Techniques for driving long screws through thick exterior foam, hitting narrow rafters?
severaltypesofnerd
| Posted in Green Building Techniques on
What techniques are you using to for accurately driving long screws through thick exterior foam, hitting narrow 2×10 type rafters? At some point the ratio becomes difficult, especially if the interior is already finished and you can’t see shiner screws and adjust!
In heavy snow load areas the screws really have to not just hit the rafter, but center in the rafter…. and the deeper the foam the greater the cumulative error is for even a small angle offset.
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my rafters were a bit easier being rough sawn and full 2" wide. I did not have much problem. You will know when you miss the rafter.
I fastened 2x4 on the flat over 6" eps.
the key to my success was I made an accurate drawing showing all the centers of rafters before installing the eps and referred to it to snap lines on the eps once down. verify you hit top and bottom of rafter with long bit first before snapping lines.
I think I pre-drilled pilot holes in the 2x4 to be sure they were square to the board.
there is no mistaking a missed rafter.
in some spots I had only 5/8 plywood and simply installed more screws.
Then the ants found it and nested in it 2.5 stories up. Apply ant kill at your foundation EVERY year.
Up over 2" thick foam starts to get a little more tricky. What I have done in the past is to make a simple jig from a scrap piece of 2x4, or other handy framing lumber. Cut a narrow (like a single kerf or two wide) slot in the narrow edge of one side of the 2x4 about halfway through the piece. Make sure the bottom of the slot is perpindicular to the face of the 2x4. I've posted some sketches of this on GBA before, so you might be able to find a drawing of what I'm describing if you search the archives.
The way to use the jig is to push a screw into the slot, up against the "bottom" of that cut, which is now the "back" of the slot. Hold the 2x4 piece flat against the rigid foam. Carefully start the screw while keep it pressed against the back of that slot, and the 2x4 pressed against the face of the rigid foam. The 2x4 and that slot will act as a guide to ensure you drive the screw perfectly straight into the foam. Now you just need to make sure you're aligned over one of the studs -- the jig will help make sure you hit the stud and don't go in at an angle by mistake and miss it.
I've usually found it relatively easy to locate the studs behind the foam, usually by marking the foam with a sharpie marker as I put it up, so that there are lines on the edges of the foam at the center of each stud. The tricky part is keeping the screw straight as you go into/through the foam, since a small amount of angle ends up being a lot of error by the time you get all the way through the foam, making it easy to miss the stud completely. The simple jig I've desribed helps to keep the screw straight and eliminate that "missing the stud because the screw went in at an angle" problem.
Bill
Any big difference between this method on walls vs roofs?
I think the simpler way is to make the first layer (could also be the middle layer) of rigid insulation 1.5" thick and embed 2x4s on flat into it over the rafters. This gives you a much wider target to hit, lets you use shorter screws and with the right sized fasteners, there is no chance of a shiner on the inside.
Don't forget to picture frame the rigid around all edges for critter protection. If they are anything like mine, they find even the smallest gap and destroy the insulation.
Good technique with the flat 2x4 targets. Now if only there were a way to keep ants from getting into the insulated cavity....
You can also skip any rigid and over-roof with new rafters ie #15 here:
https://www.greenbuildingadvisor.com/question/risk-of-rot-in-warm-roof-outlooks
But more labor but cheaper materials and no footlong fasteners to deal with. This also has the benefit of built in vent gap which is important in snow country. You can insulated with cheap batts or even better, borate treated cellulose which most pest avoid.
The vent gap is good for some snow country, but not all. My particular area has enough above freezing days that the general local consensus is that "cold roof" style vent gaps under the top decking do as much harm as good. We'd need a temperature sensitive vent I guess to cool the roof all night, but close up after daytime temperatures are above freezing.
You can wrap the edges of the foam with screen or foil tape, but it's a lot of extra work and you still have no guarantee ants won't eventually find a way in. If you're working in an attic, then you could use fiberglass (easier, cheaper, not as durable) or aluminum (more expensive, harder to work with, more insect proof) screen. If you're going to be outdoors, stainless steel screen is a much better choice in terms of longevity. A wrap of wide foil tape would also give an insect barrier. The problem is that it seems ants are extremely determined, and will eventually find some tiny little hole or scratch and then they're in.
Bill
Has anyone tried using a drill guide? I think I got this idea from an older Q&A on this forum. This Rockler version can handle up to 9-5/16” screws:
https://www.rockler.com/rockler-portable-drill-guide
matthew25,
That suggestion may have come from one of my comments. It gives you a good 90 degree hole, especially if you add a larger plywood base. What none of the jigs do is give you a good way to knowing exactly where the underlying rafters are, which is often the problem that causes misses.
I've only done it myself once, through 1 1/2" of furring and 3" of foam, into rafters that we learned the hard way were not straight but the bowed significantly and inconsistently in the middle of the span. It was a painful experience and added to my reasons for avoiding exterior foam insulation. But with precisely installed framing it should be much easier, especially if using a portable drill press jig like Matthew just shared.
Michael,
The consequences of missing are mainly annoyance in most situations, but I can't imagine using long screws on a project where the framing was exposed on the interior like the Perfect Wall House. That's my worst nightmare.
https://architizer.com/projects/perfect-wall-house/
True, though on my project we had covered the entire roof in ice and water shield as the primary air barrier and to keep rain out until we could get the Zip sheathing on and taped, so I wasn't happy about all of the extra holes, or the time required for all of the misses. But you're right--having exposed framing would have made it even worse.
I assume that's why it's painted white, and not left bare wood :).
Kyle,
Okay, you win. Bare finished framing would be a way worst nightmare.
I wonder how much an X ray tech with a portable machine would charge per hour? ha.
Might be a dumb question, but...
I'm trying to work out a reasonable assembly for an existing shop building, with a gambrel roof whose rafters are all over the place (ie, not evenly spaced, bowed, etc). Would like to do all my insulation on top, possibly also with an overroof around the sides to get additional overhang.
I am also sitting on a small fortune in short 2x4 pieces. My thought was to install blocking, on the flat, between the rafters on the inside, pushed tight to the existing sheathing, at regular intervals, say every 16" or 24". That would give me horizontal lines across the roof, where I've only got to hit a 3 1/2" target instead of a 1 1/2" target. And if I go with the overroof, I can lay out the overroof rafters however I want, rather than needing to stick with the arbitrary existing spacing. I'd use some kind of metal brackets, fastened to the blocking, on top of the roof to help secure the overroof rafters.
Pretty similar to Akos's suggestion in #3 above, I guess, except that my 2x4s on the flat would be blocking on the inside, rather than continuous on top of sheathing. More work doing it my way, but I've got this huge pile of cutoffs that I'd love to use. This bump anyone as being a bad idea? It seems like it ought to work okay?