screws for light siding on thin continuous exterior foam
sribe
| Posted in Green Building Techniques on
I’m looking at adding 2″ of exterior foam when I replace siding. I cannot go to thicker foam because of limited roof overhang. I will be using steel siding, 18″ wide vertical, 1.25lb/sqft. The tables I’ve found for screw sizing start at 4″ thick foam and heavier siding. At least I know that my plan seems feasible, but I’d rather not use grossly over-sized screws. Honestly, I’m wondering if normal 3″ deck screws wouldn’t work here? (I guess I’d have to pay attention to materials, coated vs galvanized, to make sure I’m not setting up a long-term reaction.)
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Replies
Scott,
Are you planning to install the steel cladding directly onto the foam or onto furring strips?
What type of fasteners does the steel cladding use? Are they hidden, or exposed gasketted screws?
No room for furring, so screwed directly through the foam (hydrogap between so there's some drying) into studs & horizontal nailers behind. (Odd construction, no structural sheathing, studs with 1x4s let in + horizontal blocking to support vertical siding...)
Lower half will be corrugated sheets with exposed gasketted screws. Upper half will be board and batten (that's the 18" wide stuff) with hidden screws.
Scott,
I agree you probably don't need anything like a structural screw to keep the siding in place, but as Tristan posted, you will need something with a pan or flat head, for the hidden fasteners, (not a deck screw), and you don't have any choice beyond brands for the gasketted ones.
I’ve done something like this for a cabin and used GRK cabinet-style screws. I also added narrow rips of 1/4” (nominal) plywood for furring strips as I went along placing the snap seam/standing seam/concealed fastener metal panels. This thin furring is thick enough for a reasonable rain screen gap in some climates and helps with foam compression issues, too.
Switching to the exposed-fastener corrugated panels will be tough without furring strips as nailers for the (short) gasketed screws. It may be you’ll need to re-evaluate “no room for furring” or using the corrugated panels. Or drop the 2” foam down to 1 1/2” and add furring…or some other compromise to balance the various considerations/priorities.
You can get stele bearing plates for the rigid insulation. These are just larger square steel plates used to support the clips for standing seam roofing. Search for "metal roof bearing plates".
You can get snap lock with clips that attach to these, screw the nail strip or the gasketed screws for exposed fastener panels with metal self tappers into the bearing plates.
Strapping out the wall with 3/8 plywood is still the simplest though.
One thing to keep in mind is that you are not simply supporting the weight of the siding, but the resistance to wind pressures and uplift. The image of metal roofing panels flying down the street should be playing in your mind now. Don't underestimate this.
Andy,
This summer I had a chance to do some GBA style backyard testing of wind pressure on siding while building a couple of shops. They didn't have sheathing, just house-wrap on horizontal strapping behind the metal cladding. Because the metal trim goes on before the siding, there is a period when the house-wrap is exposed to any winds, and initially I was worried it would tear loose, as it threatened to when we just had the first walls done. But once we had all four covered, as soon as we shut the doors the problem disappeared, as there was no way for the wind to exert pressure from behind.
The panels over the roof overhangs, which are exposed from below, had a really aggressive fastener schedule to keep them from blowing away
Thanks for all the replies, here's a few clarifications:
@tristanherriot Sorry for the confusion in my posts, I initially tried to boil down my question from a complicated install, then when prompted for more info mentioned the corrugated when I really shouldn't have. The profile of the corrugated is not as deep as the board & batten; there is room for furring AND we need furring to make things line up esthetically. So we are already planning furring for the corrugated, and the question was really about the board & batten.
@Andy_S Good point, as we are in a location with very high winds. @Malcom_Taylor Yes, I've already seen this effect with the lower garage level and foam board installation. But of course I don't want an install where the siding could blow off if someone opens the garage door on a windy day, LOL