I have just installed fire-rated gypsum board on the ceiling of an unheated garage. I used regular zinc plated drywall screws. In hindsight, I have realized that such screws are not suitable for use in an unheated space, and that I should have used special outdoor drywall screws instead.
The question is, what will be the consequences of this mistake? Presumably, the screws will rust, causing the drywall to eventually fall down. The question is, how long will this take? Are we talking about 25 years, 50 years, or 100 years?
If it is so humid that galvanized screws rust, then a lot of other things have probably molded at the same time, so if your ceiling falls down, it won't be because the screws have rusted away.
It should mainly be a matter of surface rust, but it depends on how humid it is. That is, how many hours per year with RH>60%, and probably an "acceleration factor" for every 10% over 60%.
So, in terms of time, it can only be a rough estimate of how long it lasts: >25 years - <100 years. Screws are usually always weaker than corresponding nails and therefore do not resist rust as long.
So, if you want to secure the roof for a long time, it's just a matter of buying screws for outdoor gypsum and screwing them between the existing screws. Or, as suggested, lowering the humidity (<60% RH).
It's annoying if you want to paint since the rust must be patinated to not go through the paint. (Proper alkyd oil paint, i.e., not water-based, works well for patinating, and probably shellac too.)
I should also add that even if the screw heads are patinated, the screws will rust over time as they will continue to rust at the junction between gypsum board and plank/sparse/framework. The remedy is a warmer and drier garage. That is, RH <60% even above the gypsum boards.
I will go ahead and put outdoor screws between the existing screws.
Keeping the garage heated or dehumidified is not an option with today's electricity prices.
Regarding painting, I think I'll apply a layer of Hagmans Rust Protection Primer on the screw heads, and then paint with alkyd oil paint. That should work, right?
I find it strange actually that zinc-plated screws don't do well in unheated spaces. We have zinc-plated screws and nails that have been in open boxes in an unheated agricultural building for sure 40 years, and they are still shiny and fine (unlike regular bright wire nails, which have rusted somewhat). But maybe it's worse if the screw is driven into something? In my experience, when you pull out an old bright wire nail that has been halfway driven into wood, it's less rusty where it has been in the wood. In the attic above the garage, there are bright wire nails that are probably about 60 years old, and only have a thin layer of surface rust. There are also cut nails that should be even older, and they too only have surface rust. Shouldn't zinc-plated screws rust less than regular bright wire nails?
Shouldn't zinc-plated screws rust less than regular wire nails?
That might be expected, but maybe the steel quality is different for nails versus screws. That could explain why they rust differently. Electroplating is for indoor use and comes in different thicknesses. The thicker the layer, the more resistant to rust. These are qualities (quality) that can vary from one manufacturer to another. My feeling is that most materials in the past were thicker and more robust. Among other things, because of poorer control in manufacturing processes, greater margins were needed.
I recently installed fire-rated gypsum board in the ceiling of an unheated garage. I used regular zinc-plated drywall screws. After the fact, I have realized that such screws are not suitable for use in an unheated space and that I should have used special outdoor drywall screws instead.
The question is, what will be the consequences of this mistake? Presumably, the screws will rust, causing the drywall to eventually fall down. The question is, how long will this take? Are we talking 25 years, 50 years, or 100 years?
Regards,
Kaj
There will be no consequences at all. The screws will more than outlast the garage.
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