Maximizing overhang using 2×4 furring installed over monopoly framed house
mason_
| Posted in General Questions on
Zone 5b, 50 psf ground snowload. Attached is a simple drawing.
I have 2×4 furring over 5″ rockwool on a “monopoly” framed house, I want to use the 2×4 furring to add the overhang, but how much overhang can I reasonably leave? It will get 1×4 furring at 18″oc in a grid on top of the 2x4s.
I left them long so I can snap a line and cut them back, but I’m having trouble finding any info on adding overhangs this way.
Any thoughts / ideas?
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I can't give you a reference for this but I don't expect you'll get more than a 12" overhang. I would hesitate at that, because I can't find any code references addressing the issue. Matt Risinger's "monopoly" house braced the overhanging boards with a horizontal piece below, triangulating it back to the wall. If you could do that you'd certainly be able to go further. But you also need to make sure those 2x4 flats are well anchored through the insulation into the rafters below. It's probably not enough just to screw them into the plywood roof deck.
Thanks for the response;
The 2x4 flats are all directly above a rafter and are secured with Simpson strong drive SDWS screws per their Exterior Foam-to-Wood Fastening guide
Triangulating is a good foolproof idea
mason,
Like JGSG I can't give you anything more than a guess, although mine would be a bit more generous - say 16" if the 2"x4"s were at 16"oc too.
One thing you might consider is solid blocking at the exterior wall so the cantilevered ends didn't have the tendency to compress the rockwool there.
Ya I would be very worried about compression at the edge and not only bowing but also a wavy eve as the rockwool does not necessarily compress evenly. Even on 1 1/2” comfortboard 80 it compressed 1/4” when fastening rainscreen on a wall for me.
The IRC allows eave overhangs up to 24" and rake overhangs up to 12" without engineering. That's more about uplift than downward load.
I have used a similar cross-framed roof several times, but never without solid bearing at the eave. The main risk I see with your design is the potential for crushing the mineral wool. Here's some rough math on why it might be ok:
If we assume that the 50 psf snow load on the eaves transfers back 4' up the roof--the classic 1/3 : 2/3 ratio--then where the load is the highest, at the edge of the mineral wool, is 125 lbs/ft. Without doing more intensive math, my gut says that the load probably travels up the rafter another 3', not 4'. At 3', the load at the edge of the roof is 133 lbs/ft. Let's say the load is probably around 130 lbs/ft.
It looks like your 2x4s are 16" o.c.. If that's the case, the 130 plf load becomes 173 lbs at each 2x4. Divide that by 3.5" to get 50 lbs/in. Let's say the load is spread just 1" up the roof; that leaves 50 lbs/sq.in. If you're using Rockwool Comfortboard 80, as most builders do in this application, that's rated for 80 psi so you have some leeway with your 50 psi load.
However, mineral wool creeps over time under sustained load. If my memory is working--never a sure bet--I believe that like foam they derate the load to 30% of the stated compressive strength when under a long-term load. Snow isn't a long-term load but let's pretend it is, to be safe. That means the 80 psi insulation is actually only good for 24 psi, about half of what we need.
But I assumed the load would be spread only 1" up the roof. In reality it's probably several inches. Let's say it's spread 4" up the mineral wool. 4" x 24 psi = 96 psi, about twice what you need.
Long story short, I think you'll be fine, but I would include an engineered wood subfascia to help keep the eaves straight. And I'd be more confident with solid support under the overhang.
"The IRC allows eave overhangs up to 24" and rake overhangs up to 12" without engineering."
I believe that is for 2x4s that are on edge, not on the flat as in mason's image.
I've seen overhangs on edge and on flat but that's a good point, it makes more sense that the code would interpret it on edge
The only IRC reference for wood-framed roofs is R802.7.1.1: https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR802.7.1.1. That explicitly relates to notching rafters, where you have to leave at least 3 1/2". There is nothing I have found in the code saying you can't use 2x4s on the flat, but your AHJ or structural engineer may have their own views on the matter.
I would also note that I think the main reason for the 2’ overhang limit is wind uplift, as a 2x10 rafter can almost certainly cantilever 3’ ( obviously depending on snow.
What I would recommend is having your littlest person (or the one matching your eve load) hang on the end and maybe bounce a little to see if you get movement, I am still skeptical that the rockwool won’t crush at the edge, having worked with it a bit.
I like the idea of an engineered wood subfascia to keep it straight. I was going to plane 2x10s and plane them into very straight 2x6's but they may still move, I'll see if I can get an engineered solution around here.
The data sheet for Comfortboard 80 provides compressive strength information, which may help address concerns about edge crushing under snow load.
Compressive Strength:
10% compression = 439 psf
25% compression = 1065 psf
The roof area is 950 sqft, and the county design requirement is a ground snow load of 50 psf. This translates to:
Total snow load = 50 psf * 950 sqft = 47,500 lbs
There are 39 vertical furring strips, each 16'4" long, positioned directly over the rafters to bear the weight. The bearing area is calculated as follows:
Bearing area = 16'4" * 3.5" * 39 pcs = 26,754 square inches
To convert square inches to square feet:
Bearing area in sqft = 26,754 square inches / 144 square inches/sqft ≈ 185.5 sqft
This means that 47,500 lbs of snow distributed over 185.5 sqft results in an additional pressure of:
Pressure = Total load / Bearing area = 47,500 lbs / 185.5 sqft ≈ 256.7 psf
If the Comfortboard is installed at 10% compression (439 psf, or 0.5" compression of 5" rockwool) and then subjected to an additional load of 256.7 psf, the total load on the 2x4s becomes:
Total load = 439 psf + 256.7 psf ≈ 695.7 psf
Using linear interpolation, this corresponds to approximately ~16.2% compression (0.81" compression).
This means that if the roof experiences a rare snow load of 50 psf, the 2x4s would compress the rockwool by an additional 0.31" compared to fully unloaded.
Is my math, mathing?
mason,
I don't think it's the uniform loading people have expressed concern about, it's what amounts to point loads on the edge of the Comfortboard where the overhangs begin to cantilever where you may see a lot more compression. Dealing with that is as easy as surrounding the insulation with a 2"x perimeter at the exterior wall.
No, because the snow load doesn't add to the initial pressure created by the fasteners, it replaces it.
With no snow load, the compressed Comfortboard is acting like a spring, pushing back against the 2x4s with a pressure of 439 psf, and the 2x4s in turn are pushing back against the fasteners.
With snow on the roof, the weight of the snow presses down on the 2x4s from the top side, balancing and canceling out part of the force from the Comfortboard below, thus reducing the force on the fasteners.
I did something like this on 2 parts of my house. I pulled off the old roof with zero overhangs, replaced purlins with 1/2" ply sheathing, leaving a ~10" overhang on the gable ends and ~12 at the eaves. I then made a lattice with both layers made of 2x4s, 24" OC both ways. The sheathing was screwed into the lattice from underneath the overhangs to make the sheathing part of the over hang structure. I then installed standing seam roofing over the lattice. 12:12 pitch (snow slides off).
On an small, 100 sf addition I built, I put 5" of foam over 3/4" ply roof sheathing, then more 3/4" sheathing, 1x4 lattice, and finally another layer of 3/4. The 3/4", lattice sandwich supports supports 12" overhangs. 3:12 pitch.
I had some problems with the 2x4 on flat warping at the overhangs while building. with some rain and drying they will want to warp upwards. the bottom layer of sheathing helped minimize the movement, I think. I ended up replacing the worst 2x4s before putting the roof on.
This is in a 60 psf snow load area. Nothing bad has happened yet.
Thanks for the first hand information!