20x34 2-story universal in upstate NY

Started by NathanS, May 13, 2016, 11:04:09 AM

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Thanks Don. I am not sure if I was clear on what my concern is. I know that the opening itself is fine - I already installed two jack and two king, which I took directly from either the code book or WFCM.

My concern was what happens when those loads are transferred down to the first floor. Should I put studs directly below? Attaching pic to hopefully clarify. I haven't found anything in the code or WFCM that directly addresses this.

Maybe that means I am worried about nothing (again  ;D)


I was the one being unclear. You do need to follow the load path down to ground. The floor should be blocked solid under the posts. There should be a stud pack under that, directly under the posts above. Since we've just created a stiff column, assume it will take the entire... lets call it 5,000 lbs from each post. I'm just throwing numbers at it, check your conditions.
Notice the column retains about 95% of buckling strength at those dimensions and height. The plates need to be capable of taking ~200 psi, they are good for ~325 psi so all is good. If you drop to 3" wide, 2 ply, it makes it, but barely on several counts.

I think the tables is WSDD will give you official backup for inspectors if needed. I haven't delved deep, it starts on pg 210 of my old copy.


Thanks for the help Don. It looks like 3 ply is good to about 7600lb and 4 ply is good to about 10100lb in this scenario.

Is WSDD (Wood structural design data?) now called NDS (National Design Specification for Wood construction)?

In hindsight the WFCM is a much better framing resource than the code books. I have not used NDS or WSDD at all though. I don't see WSDD or NDS referenced in Chapter 3 of the code.

Separate topic - We started installing our chimney pipe today. Only got the through floor box installed. The instructions call for #8 x 1/2" self tapping stainless steel screws to attach sections of stove pipe together. Could not find self tapping anywhere, so thought I'd just predrill the holes. I'm thinking the screws need to be hand fastened? My drills were stripping the threads off the screws (!). Also I really don't like the phillips head. I've virtually never stripped a square head, but feel like I'm always fighting with the phillips.


Go to chapter 44, referenced Standards. Under AFPA, the NDS (National Design Specification for Wood Construction) is listed. It is sort of the "parent" document that many others expand upon. There is also a list of code sections where a referenced standard is mentioned. It also lists the WFCM, Wood Frame Construction Manual.

I'm surprised, WSDD isn't referenced. I believe it would be respected. All of those documents were prepared by AFPA. You can find a copy of WSDD on the right column here;
That looks like it is the same as my paper copy. On pg 204 read the section on built up columns and the reduction to use.
Notice how both the WFCM and the WSDD reference the NDS... that is where the engineering "rules" for wood construction are laid out. Broadly, if something references that it is based on the NDS an inspector will respect that.

I'll go to school on you, my client ordered the chimney last week.


I have been going to the WFCM for everything I do since I was trying to understand the proper way to do roof overhangs. Also then went through WFCM to double check the walls and floor are correct. It is a much more concise manual then the code book.

Sometimes I think if I were to start learning from scratch again, where would I begin? Not sure I know... something may have been lost when things went from master builder that did the whole house, to a million sub trade specialists that know their thing and not much else. Seems a little worrysome to to turn a plumber loose on the framing. Or even not having an idea of where drains are going while you're framing. Then again maybe the carpenters do know that.

This can all be very hard to navigate. The balcony is my magnum opus.  ;D Knowing it will need all sorts of special attention. You have no idea how glad I am that we chose a simple build.

Reading the stove pipe instructions, that first floor box is to be held in place with screws or 8-d nails. They say rated to hold 60ft of stove pipe. I went with spiral shank 8d knowing it would be carrying some weight. All the instructions say - to be installed by stove pipe specialist - so you probably shouldn't list to me :D


Drew up my basic plan for the shed roof and looking for any input from anyone.

Discussed much earlier in the thread, I do not want to do a ledger board, and would rather run the rafters into the second floor wall of the house. Would plan to cut little blocks/studs at the 4 - 12 pitch, with a top plate resting on those. The rafters would then rest on that.

I excluded the blocking at the top and bottom between the rafters in the 3d images to keep the pictures as clear as possible.

Sort of thinking of this as a floor, just at a slight angle. Only other thing I'm thinking right now is that I could double up the top plate that the rafters rest on. Also should probably add the little 4 - 12 studs at each end of all the wall stud bays to help secure the rafter top plate.


You would have more wall penetration and less insulation value.  If you choose this option I would add blocks to keep the rafters from wanting to roll. 


Thanks. Yeah definitely will be blocked at top and bottom of rafters. I don't like the idea of the the ledger board because snow from the gable will land on this roof occasionally. Screws holding everything in place makes me nervous. This is 50lb snow load and we are up on a mountain - potential for more I think. I could probably put 4" of foam board on the exterior of this area to make up for thermal loss.


Looks well supported.  Run that blocking about an inch proud of the top of the rafter and flush with the outside wall, then you'll have something to nail your sheathing to.

Using Don_P's toolbox, a 2x10 rafter in SPF #2 grade can carry just over 1,000 lbs.  At 16" on center that works out to about 105 psf.  That would be more than enough for a gable roof, but as you said this roof could take some snow load from the main roof.  Having Doug fir #2 gives you another 500 lbs load carrying capability.  Not knowing what the actual snow load might be, were I in your shoes I'd try to get up to 1500 lbs per rafter, or add more rafters.  Especially considering that a sliding drift of snow will have some impact when it hits the shed roof.
My cabin build thread: Alaskan remote 16x28 1.5 story


Smart thinking on the blocking.

I already installed my joists @ 16" centers.. a little late to step down to 12" OC. Once a nail is in an LVL.. it ain't coming out. I did take Don's advice and predrill my nails this time for the joist hangers. What a difference. I wish I knew that before all the pain and suffering. Other side note, been awhile since I did simple framing like that, and amazed how much faster I am now. Knowing exactly what to do before you start.. must be 2-3 times faster. Also comfortable tight rope walking on top of a 2x6 plate now which I wasn't before.

I also already have 2x10 #1 SPF on site for the rafters. That 100 lb snow load I am good for, I figured double the code would be more than enough. Hard to think of stepping up to 2x12's right now. That much snow and I'll have to rake it off anyhow because I have a window that will be something like 2-3 feet above the top of the shed roof.

Which calculator did you use to come up with those numbers?

Other unrelated note.. the standing seam roof is much slower going than a typical exposed fastener roof. Tons of extra steps. The ridge is a real chore, involving many many steps. Not to mention that pop rivets are a lot more fussy than screws, and you really need to check every rivet to make sure it properly engages the custom fitted metal Z trim that gets mastic on top and underneath and then fastened to the flat portion of the standing seam panel. Oh yeah, and then you also caulk the vertical seams of every piece of Z trim. On my gable roof that is 116 caulkings. Freakin roof.

Also straddling a ridge for an hour or so my knees feel how a 90 year old must feel.


Nathan, I used this calculator for the beam:  http://www.forestryforum.com/members/donp/beamclc06b.htm

Just plugged in the rafter dimensions and kept upping the load until it showed failure.  Now that I think about it I was keeping dead load at 20% of live load.  Should have kept it constant.  Assumed 10 s.f. for the tributary area for a rafter, so 100 lbs dead load.

Total Load on Beam(pounds)    1700
Dead Load on Beam (pounds)   100
Span of Beam (inches)   90.5
Width of Beam   1.5
Depth of Beam   9.25
Select Species and Grade #1 SPF B+S


170 psf seems like it might be enough. :)
My cabin build thread: Alaskan remote 16x28 1.5 story


That is a heavy timber calc for timbers 5x5 and larger. It uses different, lower, base design values for many species.
Here's one way
go to the simple beam calc at timbertoolbox.com (my bandwidth  ;)). You can input design values directly. Do all your adjustments for snow, repetitive member... plus impact.

Figure trib area and snow load for the roof above. Dump that entire load times 4 (WHUMP!) for impact load on the snow landing zone of the rafter. Where it hits is a big part of bending moment.

Go to the beam design equations,awc's DA6, find the simple beam equations and use the equation to find max bending moment at any point along the beam. Apply the impact load to that spot and find max moment in the beam. You can continue longhand from there or use equivalent load by going back to my calc and play with load until you get to that max moment in the output. Now check your dimensions at those adjusted design values and moments.



When I enter info and click show result I'm not getting any results. Tried in Chrome and Firefox.

I'll think about this more tonight, but the point load is going to be something like 1.33*50*14*4 = 3733lb which I'm guessing is impossible to pass with anything but steel or unobtanium?

Across the 14ft shed roof that is a 40,000 lb load which is like designing a roof for an old growth tree to fall on it.


Yup, I'm getting a bug in that one too, click the simple beam calc to its left, you'll need to hand input the design values based on adjusting for impact, I think a 60% increase... wood can take a hit. You're impacting close to one end, the moment won't be as bad as I think you're thinking.


Good morning gentlefolk,

Don, thanks for pointing out the difference in the two different toolbox calculators.  I take it the formula are the same but the engineering properties in the dropdown are different based on joist vs beam dimensions?

I ask because I get the same results from the two beam calculators

Using the same numbers I mentioned in my post from yesterday, and using the engineering properties from the dropdown species selection for SPF #1, the uniformly loaded beam calc has a max load of 1701 lbs.

Just to be sure, I also plugged those in to the manual entry beam calc at FF Toolbox at http://www.forestryforum.com/members/donp/beamclcNDS2.htm and get the same results.

To use these tables for figuring joist or rafter loads, I should probably use a manual entry one and plug in the manufacturer numbers for the grade and dimension of the lumber, right?  If that is the case, I've looked up some specs.

Source  Grade  Fb  E    Fv 
Canadian Wood Council    SPF #1 or #2 2x10    960    1.4    135 
NELMA    SPF #1 2xAny    875    1.2    135 
NELMA    SPF #2 2xAny    775    1.1    135 
WWPA    SPF #1 2xAny    875    1.2    135 

Using 875, 1.2 and 135 for Fb, E, and Fv I get a max load for the 2x10 #1 SPF with 90.5" span of 1650 lbs.  Slightly less than the 1701 lbs but not by much.

However those values for Fb in rows 2, 3, and 4 are for any 2x dimensional lumber.  The design value adjustment for a 2x10 is to multiply the base Fb by 1.1, which yields a 2x10 Fb of 962.5.  That looks to be the source for the CWC value in row 1 of 960.  Assuming I am doing this right, I would then plug that value in to the beam calc and get a max load of 1819 lbs.

I'm working my way through this stuff, so tell me if I'm on the right track please.  There is a load adjustment factor for the duration of the load in this document.  For an impact the factor is 2.0, so in theory the rafters can withstand a force of 3600 lbs that lands in a WHUMP from the roof above.  The seven-day factor is only 1.25 though, so unless the 3600 lbs (per rafter) slides all the way off it could result in damage to the structure.

My cabin build thread: Alaskan remote 16x28 1.5 story


Replying to myself here, actually reading a little more slowly and carefully.  The WWPA adjustment page is very helpful.  So for a uniformly loaded set of rafters one would start with the base engineering values, which for most SPF is 875 psi for allowable fiberstress in bending (Fb), modulus of elasticity (E) of 1.1 million psi, and allowable horizontal shear (Fv) of 135 psi.  With a nominal depth of 10" the adjustment factor of 1.1 is applied to Fb, resulting in a value of 962.5 psi.  Because the rafters are repetitive, applying the adjustment of 1.15 bumps the Fb up to 1106.9 psi.  The duration of load adjustment is then applied.  For snow accumulation over the winter, a factor of 1.15 is applied.  This results in a Fb of 1272 psi.  Following this method, each rafter can support up to 2400 lbs for two months.

For an impact, the adjustment factor of 2.0 is applied to the size-adjusted values, resulting in an Fb of 1925 psi and Fv of 270 psi.  I don't think the repetitive factor is applied.  The maximum uniform impact load for each rafter is then 3630 lbs.

My cabin build thread: Alaskan remote 16x28 1.5 story


Just got back from cutting lots of metal so haven't had time to process everything.

But I should mail you both some fine upstate NY beer for all the help.

Will do these calcs but if 2x10 @ 16" doesn't pass it actually may be cheaper for me to double each rafter, or every other, instead of stepping up to 2x12.

Also used metal scraps to mock up my chimney flashing.. looks like rain tomorrow but hopefully doing that on Friday. Really glad I physically did a mock up because my original thought process was wrong.


Chugiak I doubled checked numbers straight out of the NDS book. Identical to the numbers you used.


pg 35
SPF (Southern)
Bending Stress = 875 = Fb
Modulus of Elasticity = 1.2 = E
Horizontal Shear = 135 = Fv

pg 30 has the adjustment factors you used from WWPA.

Repetitive Member = 1.15
10" nominal thickness = 1.1
Load durations; 2 month (snow) = 1.15, impact = 2.0

Everything same as you so far.

So for my max allowable impact calculation those factors get applied to -

Fb = 875 * 1.15 * 1.1 * 2.0 = 2213.75
Fv = 135 * 2.0 = 270 (not clear that the horizontal shear is supposed to get the impact adjustment?)

With the 2 ft overhang on the gable, the snow would fall around uh.. 32" from the edge?

So using Simple beam with a concentrated load @ 32", it looks like it is good for 2289lb in point load.

Gable roof is 14ft long. with 50lb snow load. So we do have the 14*50*1.33 = 931lbs Don mentioned quadrupling (!) this number to 3724 lbs.

2x12 is good to 3386lb

Double 2x10 passes the bending test to over 4500lb, fails deflection at that amount but don't think that matters.

I am a little skeptical that we are treating sliding snow (even if some ice) the same as a solid object.



I am a little skeptical that we are treating sliding snow (even if some ice) the same as a solid object.

You have to keep in mind that you are going from a 7/12 roof to a in your words "Sort of thinking of this as a floor" so the snow will slow or even stop on the lower pitch.  If in fact the addition was a continuation of the main roof the majority would probably slide off.  A lot depends on several factors.  Type of snow, heat from the sun, inside temperature penetration to the roof, outside temperature and roof pitch.  My roof is 10/12 transitioning to  3/12 and when it does slide don't be in front of it.  It had slid out into the yard some 15-20 feet one year. 


I've never checked this but am seeing a need to learn more about it.

The math is the same for both calcs, those dropdown lists in the calcs make things easy in one regard and limited in another. Steel, another elastic material, uses the same equations, the section properties and the design values, two sets of variables, change quite a bit.

This calc will do the point load where the snow lands coming off the overhang above;
The equations in that graphic are what the calc is using, that is from DA6, the people who write the NDS. That is all those calcs are, most of them are just DA6 written in javascript so I wouldn't make dumb longhand mistakes.
Anyway, that will give the point load moment.
I'm not liking the output at all, it seems over the top... and I'll blame my 4x fudge factor for impact. I'm going from a comment, that needs nailing down. Using 3724 lbs and dropping the snow 2' out and coming up with a max moment of ~5473 ft-lbs

We also have a uniform load moment from the snow that is on the lower roof to begin with.
Using this calc
With a static, uniform, load of 466lbs I'm getting a 440 ft lb moment
When you look at the moment diagram for this beam it is a nice uniform curve. Max moment on a uniformly loaded beam is in the center. The area of concern is at about the quarter point. Brutally basic, the moment at the location of impact is about half the max moment... call it 250 ft-lbs, it is a curve.

Add the moments at that location 5473'# + 250'#= 5823 ft-lbs at the point 2' out on the rafter.

You can go back to the variable location point load calc and start upping the load until the max moment reads about 5823 ft lbs to get a solution to that combined loading scenario. I'm close calling the point load 4000 lbs. ... but I'd like something better for the impact load adjustment factor.


Thanks Don. That helps tie things together for me. I wasn't quite sure how to back into the combined static load and point of impact load. Makes sense to me now.

What I mean when I said I'm skeptical of treating snow the same as a solid object is that I can drop 15 lbs of snow on my head from 10 feet up and just be cold, but if I do the same with a bowling ball.. well see ya later.

I think this is getting into force of impact physics. It's like a car buckling in a crash makes it safer, similar things are going on with the snow I think. When the front of the pile is stopped by the roof, and so on, it's not transferring the weight all at once.

It looks like 2x12 @ 16" would pass bending if the point moment was 4000lbs


I'm enjoying digging into the nitty gritty details of the beam calculations, but I don't want to derail the discussion from Nathan's issues.  One of the unknowns at this point is what the design snow load will be for each of the roof sections.  It may be worth the effort to delve into the ASCE publication on wind and snow loads, because it applies several factors to the ground snow load to arrive at a roof design load.  I'm working on a calc sheet for my cabin that goes like this:

pf = 0.7 * Ce * Ct * Cs * I * pg 


  • pf = roof design load, psf
  • Ce = exposure factor
  • Ct = thermal factor
  • Cs = roof slope factor
  • I = importance factor
  • pg = ground snow load

The ASCE is very proud of their publication and would love to charge you $165 for the 2010 version, Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) .  I managed to find a version of the 2002 online and if you google around you may be able to get it, in whole or part.

My final roof load is 31.75 psf, reduced from a ground snow load of 70 psf.  The wind exposure, thermal regime, and roof pitch and material all contributed to a reduction of my snow load.  My roof is just a simple gable and your main roof will probably have a similar reduction.  The shed roof is a whole other matter.  It wouldn't surprise me if it ends up with an overall factor greater than 1.  I think before you go much further it is worth your time to get a more accurate estimate of your two roof design loads.

Edit: Here's an article on changes in the 2005 version that adress unbalanced snow load and sliding snow.
My cabin build thread: Alaskan remote 16x28 1.5 story


Quote from: ChugiakTinkerer on October 13, 2016, 11:14:57 AM
The ASCE is very proud of their publication and would love to charge you $165 for the 2010 version


It is strange to me that some code - or more, the sources of the code, are behind a pay wall. Had a similar experience recently trying to look at some diagrams for the British Columbia code.

I do appreciate all the input. I was at the house earlier, and added up how much 2x10 stock I have left, I think I have enough to double all 8 rafters. That would give me a max moment @ 24" of 7891 ft lbs.

Wound up with a lot of extra 2x10 because when I did my estimate for header material I assumed every window needed a double 2x10, which was actually only necessary for the 5' windows. Going out and buying 8 2x12's would cost me probably another $160'ish.


Making progress on the roof. Hopefully done soon. I am having the windows delivered on Tuesday.

Flashing the chimney was a not fun. All of our chimney parts are "Ventis" (made in Scranton PA) and seem like really high quality. Everything stainless steel. The flashing seems a lot nicer than what I've seen in stores. The top side is triangular and slides underneath the panels. Making sure those cuts were accurate was very time consuming and difficult to do. I chose to do everything as in place as possible because it's less chance to make a mistake with a tape measure. Anyway, the flashing is basically installed shingle style. I cut the off the roof seam on the bottom edge of the flashing and caulked the inside, and made folds with the metal to keep the rain out. All edges of flashing boot sealed with mastic/butyl tape. 1" wide, good stuff we've been using elsewhere on the roof. Also used polyurethane caulk as a backup in a few areas.

Additionally, hidden on the inside of the flashing I made all my metal cuts 1" small, and bent them upward. If any water somehow gets in there it will have a hard time getting down the chimney opening.

Anyway, that made for a long couple days. Chimney itself goes together very easily. At first I thought no screws were included, but I found them in the bottom of the boxes. The included self tapping screws worked like a charm. The chimney extends about 2.5' above the ridge. We had to install a stabilizer since we have about 7' of pipe above the roof. That was not a bad job.

No good detail pics up on the roof because it's just too much work. Hard to describe how to do some of this stuff by writing. Doing all the standing seam panels more than prepared me for the job though.

As long as we don't get rained out, tomorrow we have to do the main stack vent and radon vent through the roof. I really don't like poking holes in this roof.. it should be a lifetime roof, and the boots especially will not last that long. I think it is what it is though. Was thinking about wrapping the pipes in copper but frankly I don't have time for that right now. I want to get this roof done.


Quote from: NathanS on October 15, 2016, 04:56:51 PM

Gorgeous!  This is a great place to take photos from.  Were I in your shoes I'd probably be swooning every time I saw the place from this perspective.
My cabin build thread: Alaskan remote 16x28 1.5 story