Sizing Ceiling Joists for Attics

[jar546]

Educational:

A builder submits plans for a new home. It is a small ranch 24' wide by 60' long with a simple gable roof that has a 6/12 pitch. All of the ceilings will be flat, nothing fancy, no pan/tray, vaulted, etc.

In order to save money for the homeowner, the builder has opted to declare the attic non-storage space on the submitted documents and utilize Table 802.4(1) The 2009 IRC applies for this house.

The longest span for ceiling joists in the entire house is the master bedroom with a span of 16'8". The builder has specified 2x6 SPF 16" o.c.

Can this be done prescriptively per the 2009 IRC?

 

[Jobsaver]

I'll bite, and suffer the usual sting of humiliation after I am once again corrected. I operate under the teaching that the correct chart to use for the subject home, for an occupied space, would be 802.4(2) having a dead load of 10 psf for lumber, drywall, and insulation.

 

[jar546]

I'll bite, and suffer the usual sting of humiliation after I am once again corrected. I operate under the teaching that the correct chart to use for the subject home, for an occupied space, would be 802.4(2) having a dead load of 10 psf for lumber, drywall, and insulation.

Never humiliation here. We all may learn something or some of us or most of us or maybe no one but let's hope not on the last one. I can only hope that some do learn from this exercise and it may just be me too.

Anyway. What code verbiage is telling you to use Table 802.4(2)? Please be specific about your answer whether or not it is correct which we don't know at this point.

 

[TheCommish]

is the space easily developed for living or sleeping space, if not treat like a truss 10psf bottom load for deas, no storage allowed, access for mechanical s only.

If a walk up attic with floor use proper size joist for potential load, light storage, or bedroom 30psf

 

[Jobsaver]

What code verbiage is telling you to use Table 802.4(2)? Please be specific about your answer whether or not it is correct which we don't know at this point.

Can't say. I may have been taught wrong, but have been taught that the average or typical dead load for the building materials used or required in my area for an occupied space in a SFD is about 7 lbs per square foot. Or in other words, the framing, 1/2" drywall, joint compound, fasteners, paint and required insulation typically average about 7 lbs per square foot. Table 802.4(1) reflects a dead load of 5 psf. Table 802.4(2) reflects a dead load of 10 psf.

 

[jar546]

Great information and feedback. I would like to get feedback from an experienced residential plan reviewer or inspector familiar with the 2009. I wonder if there are any members of this forum that perform inspections on homes such as this on a daily basis?

There is often a difference between what we are taught and what the code says.

OK, enough prodding the cattle. Still waiting for an answer explaining exactly why or why not. Thanks!!!

 

[jar546]

For the sake of keeping this thread alive so that it can be debated or we all learn something, I will post my opinion now that this thread sat dormant for 24 hours and still no definitive answer.

Table 301.5 is essentially the answer. Here is what it says about attics with no storage and attics with limited storage within the footnotes.

b. Attics without storage are those where the maximum clear height betweenjoist and rafter is less than 42 inches, or where there are not two or more adjacent

trusses with the same web configuration capable of containing a rectangle

42 inches high by 2 feet wide, or greater, located within the plane of the

truss. For attics without storage, this live load need not be assumed to act

concurrently with any other live load requirements.

g. For attics with limited storage and constructed with trusses, this live loadneed be applied only to those portions of the bottom chord where there are

two or more adjacent trusses with the same web configuration capable of

containing a rectangle 42 inches high or greater by 2 feet wide or greater,

located within the plane of the truss. The rectangle shall fit between the top

of the bottom chord and the bottom of any other truss member, provided that

each of the following criteria is met.

1. The attic area is accessible by a pull-down stairway or framed opening

in accordance with Section R807.1.

2. The truss has a bottom chord pitch less than 2:12.

3. Required insulation depth is less than the bottom chordmemberdepth.

The bottom chords of trusses meeting the above criteria for limited storage

shall be designed for the greater of the actual imposed dead load or 10

psf, uniformly distributed over the entire span.

So this comes down to an attic height determination that drives the requirement. In the example that I gave, it is a 24' wide home with a 6/12 pitch. That comes to about 72" of head clearance which is equal to or greater than 42", therefore it cannot be an attic with no storage.

Therefore, in attics with 42" or more of vertical space from the top of the ceiling joist to the bottom of the rafter, you cannot declare it no storage and must use the limited storage span charts for ceiling joists.

 

[Jobsaver]

This reminds me of the "what constitutes a bedroom" debate and threads, (room with a closet, etc.). I am still on the 2006. More controversy on the horizon.

 

[globe trekker]

So now we're cattle huh?

.

 

[SCBO1]

Jar, Thanks!

So often I just go to the span charts, I'm glade you alerted us to refer to R301.5.

pc1

 

[jar546]

Footnotes, footnotes, footnotes

 

[steveray]

If it has an access...it will be storage.....figure it as so.....R802.4(1) and (2) address without storage 10LL and with 20 LL....