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Old 09-21-2009, 01:34 PM   #1
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Remodeling Attic Space


I have a one story home which used to have a somewhat finished attic room. The attic was poorly built so we took it all out. Now that I have a big empty space I want to be able to build a new bedroom making sure that existing structure will support it.

Attic space is 25 x 30
Bedroom dimensions will be centered at 15 x 24
Existing floor joists are 2x6 16" oc and roughly span 12 feet on both sides with supporting wall below.
I will have knee walls all the way around the room approximately 5 feet away from exterior walls.

I already know my floor joists are not strong enough so I was thinking about sistering a 2x8 next to each existing joist. According to a floor joist span calculator. 2x8 Spruce #2 joist at 16"OC with a deflection of L/360 have a maximum span of 13.5'.

My question is, considering the knee walls being in the middle of the joists, will 2x8's be enough?

Thank you.


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Old 09-21-2009, 05:47 PM   #2
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Remodeling Attic Space


1. A spruce 2x8, 16"o.c. will carry 40# live load and 10# dead load and span max. 11'7"

2. Spruce 2x6, 16"o.c' span 12.5' will carry 403# Spruce 2x8, 16"o.c. span 12.5' will carry 702#
403 + 702 = 1105# / 12.5= 88.4# per lin. ft. / 16" = 66.5# per square foot if no holes in middle 1/3 of joists.
3 How much headroom do you have to the collar ties?
4. Where are the collar ties located from the peak outwards horizontally?
Be safe, Gary

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Old 09-21-2009, 06:39 PM   #3
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Remodeling Attic Space


Thanks for your reply Gary. I have just over 7.5 feet to the collar ties. They are 22" below the peak of the roof and measure 6 feet long. So are you saying that 2x8's should be enough?
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Old 09-21-2009, 07:39 PM   #4
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Remodeling Attic Space


Yes, 2x8's would be enough nailed to the 2x6's, if your knee walls do not carry much of the roof load. If they did, you would consult a Structural Engineer in your local area, as he would know your wind and snow loads on the roof structure. As stated using listed materials, you could safely carry 66# load, instead of code minimum 40#. Be sure to furr down the room rafters for required insulation for your location, leaving 1" next to the roof sheathing/insulation for ventilation air flow. Or use extruded foam board to add minimal furring and meet your energy code. Ridge vents (continuous) with baffles, balanced soffit venting sized for your roof (25 x 30= 750 / 150= 5sq.ft.= 1/2 ridge, 1/2 soffit) sealing all ceiling penetrations, etc. Any other questions? Or just click on my name in blue, as I have answered this many times before. Be safe, Gary
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Old 09-21-2009, 08:06 PM   #5
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Good info, thanks again.
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