Small Cabin

Small Cabin Forum
 - Forums - Register/Sign Up - Reply - Search - Statistics -

Small Cabin Forum / Cabin Construction / Roofing Rafter Spacing / Purlins / Sheathing?
. 1 . 2 . >>
Author Message
AKfisher
Member
# Posted: 16 Jul 2018 20:52
Reply 


I am in the process of building a 16x24' D log cabin, it will have a 7' covered porch as well that will also be dryed in for a loft. So effectively it will be 16x31' and then add on a 2' overhang and the ridge will be 35' long.
My questions are:
What should my rafter spacing be if I plan on a 12/12 pitch and 2x8 common lumber?

What size sheathing should be used if I plan on using concealed fastener 24 gauge steel roofing?

Does the sheathing need to have 1x3" purlins?

As of right now I was planning on having 2x8-16 rafters, they would be cut down to about 15', this will give me a 2'3" eave overhang which should keep the logs out of the weather. The cabin will have a 2' knee wall as well.
IMG_1540.jpg
IMG_1540.jpg


DaveBell
Member
# Posted: 16 Jul 2018 21:56
Reply 


Quoting: AKfisher
What should my rafter spacing be if I plan on a 12/12 pitch and 2x8 common lumber?


What is the snow load? If you don't know that, what is the nearest town?

AKfisher
Member
# Posted: 16 Jul 2018 22:24
Reply 


120 is the ground snow load in my area.

Gary O
Member
# Posted: 16 Jul 2018 23:05
Reply 


Very interesting project
I love a log cabin
No guts to build one, however

please keep us posted, AK

cheers to yer experience



DaveBell
Member
# Posted: 16 Jul 2018 23:42
Reply 


Table R802.5.1(8)
70 PSF Ground Snow Load,
Dead Load 20 PSF,
shows:
Spruce-Pine-Fur or
Douglas fir-larch,
#2 or better, 2x10 at 12"

2x8 at 12" would have to be SS Select for both species.

You might want to go 2x10 or even 2x12.
2x12 could go 16" spacing.

I think 2x8 might collapse on you.

ICC
Member
# Posted: 17 Jul 2018 00:22 - Edited by: ICC
Reply 


Here is a screenshot from the AWC calculator android app. 2x8 can work, depending on species, grade, etc. But, will the roof be insulated in the rafters or in a flat ceiling? 2x8 do not have much room for insulation.

Sheathing- osb or plywood--- 7/16 is okay, but 5/8 gives more for the roof fasteners to bite into. Metal can be installed on sheathing, no purlins needed, although I know some folks like them. A weather barrier, tar paper or a tyvek like synthetic would be used o the sheathing and the metal on that.
AWC screenshot
AWC screenshot


ICC
Member
# Posted: 17 Jul 2018 00:25
Reply 


Knowing the species to be used, grade etc will permit more accurate results. The AWC calc is online . As well there are android and ios apps.

ICC
Member
# Posted: 17 Jul 2018 00:33
Reply 


With a 2 foot eve overhang the rafter tail loads can be high enough to stress the rafter, especially with the birdsmouth notch out. Another reason to upsize the rafters when dealing with high snow loads.

I know there is a belief that with a 12/12 pitch and metal, the snow will slide off and not be piling up. However if the roof is not well insulated sometimes snow will melt a bit and then freeze as the temps get colder. Then the snow will not slide.

AKfisher
Member
# Posted: 17 Jul 2018 02:18
Reply 


Quoting: ICC
Here is a screenshot from the AWC calculator android app. 2x8 can work, depending on species, grade, etc. But, will the roof be insulated in the rafters or in a flat ceiling? 2x8 do not have much room for insulation.

Sheathing- osb or plywood--- 7/16 is okay, but 5/8 gives more for the roof fasteners to bite into. Metal can be installed on sheathing, no purlins needed, although I know some folks like them. A weather barrier, tar paper or a tyvek like synthetic wo



I will be insulating with batts between the rafters, cathedral style. I am thinking your right, 2x8 won’t leave me much insulation. I was thinking a 1/2 plywood sheathing and then felt, then the concealed fastener roofing panels. It will basically be a hot roof since it won’t be vented unless I put in baffles.

toyota_mdt_tech
Member
# Posted: 17 Jul 2018 08:56
Reply 


Quoting: AKfisher
I was thinking a 1/2 plywood sheathing and then felt, then the concealed fastener roofing panels. It will basically be a hot roof since it won’t be vented unless I put in baffles.


This sounds right, use #30 felt, not the 15. Use fat screws, go well past the wood. for max bit. I suspect you mean visible on top, why you want to go with standing seam type.

DaveBell
Member
# Posted: 17 Jul 2018 09:54 - Edited by: DaveBell
Reply 


His cabin span of 16, 12/12, 2 overhang, provides slope of 14-1. 14-1, minus overhang = 12-1 each rafter.

The Maximum Horizontal Span is:
11 ft. 0 in. using the online calc. 2x8, Select Spruce.
That's each rafter, right? His is 12-1.

If we relate that to the table in chapter 8, the max horizontal span is the slope length, not 1/2 the cabin span length.

I don't know why the table and the calculator don't agree more closely.

Gary O
Member
# Posted: 17 Jul 2018 12:00
Reply 


This is strictly just my opinion (based on my logic)
It’s a 16’ wide log cabin
Overbuild it
12/12 (45° pitch) is plenty for snow load (with a metal roof)
Even 4x4 beams crossed with 2x6s is plenty for a roof, and makes an attractive ceiling


Sheathe the top of that with 7/16 max, then 30 lb felt
Then metal




Done

happy

AKfisher
Member
# Posted: 17 Jul 2018 12:18
Reply 


Quoting: toyota_mdt_tech
This sounds right, use #30 felt, not the 15. Use fat screws, go well past the wood. for max bit. I suspect you mean visible on top, why you want to go with standing seam type.


I was thinking standing seam (hidden fasteners) because of the look, haven't been a big fan of the corrugated metal since the last time I used it... But now it has me wondering, what is the difference between the two?

FYI: we get very little wind where we are, so the uplift is low.

AKfisher
Member
# Posted: 17 Jul 2018 12:19
Reply 


Quoting: Gary O
This is strictly just my opinion (based on my logic)It’s a 16’ wide log cabinOverbuild it12/12 (45° pitch) is plenty for snow load (with a metal roof)Even 4x4 beams crossed with 2x6s is plenty for a roof, and makes an attractive ceiling


Looks nice Gary! What type of metal did you use?

ICC
Member
# Posted: 17 Jul 2018 12:33 - Edited by: ICC
Reply 


Quoting: DaveBell
The Maximum Horizontal Span is:
11 ft. 0 in. using the online calc. 2x8, Select Spruce.
That's each rafter, right? His is 12-1.

If we relate that to the table in chapter 8, the max horizontal span is the slope length, not 1/2 the cabin span length.


I don't follow you.... I must be misunderstanding something. ??? Have you references you can link to?
All my working life rafter span has been 1/2 the horizontal distance from one side wall to the other side wall.
Or, to put it another way, from a plumb line dropped from the peak, measured horizontally to the inside of the side wall.
Like this---


above image from here, right at the article end.

Same info here --- this is the tutorial page for the AWC calculator. Notes indicate---
Figure 6
Use the horizontal projection of a rafter, not its actual length, when figuring rafter s
pan

ICC
Member
# Posted: 17 Jul 2018 13:00 - Edited by: ICC
Reply 


IRC, Chapter 8--- screenshot below
It states to use the rafter horizontal projection as the span.

When I run the AWC calc for values in the table 805.5.1(6),
I get the same rafter specs as in the table
IRC-8
IRC-8


DaveBell
Member
# Posted: 17 Jul 2018 14:04
Reply 


We're good.
2x8, 16 OC, Spruce #2, span 10-1, 70 PSF
So 120 PSF would be 8-1, 1/2 of his 16 span.

AKfisher
Member
# Posted: 17 Jul 2018 14:41
Reply 


Quoting: DaveBell
We're good. 2x8, 16 OC, Spruce #2, span 10-1, 70 PSFSo 120 PSF would be 8-1, 1/2 of his 16 span.


Thanks Dave,

If I upsize to 2x10s, can I space them 24 OC?

ICC
Member
# Posted: 17 Jul 2018 15:28 - Edited by: ICC
Reply 


Tell us what species you have available as well as the grade.Most commonly available lumber is #2, but the species may vary widely. There are usually slight differences between species, but may as well calc the right one. Or D/L the app (or use the online AWC calc) and see how easy it is to use; plus you have the fun of comparing sizes, spacing, etc. The tutorial referenced above is useful.

For S-P-F #2 2x10 are good at 24" OC

AKfisher
Member
# Posted: 17 Jul 2018 15:39
Reply 


Quoting: ICC
Tell us what species you have available as well as the grade.Most commonly available lumber is #2, but the species may vary widely. There are usually slight differences between species, but may as well calc the right one. Or D/L the app (or use the online AWC calc) and see how easy it is to use; plus you have the fun of comparing sizes, spacing, etc. The tutorial referenced above is useful.


Spruce / Pine / or Fir will be the species, #2 grade.
I ran the calculator with:
2x10
24 OC
L/240
120 Snow load
15 Live load

= 8'2" span.

DaveBell
Member
# Posted: 17 Jul 2018 16:27
Reply 


ICC, I just checked the Home Depot in Anchorage, AK for species.

ICC
Member
# Posted: 17 Jul 2018 17:32
Reply 




Gary O
Member
# Posted: 17 Jul 2018 22:59
Reply 


Quoting: AKfisher
Looks nice Gary! What type of metal did you use?

26g Strata rib
36” widths

I chose the weathered copper

It’s not cheap

But it is good


NorthRick
Member
# Posted: 18 Jul 2018 00:50
Reply 


Quoting: AKfisher
Spruce / Pine / or Fir will be the species, #2 grade.


I usually see Spruce instead of Pine up here. Not sure if that makes any difference.

ICC
Member
# Posted: 18 Jul 2018 09:34 - Edited by: ICC
Reply 


The grading stamp is what you go by. Spruce should be stamped S-P-F.

document

There is a wealth of information in the grade stamp; as well as the species there is the mill #, grade # including if it is for stud use only, if it was kiln dried, heat treated, what the moisture level was dried to before leaving the mill, if it was machine tested, grading organization, ....

Canadian SPF article, applies to USA as well

http://stimson_lumber.s3.amazonaws.com/products/literature/Understanding_Grade_Stamps .pdf

https://www.thisoldhouse.com/ideas/how-to-read-lumber-stamp

http://www.alsc.org/greenbook%20collection/lumberprogram_facsimile.pdf

AKfisher
Member
# Posted: 6 Aug 2018 20:47
Reply 


Gents, I have another question. The ridge board that will be holding up the rafters. I was thinking double up 2x10s glued and screwed would be sufficient but would like you opinions. The Ridge will be 35' long and in 3 sections. Working from front of cabin to back: 9' section which includes the 2' overhang will be supported by the Log gable and 8" post inside. Second section will be 16' spanning from the 8" post across the cathedral ceiling to the other loft that will have another 8" post. Third 10' section will span from the back 8" post to the log gable and 2' overhang.

Will double 2x10s be sufficient?

Borrego
Member
# Posted: 6 Aug 2018 21:37
Reply 


Quoting: AKfisher
Will double 2x10s be sufficient?


Why don't you just use a 4x10?

ICC
Member
# Posted: 6 Aug 2018 23:35
Reply 


Quoting: AKfisher
Will double 2x10s be sufficient?


More info needed.

1. You call it a ridge board, but also say it will be "holding up the rafters". That sounds like it will be a ridge beam. Boards just give something to help nail the rafter tops together and provide spacing. Beams carry roof loads.

2. If a beam, what ground snow load (PSF) would this be carrying? The answer to that might be buried someplace in the previous 20-some replies but I am too lazy to look.

AKfisher
Member
# Posted: 7 Aug 2018 01:47 - Edited by: AKfisher
Reply 


12:12 pitch roof. It will be ridge board/beam, osb sheathing, felt, and standing seam tin roof. Most likely no snow load even though the snow load is 120 on the ground. I believe very little will accumulate on the tin. I will have r30 wool insulation and t&g spruce for the ceiling.

ICC
Member
# Posted: 7 Aug 2018 11:19
Reply 


Quoting: AKfisher
It will be ridge board/beam,


It is one or the other, not an either/or.

As stated above, one (a ridge board) is simply a nailer, needs no strength at all. The other (a ridge beam) needs to support load like the roofing materials, plus wind, rain, snow. Not only that a beam needs those supports in mid span to be well supported right down to the foundation. Before running some beam loads, what exactly have you got going on? I know that with 1 12:12 pitch it is thought snow will slide off. Mostly it does. Sometimes weather conspires to lay down sleet that freezes and then the snow piles on top and does not slide. Can vary.

. 1 . 2 . >>
Your reply
Bold Style  Italic Style  Underlined Style  Thumbnail Image Link  Large Image Link  URL Link           :) ;) :-( :confused: More smilies...

» Username  » Password 
Only registered users can post here. Please enter your login/password details before posting a message, or register here first.