weep screeds are essential for exterior stucco applications because they provide both ventilation and a drainage path for any trapped moisture.
Proper Waterproofing for Decks: In typical deck systems, the sequence involves installing a waterproof metal flashing that covers the deck surface and turns up the wall (L shaped flashing). A weep screed is then installed over this flashing, counterflashing the piece below it. Layering like this ensures that water sheds properly and doesn’t infiltrate the stucco or decking system.
Lightweight Waterproof Decks: For lightweight waterproof decks, having a robust and correctly installed waterproofing system is critical. I have installed many such decks where this scenario applies, and the typical approach involves using a 4x6-inch metal flashing first, which covers the deck surface and runs up the wall.
The weep screed is then installed over this flashing, providing proper counterflashing and allowing for effective drainage and waterproofing.
The Contractor’s Solution: The method your contractor used—stapling FHA 7 weep screed to L metal and sealing the top edges, follows a more logical and widely used approach. This method creates a better water-shedding system and is closer to "standard practices" for waterproofing decks with stucco walls.
I struggle to find a scenario where deck diato metal would be appropriate for a deck-related stucco repair, especially if waterproofing is a concern. It might work in certain decorative or interior applications where drainage isn’t necessary, but for exterior deck systems, I’d always recommend sticking to tried-and-true flashing methods with weep screed and proper counterflashing techniques.
Thanks for bringing up such an interesting topic, Alexi! Let me know if you have any follow-up questions.
