Author Topic: My turn for a question please sizing ceiling/floor?  (Read 10166 times)

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Offline Willy

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My turn for a question please sizing ceiling/floor?
« on: March 21, 2008, 12:18:47 PM »
I will try to make this simple in my discription? Approved Cabin Plans had 2x8 floor joists on 16" centers spanning 12 ft outside to outside on 2 rows of peir blocks. Later I sized them up to 2x10s on 16 inch centers. The ceiling above was 2X6 for the attic space and now I am going to up/grade them to 2X8s. All wood structural select grade. Walls are 2x6 16" on centers and used to be 2x4s for this cabin.

Now the 2x6 ceiling spans 12 ft also and sit on the outside 2x6 walls. Above these walls the rafters sit for the shed roof rated at 45 lb snow load. I have 6 ft wide porch that comes off the cabin and that roof will attach to the shed roof rafter and have its over supporting beam and peir on the same center as the cabin. OK this is how it is figured to be built now.

What I want to do/change is enclose only 10 ft of the 6 ft wide porch and add it as open space to the cabins 12 ft wide area making it 18 ft wide instead.

To do this I was going to span the 18 ft wide floor with full length 18 ft- 2x10 floor joists on 16" centers & 3/4" plywood sitting on 3 rows of approved peirs with approved beams (3-2x8s 6ft on centers) bolted to them. This is not a problem to do cause it is stronger than before and no real change cause the porch will have up/graded (2x6 to 2x10s) joists. Instead of a open porch roof sitting on beams with posts going down to beams as big as the ones holding the cabin up on the same peir centers I will frame 3-2x6 stud walls above the 2x10 floor joists to hold up the porch roof in this 6 ft x 10 ft long area. All this stuff is no problem to do and more than stronge enought and also to code.

Here is the real change the high side of the 6/12 pitch - 12 ft wide shed roof 2x10 rafters used to sit on the 2x6 wall I am moving out 6 ft. Now the shed roof rafters will sit on a 6 ft high 2x6 stud wall sitting on top of the up/graded 2x8 ceiling joists on 16" centers spaning 18 ft instead, but 6 ft away from the outside wall. The 6 ft wall below the rafters end will not have a bearing wall directly below to the lower floor except on the outside walls.

The 2X8 ceiling joists will have a bearing bath room 2x6 wall below them the full length of the 10 ft long area inside the cabin 6 ft out from the low back wall holding up the other ends of the shed roof rafters. This wall will be sitting on top of the up/graded 2x10 floor joists on 16" centers. So realy the 2x8 ceiling joists will be spaning only 12 ft again with the rafter 2x6 wall above in the middle of it.
 
After saying/asking all this what I want to know is do you see a problem supporting the ridge of the 2 separate shed roofs on top of the 2x6 bearing wall which will be now supported by the 2x8 ceiling joists below.

It seems to me to be like a giant truss made out of  2-2x10s and 1-18ft 2x8 with a 2x6 in the middle yet still has bearing walls below it?  Man I hope this makes sence cause it took a long time to type! Here is a picture of the cabin and you can see the porch on the front I want to enclose and the roof over the porch will go the full length of the building. Mark




One reason for the 2x8 joists on the ceiling is this area is a loft now above the kitchen bathroom and the shed roof was raised 2 ft on each end(front and rear) in this 10 ft long area. So if you now look at the front of the cabin there is a 2 ft higher wall above the porch roof on the last 10 ft of the 24 ft long cabin kitchen end. The porch roof will stay the same all the way down the 24 ft long cabin. Thanks, Mark


Offline MountainDon

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Re: My turn for a question please sizing ceiling/floor?
« Reply #1 on: March 21, 2008, 12:52:17 PM »
I have to re-read this later. However, I had a couple first thoughts. Not sure if they're good ideas.  ???

This is about the end of the cabin where you want to increase the width and enclose a portion of the porch as an enclosed portion of the cabin. Why not build that end with a regular rafter roof, with unequal pitches? You could still have a supporting wall underneath if needed.

Remember an outside, exposed to the weather, porch floor should be an inch to an inch and a half lower than the main cabin floor. So any such porch should be built like an add-on, not using joists that continue back under the main enclosed cabin.


Anyhow, gotta run. Back later
Don

Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Willy

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Re: My turn for a question please sizing ceiling/floor?
« Reply #2 on: March 21, 2008, 01:33:55 PM »
I have to re-read this later. However, I had a couple first thoughts. Not sure if they're good ideas.  ???
Anyhow, gotta run. Back later
Don
Let me re/phrase the porch thing a little. The porch was going to be a add on 18 ft long on the front of the cabin sitting on it's own joists lower then the floor in the cabin. The roof of the porch is of a shed roof rafter design and open inside it from end to end but sided closed. The 12 by 24 cabin is totally sided and the porch added to the front. In this porch area it will have a sofet covered ceiling and plywood floor inside. I will be able to access it as storage space from inside the cabin up in the loft thru a door.

Now insted of building the bottom part of the porch 18 ft long I will build it 14 ft long. The joists for the porch floor will be attached to the rim joist on the cabin with a ledger board. The roof line of the porch will continiou the full length of the building and that cut out section on the picture will not be cut out.

Now the 10 ft long area that was going to be a porch is going to be added into the cabin insted and framed the same way making the floor joist from the building go out 6 ft longer in this area. So the cabin will be wider in this 10 ft long area by 6 ft on the lower floor.

I am sorta cheating the building dept out of 60 sq ft on the lower floor plus they do not count the 6X24 ft (144 sq ft)storage area above the porch, nor does the 10 X 12 (120 sq ft) loft count as living space so permit fees drop around $500.00 and taxable space is lower to thinking the cabin is only 288 sq ft instead of 552 sq ft. For what I take off the porch the fees saved are a wash for adding to the lower floor.

The rest of the 12x14 cabin is open ceiling with 8 ft tall walls on the back and 12 ft tall walls on the porch side. The 10 ft long area(stairs-kitchen-bathroom) has a 10 ft tall(2 ft into the loft) back wall 14 ft tall ridge(6 ft into the loft) and 8 ft front wall meeting the floor of the 24 ft long porch attic space. Mark
« Last Edit: March 21, 2008, 02:57:44 PM by MountainDon »

Offline MountainDon

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Re: My turn for a question please sizing ceiling/floor?
« Reply #3 on: March 21, 2008, 03:15:40 PM »
Let's see if I have a handle on this? And for now, let's leave the roof out of it.

There's a 12 x 24 ft main cabin.

The "as drawn porch" footprint is being expanded to 24 feet long x 6 ft wide. A  10 x 6 ft portion of that will be built with the main cabin floor joists being extended the extra 6 ft. This will be built with insulated walls, access from the inside main cabin.

That leaves a 14 x 6 ft area to be a storage area. Here's where I get lost.... 
Quote
I will be able to access it as storage space from inside the cabin up in the loft thru a door.
Is this an open air storage area or a walled off area, like another room? Access from the loft? Sorry, maybe I'm a little slow with turning the word description into a mental image today.  d*   I need a clear image.  :-[

I think best from the bottom up. The building.  :D
« Last Edit: March 21, 2008, 03:38:35 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Willy

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Re: My turn for a question with pictures now too.
« Reply #4 on: March 21, 2008, 04:04:10 PM »
Let's see if I have a handle on this? And for now, let's leave the roof out of it.

There's a 12 x 24 ft main cabin.

The "as drawn porch" footprint is being expanded to 24 feet long x 6 ft wide. A  10 x 6 ft portion of that will be built with the main cabin floor joists being extended the extra 6 ft.

That leaves a 14 x 6 ft area to be a storage area. Here's where I get lost.... 
Quote
I will be able to access it as storage space from inside the cabin up in the loft thru a door.
Is this an open air storage area or a walled off area, like another room? Access from the loft? Sorry, maybe I'm a little slow with turning the word description into a mental image today.  d*
OK less words are better sometimes just some don't give enought information in a question.
Yes the plans were for a 12X24 cabin and a porch that was 6X18 in size.

Now I will build the cabin as planed for the first 14 ft in the floor plan but expand to 18 ft wide on the last 10 ft of the cabin(kitchen area). This makes a "L" shape floor plan sorta.

Now the porch won't have room to be 18 ft long cause the cabin part is in the way, being built wider at the kitchen end now by 6 ft and 10 ft long. So I will build the porch 14 ft long and 6 ft wide and keep the porch roof design going the full length (24 ft) of the building and over top of the widened kitchen area(now the roof for it) that used up the space for the porch.

So since I moved the lower 10 ft long wall (kitchen area) out 6ft that the roofs rafters short wall up in the loft (highest roof point on the cabin) sat above it does not have a bearing wall on the lower floor below it now. The 6 ft high wall up in the loft holding up the rafters sits only on the 2X8 floor joists of the loft now. That rafter wall is located 6 ft from the outside wall and in the middle of a 12 ft wide span of the joists.

Is this a problem only using the 2x8 loft floor joists to hold up the 6 ft tall rafters wall in the loft? It does not have a bearing wall located right below it now to the floor below. It is now 6 ft further out on the build at the eve edge of the porch roof.

I will draw a side view and post it for you of what I am tring to discribe. Mark

Here is a link to 5 drawing that show what I am talking about. Having a 45 lb snow load do you see a problem with the 2x8 loft joists holding up the rafters wall in the loft? It is in the middle of a 12 ft span.
http://ImageEvent.com/willy/cabinmods
« Last Edit: March 21, 2008, 04:57:57 PM by Willy »

Offline MountainDon

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Re: My turn for a question please sizing ceiling/floor?
« Reply #5 on: March 21, 2008, 05:04:59 PM »
posted before seeing the added edit above:

Is this a problem only using the 2x8 loft floor joists to hold up the 6 ft tall rafters wall in the loft? It does not have a bearing wall located right below it now to the floor below.

This Span Calculator indicates that for that span, joists at 16" OC, 30 lb. live load, and a deflection of L/360, you would need DougFir Select Structural 2x10. Number 1 or 2 grades would need 2x12's for that length of clear span. The sizes don't change with L/240 either.

Using a header beam across that space where the wall used to be would help hold up the joists to allow smaller sizing, as long as the beam was properly supported at each end. It would rob you of headroom.

Same calculator shows 2x8 #2 DF okay for the 12 foot span, or 2x6 Select DF.

Increasing width is always more trouble than adding length.

At least that's what I come up with.
That's without holding up that section of roof, that is shown with the Hmmm in the below drawing.

« Last Edit: March 21, 2008, 05:37:00 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline MountainDon

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Re: My turn for a question please sizing ceiling/floor?
« Reply #6 on: March 21, 2008, 05:27:40 PM »
If the red circled wall ? in the drawing below is under the 18 ft total span width then if it's built as load bearing you would be okay with 2x8's IF the 2x8 joists did not have to support that wall section that goes up to the roof. Hmmm



That would probably need some engineering calculations. That would be holding up your snow load.




Do you have a drawn floor plan with all the walls, etc?
« Last Edit: March 21, 2008, 05:43:25 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Willy

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Re: My turn for a question please sizing ceiling/floor?
« Reply #7 on: March 21, 2008, 05:44:31 PM »
If the red circled wall ? in the drawing below is under the 18 ft total span width then if it's built as load bearing you would be okay with 2x8's IF the 2x8 joists did not have to support that wall section that goes up to the roof. Hmmm



That would probably need some engineering calculations. That would be holding up your snow load.

Do you have a drawn floor plan with all the walls, etc?
Wow just looking at your red lines I figured out what to do!! I will remove that old bathroom wall near the stairs since my stairs are free standing anyways and put it under the rafter wall. Then make that porch area my bath room and a closet insted. The 12 ft wide area under the loft will be my bigger kitchen/dining are I wanted to make. Easy to move a kitchen before it is built! Now if I could span the 12 ft area with 2X6s it would be cheaper since it is only a loft. If not I will go with 2x8s over the kitchen. What do you think? Look at your span tables the 2x6s will only be the loft floor and not holding up the roof. There span will be 11 ft between the 2 walls on 16" centers and 3/4" plywood on top of them. Mark

Offline ScottA

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Re: My turn for a question please sizing ceiling/floor?
« Reply #8 on: March 21, 2008, 05:51:27 PM »
I would make sure that load bearing wall had a beam under it in the crawl space and not rely on the floor joists to support it.

Offline MountainDon

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Re: My turn for a question please sizing ceiling/floor?
« Reply #9 on: March 21, 2008, 05:57:37 PM »
Well there ya' go! I always find drawings help me see things better.

With moving the kitchen over, having an 11 ft span, using joists 16" OC, 30 lb. load (the usual value for a loft), L/360 deflection, the calculator says: 

DougFir Select Structural = 2x6 (but that's at the upper limit of recommendations)
pretty much anything else = 2x8

How's that DF Select price out against lesser grades/species?



Re: Scott's comment above.   Mark, you mentioned three girders/beams under the main floor joists. Were you going to space them equally across the width, or place the middle one under the 24 foot "mid" wall??

I was assuming you'd leave it under what used to be a completely exterior wall. ???  I shouldn't assume.  n*
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Willy

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Re: My turn for a question please sizing ceiling/floor?
« Reply #10 on: March 21, 2008, 06:30:21 PM »
I would make sure that load bearing wall had a beam under it in the crawl space and not rely on the floor joists to support it.
That load bearing wall has 5 - 6x6 posts set in concrete with 3-2X8s sitting on top going the long way of the cabin on the outside walls. The floor joists are 2X10 crossing the 12 ft wide floor. The porch has 3- 6x6 posts and the same beam sitting on the but 2X6s for joists 6 ft long. The new bathroom area will have 2x10s on 3 - 6x6 posts and the same beam just a littl higher than the porches beam cause the porch will sit lower then the house floor. Mark

Offline Willy

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Re: My turn for a question please sizing ceiling/floor?
« Reply #11 on: March 21, 2008, 06:35:12 PM »
Well there ya' go! I always find drawings help me see things better.

With moving the kitchen over, having an 11 ft span, using joists 16" OC, 30 lb. load (the usual value for a loft), L/360 deflection, the calculator says: 

DougFir Select Structural = 2x6 (but that's at the upper limit of recommendations)
pretty much anything else = 2x8

How's that DF Select price out against lesser grades/species?



Re: Scott's comment above.   Mark, you mentioned three girders/beams under the main floor joists. Were you going to space them equally across the width, or place the middle one under the 24 foot "mid" wall??

I was assuming you'd leave it under what used to be a completely exterior wall. ???  I shouldn't assume.  n*
Believe it or not my supplier will sell me Select Structural wood for cheaper than #2 or better so all my beams, joists and rafters will be select grade. I have been buying from them over 12 years and get contractor prices along with picking thru my wood taking only the good stuff!! Yes I will leave that wall over the piers & beam that were for the outside wall and just build a set heavier (3 peirs) for the 10 ft wall and 3 peirs for the 14 ft long porch. Mark
« Last Edit: March 21, 2008, 07:32:17 PM by Willy »