Attic Truss Loading

[XR250]

I have some customers that have a 7 year old house with an attic served by a pull-down stairs. The roof/attic is pre-engineered trusses spaced 24" O.C. The attic space is 8 ft. wide x 8 ft. tall x 20 ft. long (10 trusses) . The trusses span 38 ft. and have 2x6 top and bottom chords. The truss drawings are unavailable.
They want to use this space as a bedroom. What are the chances the trusses were designed for a 30 PSF live load in the attic area?

IRC says this...

Uninhabitable attics with limited storage are those where the clear height


between joists and rafters is not greater than 42 inches, or where there are


two or more adjacent trusses with web configurations capable of


accommodating an assumed rectangle 42 inches in height by 24 inches in


width, or greater, within the plane of the trusses.


That is for 20 PSF live load. So I would assume if the space is taller than they would have to use 30 PSF?

 

[jerseyshore]

I have some customers that have a 7 year old house with an attic served by a pull-down stairs. The roof/attic is pre-engineered trusses spaced 24" O.C. The attic space is 8 ft. wide x 8 ft. tall x 20 ft. long (10 trusses) . The trusses span 38 ft. and have 2x6 top and bottom chords. The truss drawings are unavailable.
They want to use this space as a bedroom. What are the chances the trusses were designed for a 30 PSF live load in the attic area?

IRC says this...

Uninhabitable attics with limited storage are those where the clear height


between joists and rafters is not greater than 42 inches, or where there are


two or more adjacent trusses with web configurations capable of


accommodating an assumed rectangle 42 inches in height by 24 inches in


width, or greater, within the plane of the trusses.


That is for 20 PSF live load. So I would assume if the space is taller than they would have to use 30 PSF?

For older houses I feel like those are lucky to work for 10 psf DL/ 10 psf LL.

For a newer build, I'd expect (hope) that it works for 10 psf DL/ 20 psf LL if it has a pull down stair, but I would not be surprised if it was only 10/10.

30 psf LL would be really surprising though unless it had a fixed stair.

 

[XR250]

Just talked to one of the local truss companies. They said most likely 20 PSF LL - possibly 30 PSF but since it was a. production builder they would bet 20 PSF.

 

[phamENG]

I'd say zero chance they did 30psf. Especially if you've only got 2x6 bottom chords. Occupied space is usually 2x10, at least under the occupied areas, I've seen them run a panel point or two past for longer spans.

 

[LittleInch]

The attic space is 8ft x 8ft x 20ft long but the trusses span 38 feet?

Needs a sketch here.

 

[jdgengineer]

I have some customers that have a 7 year old house with an attic served by a pull-down stairs. The roof/attic is pre-engineered trusses spaced 24" O.C. The attic space is 8 ft. wide x 8 ft. tall x 20 ft. long (10 trusses) . The trusses span 38 ft. and have 2x6 top and bottom chords. The truss drawings are unavailable.
They want to use this space as a bedroom. What are the chances the trusses were designed for a 30 PSF live load in the attic area?

IRC says this...

Uninhabitable attics with limited storage are those where the clear height


between joists and rafters is not greater than 42 inches, or where there are


two or more adjacent trusses with web configurations capable of


accommodating an assumed rectangle 42 inches in height by 24 inches in


width, or greater, within the plane of the trusses.


That is for 20 PSF live load. So I would assume if the space is taller than they would have to use 30 PSF?

I think you had a typo from the definition of "limited storage" in the IRC. It is defined "where clear height between joists and rafters is 42 inches or GREATER...", not less than. For less than 42 inches I believe is the attic is defined as no storage (10 psf, nonconcurrent live load).

If there is a clearly defined attic in the truss (which it sounds like there is). I think there is possibly a chance it was designed for higher than 20 psf load, but it would be hard to confirm without the truss drawings. Is there a way to get in touch with the original truss designer? For a 7 year old house I would think they would be reachable.

 

[XR250]

I think you had a typo from the definition of "limited storage" in the IRC. It is defined "where clear height between joists and rafters is 42 inches or GREATER...", not less than.

I cut and paste it from a digital version.

 

[jdgengineer]

I cut and paste it from a digital version.

Odd. It's definitely a typo.

Which digital version? ICC's website and upcodes both say greater. (Footnote g, Table R301.5)

 

[jdgengineer]

Have you checked to see if the building department has records? Around here, truss shop drawings is a required building department submittal and the building department keeps copies of all submittals. Anyone can go in and view (but not copy without permission) them since they are public records.

 

[XR250]

Odd. It's definitely a typo.

Which digital version? ICC's website and upcodes both say greater. (Footnote g, Table R301.5)

Have you checked to see if the building department has records? Around here, truss shop drawings is a required building department submittal and the building department keeps copies of all submittals. Anyone can go in and view (but not copy without permission) them since they are public records.

It's a PDF copy that was given to me a few years ago. I'll make note of the error - thanks. Yes, the building department had been contacted to no avail.

 

[gte447f]

I would say 20 psf tops, unless you want to analyze and verify capacity.

If you were designing these on the cheap you might even say only 10 psf if the required insulation depth is greater than the depth of the 2x6 bottom chord (per IRC table footnote).

 

[XR250]

I would say 20 psf tops, unless you want to analyze and verify capacity.

If you were designing these on the cheap you might even say only 10 psf if the required insulation depth is greater than the depth of the 2x6 bottom chord (per IRC table footnote).

The area in question is over the garage so it is not insulated.

 

[gte447f]

The area in question is over the garage so it is not insulated.

Then I would say the proper design live load should have been 20psf.

 

[Ron247]

If you are saying there is an open rectangle in the truss that is 8'x8' that is 20 feet long but it was intended for storage, it would be 20 psf. The lack of a fixed stair or it being for storage (uninhabitable) kicks it out of the 30 psf world from what I am seeing. You are left with the 20 psf likelihood. How long is the 2x6 bottom chord between gussets?




Anyone know why attic LL does not have to be combined with Roof LL?

 

[Ron247]

 

[XR250]

How long is the 2x6 bottom chord between gussets?

8 ft. clear.
Honestly, there is a wall supported on a foundation right below and next to the edge of the attic space and there are a diagonal and vertical web intersecting on the wall. I will likely just sister those webs and sister the bottom chord with a 2x10 (so they can insulate under it) and call it a day. Adding 10 PSF over and 8 ft. length of a 38 ft. truss is not going to matter. And let's face it, if the room sees more than 10 PSF live load, I would be surprised. In our area, the trusses are designed with a TC and BC dead load of 10 PSF each. It is typically about 7 PSF for the TC with two layers of shingles and the bottom chord is about 4 (insulation and sheetrock) over the remainder of the truss so there is some additional capacity there anyway. There is no storage possible in the other portions of the truss.

 

[Ron247]

SYP 2x6 at 24" on center are good for 8'-6" with 10 psf DL and 30 psf LL. Only question to add is how much tension is present due to it being a bottom chord.

 

[XR250]

SYP 2x6 at 24" on center are good for 8'-6" with 10 psf DL and 30 psf LL. Only question to add is how much tension is present due to it being a bottom chord.

i figure the wall is helping to mitigate a fair bit of the tension in the bottom chord of the truss.

 

[jdgengineer]

If you are saying there is an open rectangle in the truss that is 8'x8' that is 20 feet long but it was intended for storage, it would be 20 psf. The lack of a fixed stair or it being for storage (uninhabitable) kicks it out of the 30 psf world from what I am seeing. You are left with the 20 psf likelihood. How long is the 2x6 bottom chord between gussets?

View attachment 10509

View attachment 10511
Anyone know why attic LL does not have to be combined with Roof LL?

I believe the 10 psf non concurrent live load is not combined with roof live as it is intended to represent someone crawling in the attic which is unlikely to occur at the same time as the roof live load.

20 psf storage loading should be combined.