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Author Topic: Roof Rafter  (Read 700 times)

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Offline bannerd

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Roof Rafter
« on: July 19, 2021, 06:59:12 AM »
Hey all, I'm having a hard time getting the numbers here.  Using Don_P's calculator it says I pass but I want to double check.  I'm looking to run a 4x6 @ 16ft on a 8/12 roof pitch.  The roof rafter (4x6) will be resting on a 8x8 around the 11ft mark(124 inches to be exact).  The other five feet will then be pegged with no ridge beam.

Textik - ASCII diagrams editor

That is a rough idea so you can get a picture.  The rafters will be 24" OC, the roof materials are either tin or shingles.  This location does get about 4-5ft of snow so there is going to be quite a load on this roof.  I estimated that the load per sq ft could be around 115lbs which landed me around 12600 of thrust.  The building is 30ft wide so each side will be 16ft roughly.

Let me know, Thanks!  This is for my new house build and we're getting close to getting a roof on!  8)

Offline Don P

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Re: Roof Rafter
« Reply #1 on: July 19, 2021, 09:49:52 AM »
For the 4x6 rafter you would need to enter span, load, species and grade... you didn't give species and grade. 11' span x 2' width x 115 lbs/sf=2530 lbs vertical, bending, load per rafter. 

With the arrangement I think I'm seeing there is no thrust, well internal to the entire component there is tension and compression... but what am I looking at here? The sketch appears to be a queenpost truss. Is this what each 2' oc rafter component looks like? If so I'm not understanding the 8x8 under a 4x6. A better sketch of the entire roof framing plan would help, I'm not getting what you are up to yet.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline bannerd

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Re: Roof Rafter
« Reply #2 on: July 19, 2021, 10:04:06 AM »
Here is a rough sketch.  The species of wood is red pine or eastern white pine.  I also have been using hemlock where needed.



 

Not sure why it came in crooked..  Here is the project so far;



 

Lots to do before winter comes, so far behind.

Offline bannerd

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Re: Roof Rafter
« Reply #3 on: July 19, 2021, 10:10:59 AM »
This is green wood and rough cut lumber from my sawmill and another down the road.  We plane the wood and then treat with lifetime and seal-once.  This weekend we're hoping to get the rafters up and start laying down ship lap over the rafters.  This will be another load on them but I think we should be well over the limit here.  Ship lap is 1" boards 1x8x14's which will cover the entire roof.

Offline Don P

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Re: Roof Rafter
« Reply #4 on: July 19, 2021, 03:55:09 PM »
Click this link and double check your snow load;
Ground snow load by zip

From the pic and sketch it looks like the "queenposts" are going to be stacked over the posts below? That would make the span of the 8x8's underneath and supporting the rafters at the 124" point about a 16' span... the distance between those "queenposts"? That is another beam that needs to be checked. I'd go with hemlock on those if possible to get better knot scatter but I suspect that pair of beams is where the problem is gonna be.

I'm down with a foggy eye they plucked some metal and rust out of this morning and am supposed to stay outta the dust so I'll start sketching to explain this if you can check snow loads and confirm or correct my dimension assumptions.

The future is a foreign country, they will do things differently there - Simon Winchester

Offline Don P

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Re: Roof Rafter
« Reply #5 on: July 19, 2021, 08:49:24 PM »
Here's some pics of what I hope is about what you've got;


 


The rafter load is the horizontal distance from the upper to lower plate, ~10' x halfway from one rafter to the next, 2' x (snow load +dead load ~10psf)
This is a pic of that tributary area;


 
Since it is less than 5x5 use this dimensional lumber calc;
Design for Bending (forestryforum.com)
Looks like you are good to about 1350 lbs per rafter in #2EWP or 67.5 total load, or ~57 psf snow load... might not be big enough for that lake effect stuff.



The upper plate in red is supporting the rafters from the ridge to the midpoint between that beam and the wall plate, ~10' x the span between posts ~16'. So 160 square feet x the (LL + DL)


 
This is larger than 5x5 timber so go to this calc;
Design for Bending (forestryforum.com)
Looks like in #1 Hemlock its good for around 4000 lbs total or 2400 lbs snow... divide by 160 sf= a 15 lb snow load... that definitely isn't going to work.


The wall plate is supporting the lower half of the rafter, ~5' + the overhang. If you are supporting it with a stud under each rafter no bending check needed but if it has spans supporting the rafters, it is light, check that as well.

The future is a foreign country, they will do things differently there - Simon Winchester

Offline bannerd

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Re: Roof Rafter
« Reply #6 on: July 20, 2021, 07:09:12 AM »
Wow Thanks Don,

That is exactly what we have going on. Our snow load is in the 60 pound range per sf, we do get a ton of snow.  The roof plate at 4x8 is going to be supported by studs which I believe will be the kids rooms with a ton of shelving in between.  I wouldn't have thought the 8x8 would be a weak point here so It's back to the drawing board for me.  

I'm showing our max span can be no longer than 128 inches which is not good because it's 16 ft span on that 8x8.  We do have a stair way that needs to go into the middle so an 8x8 there would put that span at 110 inches.  The stair case literally will go down into the basement and sit on footers.  I'm not worried about vertical load on these 8x8's as it's way over 50,000lbs if I did the math correct.  What worries me now is the great room which has a span of 15 ft, I could put long knees there which would redirect that span onto the 8x8 and reduce that to 11 ft when I measure it.

These calculators are very helpful and I'll be using them more now.  Hope your eye gets better, time to get to work.

Anyone know the strength of red pine?  A lot of these beams will be red pine which should have more strength than the white pine.



Offline Don P

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Re: Roof Rafter
« Reply #7 on: July 20, 2021, 07:18:52 AM »
Knees don't really do a good job with that,especially when working into a green post that will shrink in width. Consider long floor to beam angled "posts" to reduce the span.

Got a buff and polish on the eye this morn then back to the woods, memo to self, wear those safety glasses  :-[

Red pine is in the same strength range as EWP, the knot structure is pretty much the same, a whorl of knots every year, be careful with grading on either of those, that is a real point of weakness. It is in the heavy timber calc dropdown. In the dimensional calc it is grouped with eastern softwoods (which I neglected to include), #2 Fb 575, E 1.1, Fv 140. #1 Fb 775, E 1.1, Fv 140... in other words punch in EWP, shear is slightly higher but that isn't going to be the control here.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline bannerd

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Re: Roof Rafter
« Reply #8 on: July 20, 2021, 08:10:27 AM »
Right on, I'll have to take pictures of progress.  I try to work on the house after work and mostly on weekends so I don't have a lot of time.  Just me doing it which is always fun.



Offline Don P

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Re: Roof Rafter
« Reply #9 on: July 20, 2021, 10:08:04 PM »
Something along these lines is what I'm thinking. The full height "brace" is then essentially a post bearing long grain the whole way, shrinkage isn't going to cause support problems for the beam above.



 
The future is a foreign country, they will do things differently there - Simon Winchester

Offline bannerd

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Re: Roof Rafter
« Reply #10 on: July 21, 2021, 01:41:42 PM »
That is what I was thinking for our great room that will have no floor but an open ceiling.  The rest I can make work as we already have beams in position that would put the load right to the ground.  I'm still laying ship lap but will get some pictures of the beams.

floor is crucial for the gin pole.

We also have a massive column (concrete) that was put in for a soap stone/ mass heater so we have a lot of options.  I'm just glad I caught this/questioned it.

You're pretty dang handy with sketch up, I've tried to use it but ended up with an "eye sore"  :D

Offline Don P

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Re: Roof Rafter
« Reply #11 on: July 21, 2021, 09:05:53 PM »
Yeah there is a learning curve with sketchup, I can't do half of what it is capable of.

Don't forget to distribute/ think about what is under the gin pole, I've, uh, punched holes through a floor or two when forgetting to look down :D.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Mattjohndeere2

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Re: Roof Rafter
« Reply #12 on: July 28, 2021, 01:12:14 PM »
Hey bannerd, nice place your building there. I'm about 4 hours south of you, if you get in a pinch and need a hand for a weekend I wouldn't mind lending some manpower, just let me know.


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