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Cordwood Home, help with rafters and top plate sizing

Started by BraschBrothers, March 27, 2017, 04:16:32 PM

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BraschBrothers

Hello all, I have been planning on building a log cabin for years and after much debate with my wife, we have changed our plans to a cordwood house.

Plans as of now, we intend on a slipform like foundation, 24 inches below ground (frost depth) and 18 inches above ground. We are building the walls 24" thick and tying the logs together with rebar tie wire and cementing in between the logs with premix. As of now, we are planning a 30'x30' building, peaked on the non load bearing walls with a top plate and rafters butting into the top plate on either side. (excessive explanation in case its not called a top plate, at least you should still be able to understand. On top of the load bearing walls i intend on having a wall plate sitting on top of the cordwood and having the rafters resting on top of the wall plate. Does anyone see anything wrong or questionable with what im planning or have anything that they would recommend?

Things i need help with are rafter and top plate sizing. I have read quite a few posts on it and did my best to follow along but im just not getting it. The trees that are local seem to be douglas fir, western white pine, hemlock & yellow pine (I did not buy the land yet, i am flying out to see it tomorrow). The ground snow load is 104 psf, wind load 90mph, live load 55 psf and frost depth 24inches. (I am looking in north idaho). Would anyone be willing to walk me through Dons beam and stringer tool, assuming thats the right tool for rafters and top plates? Thank you for your time and willingness to help.

God Bless, Clint
John 3:16

BraschBrothers

One thing that i did grasp from Jim Rogers was that a typical timber from is designed from the top down, getting larger as it goes down. As this is a cordwood, I dont know that that will apply as the only real beams are going to be the top plate and rafters which will distribute their loads through the cordwood. My intention was to have the top plate resting on the side walls and the rafters connecting to the top plate on the top and the wall plate on the bottom. Would that make the side walls load bearing?
John 3:16

BraschBrothers

The span of the top plate will be 36ft (may be supported in the middle, depending on the resulting size of the beam) and the rafters will be 18ft from top plate to wall plate.
John 3:16

BraschBrothers

I could have made all of these one post but i am going over https://forestryforum.com/board/index.php/topic,50714.0.html and trying to answer any questions that anyone may have before they are asked.

The roof total square area will be 30'x21' or 630sf per side with a 8/12 roof slope. My current plans are to have the rafters, covered with plywood, covered with homedepot metal roof.
John 3:16

BraschBrothers

The intended spacing of the rafters is 3 foot, making the load per rafter 3ft wide x 18 ft long = 54sf x (55 live load plus 540(10lb per sq ft) dead) which equals 32,130 lbs held up by each rafter???? Something doesnt sound right.
John 3:16

BraschBrothers

Ok so assuming that my math is correct or at least my understanding of Jim Rogers math is correct, the only direction i can think to go is adding purlins between each of the rafters to minimize the load. Does it work that way? Do purlins, lets say 4 of them along the 18' span lessen the load on the rafter or is the load lessened only by the trusses (king post, etc)?
John 3:16

Don P

Terms first, what you are calling a top plate is the ridge beam.
It can be framed a couple of ways. If there are rafter ties close to the lower end of the rafters, preferably on the wall top plates, what I call a top plate. The ties would run across the room connecting the opposing rafters together to form a rigid triangle. In that scenario the ridge can be a board or even non existent as in a truss roof.

If there are no ties connecting and restraining the rafter feet from spreading, then the rafters need to be hanging from a rigid ridge beam. If the ridge doesn't sag the rafters won't spread and shove your walls outward.

So, which way are you going there, rafter ties or wide open cathedral ceiling?

Ljohnsaw

Quote from: BraschBrothers on March 27, 2017, 04:50:07 PM
The intended spacing of the rafters is 3 foot, making the load per rafter 3ft wide x 18 ft long = 54sf x (55 live load plus 540(10lb per sq ft) dead) which equals 32,130 lbs held up by each rafter???? Something doesn't sound right.
Correct, something is not right.  You stated the snow load was 104lb/sq-ft.  BUT, you have to take into account the actual area of the roof as seen from above, not the length of the rafters/metal.  THEN, you need to look at the slope of the roof and the roofing material.  For metal, you get a bit of a discount on the snow load as it is slippery and will tend to shed the snow (if steep enough). And finally, whether the roof is considered warm or cold.  A warm roof will obviously shed the snow faster then a cold roof, unless an ice dam forms on the eaves.  Soooo, not to easy to answer.  ASSUMING that you still have the 104 lb/sq-ft plus the 55 lb/sq-ft live load (you also need to add in the weight of the wood and metal...), you are looking at 159/sq-ft or 8,586 lb / rafter.  Still pretty hefty!  For your rafters loading at 9,000 lbs, I used the tool box and it came up with a massive 5x20 or 6x18.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Don P

55psf live load(snow) + 10psf dead (you might want to bump that to 15psf for timber and 2x timber decking)

15 feet of horizontal rafter span x 3' spacing=45 sf x 70 psf. carry on :)

BraschBrothers

Thank you guys, Don and ljohnsaw. I realize my mistake, after thinking about it and going over your answers  :P i came up with 18 ft length x 3 ft spacing = 54 sq ft. 56 live load, 15 dead load, 70 psf load x 54 sf = 3780 lb load per rafter. Don, I have been planning on having a ridge beam only with a cathedrial ceiling but after thinking about your answer/question i think i am going to do ridge beam with rafter ties, not all the way down to the wall top plates but if it helps strengthen the roof, why not?

One thing that i still question is if i put purlins between the rafters, will that lower the load per rafter and how do you figure that into the equation? ???
John 3:16

BraschBrothers

I am flying out to look at the land tomorrow morning  8) so if anyone has any advise, i would still love to have it but wont be able to reply for a few days. Thanks again for the willingness to help, God Bless, Clint  :)
John 3:16

Ljohnsaw

Quote from: BraschBrothers on March 27, 2017, 11:59:51 PM
Thank you guys, Don and ljohnsaw. I realize my mistake, after thinking about it and going over your answers  :P i came up with 18 ft length x 3 ft spacing = 54 sq ft. 56 live load, 15 dead load, 70 psf load x 54 sf = 3780 lb load per rafter. Don, I have been planning on having a ridge beam only with a cathedrial ceiling but after thinking about your answer/question i think i am going to do ridge beam with rafter ties, not all the way down to the wall top plates but if it helps strengthen the roof, why not?

There has been quite a bit of discussion on this one.  When is it a rafter tie and when is it a ceiling joist?  If it is not in the right place it will weaken the structure.  Jim, IIRC, talked about it a lot.

Quote from: BraschBrothers on March 27, 2017, 11:59:51 PM
One thing that i still question is if i put purlins between the rafters, will that lower the load per rafter and how do you figure that into the equation? ???
What do you mean "between"?  Like a piece of blocking? (then it would be called blocking, not a purlin)  If so, doesn't change the loading (other than the increase in weight due to that member).  Sitting on top perpendicular to the rafter direction?  No change again, but this could be called a purlin and used to support the roofing tin.  If it is below and has a post supporting it at some point(s), then the span dimension will be cut down and the subsequent loading on each portion of the rafter. You would calculate each unsupported segment of the rafter separately like they were independent.  That is what I think of when you say purlin.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Don P

For raising the tie;

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%208.html

Scroll down to the footnote under table 802.5.1(1). It is showing how much to derate a rafter span as you raise the tie off the plate. The tie begins to put an additional bending stress into the rafter and the rafter needs to be larger for the same spans to resist that additional bending stress. Remember in a masonry structure you don't want lateral load pushing against that wall, keep the ties as low as possible.

As the tie is raised there is also additional load on the connection between the rafter and the tie. Read the footnote to the heeljoint connection table, 802.5.1(9) in that same link.

If the building is 30' wide and has a simple gable roof the rafter span is roughly 15' rather than 18', rafter span for live load is taken as a horizontal measurement rather than along the rafter. Correctly the dead load is taken as the measurement along the rafter.

Assuming the tie is at the plate, try 6x10 in #1 dougfir.

BraschBrothers

Ok, Ive done a bit more figuring. If you dont mind, let me know if i got it right this time. So, to lessen the load on each timber, i changed the design to 25x30 instead of 30x30. I settled on a 8/12 roof pitch, which equates to a 15.03 foot span from ridge beam to wall top plate with 2 ft spacing instead of 3. With a 56lb live load and 10lb dead load, total load is 66. Square footage covered by each beam is 15 x 2 = 30. 30x66=1980. 1980 load / 15 span =  132 lbs each linear foot has to support. Donp, i found your fiberstress list and the 1986 wsdd and found the span calcs for 15 foot. Using the fiberstress list i found that douglas fir #1 has a fb of 1350, with that i used page 106 of the wsdd, 2nd row, 5th column to determine that a 3x10 with 1300fb has a maximum linear foot load of 137 and a total distributed load of 2059. Sounds good to me, am i missing anything or does that pretty much work? I think i finally understand it.
John 3:16

Don P

You're being more conservative than you have to be at a quick first look,
Quoterafter span for live load is taken as a horizontal measurement rather than along the rafter. Correctly the dead load is taken as the measurement along the rafter.
Rafter span is ~12' on a 25' wide building when measuring from inside the walls. I'll check back in in a little bit and pull out my wsdd, just up from the shop for a snack   :)

BraschBrothers

Don, i cant access your link in your previous message, 2015 codes, i dont have access to it, but regarding your last message i am guessing that one would use the span of 12 only if there a wall top plate mounted tie between the rafters. If there is no tie, ie: cathedral ceilings, would you then use 15 as the span? I didnt really follow the horizontal measurement rather than along the rafter, but i think i understand it now. basically you would imagine there is a tie from rafter to rafter sitting on the wall top plate and you would just measure to the center arriving at 12.5, am i understanding that correctly?

I intend on having a half loft with knee walls so the section over the loft would not work with rafter ties and the section over the open area would probably get crowded pretty quickly with rafter ties every 2 ft. Donp, i found you explaining this to another fella here countryplans dot com/smf/index.php?topic=11261.0 What i gathered from that is that i can have no rafter ties as long as i have a stiff ridge beam not board.

ijohnsaw, i found a similar article to what you were talking about with the rafter tie height while trying to comprehend what donp just told me. homeownersnetwork dot com/booktopic/ceiling-joist-rafter-spans/
John 3:16

ChugiakTinkerer

It looks like the ICC web site has changed some of their links.  See if this one works: https://codes.iccsafe.org/public/document/code/553/9838540

The rafter span doesn't change if you have rafter ties or not.  The rafter itself will do the exact job it needs to in either case.  What you will encounter in the area with a cathedral ceiling is that the walls will bow out as a result of the outward thrust.  If you want a cathedral ceiling, you can have a structural ridge or you can have scissor trusses.  Such trusses are engineered to withstand the outward thrust forces.

You could effectively have rafter ties by having an open ceiling but having a couple timber beams spanning across the room.  This wouldn't be in strict compliance with the code so if there is a building inspector involved then you'd need to get that approval.  I've also read about folks running steel cables across to serve as rafter ties.
Woodland Mills HM130

Don P

As CT said rafter span is taken as a horizontal measure irrespective of whether there is a tie or not.
Walking and talking my way through, let's use your 12.5' for half of building width x 3' rafter spacing =37.5 square feet of tributary area loading the rafter x 70 lbs per square foot load (I'm calling dead load ~15psf)=2605 lbs or ~210plf

OK, now its going to get down into the weeds. Notice the heading on my Fb list, that is for "Timbers 5x5 and larger". The correct table to use at these dimensions is "Dimensional lumber 2"-4" thick" (these tables are in the "Supplement to the NDS" on the awc.org website). In table 4A, the dimensional lumber table,#1 Dougfir has a base design value for Fb of 1000 psi. It can be adjusted upward 10% for size factor and bumped 15% for short load duration (snow) giving an adjusted Fb of 1256 psi. Going back to WSDD and interpolating between the tables for 12' and 13' spans I'm comfortable at a 4x10 (3.5 x 9.25 dressed). Dougfir is one of the only, it may be the only species where I've noticed the heavy timber has higher design values than the dimensional lumber, usually the opposite is true.

I went back and plugged all of this into my span calc just as another check;
https://forestryforum.com/members/donp/beamclcNDS2.htm
entries;
load 2605
span 150
width 3.5
depth 9.25
Fb 1265
E 1.7
Fv 180
Hit "show result"

I left my calc in the shop so that was longhand, might want to check but I think we're good. I bumped the dead load to ~15psf because that weight is taken along the axis of the rafter and you are also probably going to be heavier than normal construction on that roof (check R803 in the code, the roof deck is 1-1/2" thick T&G). The live load (snow) is taken on the horizontal projection because gravity acts vertically and snow also falls vertically, so we are looking at that roof from straight above, the snow is not going to come in and lay perpendicular to an 8/12 pitch, make sense? Holler if I lost you anywhere in that long mud puddle  :)

BraschBrothers

ok, all of that makes sense. I did double check your work don, spot on. And that link does work for me for the code book, thanks  :laugh: And the snow explanation does make it easier to understand.

Im pretty solid on the rafters and how we got there, 4x10 doug fir, 3' spacing.

I have no clue how i would even begin to determine loads for a stiff ridge beam, covering 30' inside wall diameter, 34' outside diameter with a center support post. Does that take into consideration the snow load or just dead load of the rafters or what? What is better, butting the opposing rafters over the ridge beam or lapping them?
Thank you all for all your help, I really appreciate it.

John 3:16

BraschBrothers

Don, i found you explaining it here http://countryplans dot com/smf/index.php?topic=14547.0, seriously, i search for what i am trying to understand and most boards that have a post regarding what im researching, there you are explaining to someone else exactly what im looking for. Thank you for all that you do, i am seriously blessed by your willingness to help. In that post, the guy was building a structural ridge beam on a 12 x 18 cabin, you advised that the formula was 6x18 x live and dead loads. where does the 6 come from? Is it .25 of the width of the roof per side times length of the roof always?
John 3:16

Don P

Thanks for the kind words. Practice clears the cobwebs :)

For a simple symmetrical gable roof with a ridge beam yes. Half of each rafter bears on the ridge, half bears on the wall, so each wall bears 1/4 of the overall load and the ridge bears half the load.

With a 30' building and a center post under the ridge beam, the beam span is 15'. The building width is 25' so our tributary width is 12.5'. The tributary area then is 12.5'x 15'. I think you can work it from there, happy to double check you.

Next problem for the back burner while its in mind. Those beam supports are carrying serious load, especially the center post, support straight down to ground if you can. Think about other loads along the way from floors supported by those same supports. You can generally assume the soil can support 2,000 psf and load travels through the footing at a 45 degree angle (you'll punch through if too wide and thin). That should help with footing sizes.

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