Attic joists flex noticeably under my weight - implications for construction?

[gtae07]

I started placing the ceiling joists in this weekend, and noticed that when I hang on them there's noticeable deflection/bounce. They were sized right at the IRC's limit for uninhabited attics without storage, as the overhead clearance will be very low and not worth the trouble for storing much of anything. My concern is that I've decked over the center portion so it can serve as a temporary platform for setting the ridge beam and rafters, and installing the sheathing on the gable ends. Is it going to be a problem? I can limit it to one person on the platform at a time.

Once the rafters are in (they're oversized; hurricane manual sizes them to 2/3 of the 20psf snow load span, and we don't get snow here) I was considering tying them to the joists, creating a truss of sorts, but that doesn't help me before they're installed. I can also put in temporary columns down the center if I have to, but that'll eat a day plus the lumber.

I'm not concerned about it once the roof is in place, just during the construction phase.

 

[dutchgray]

I wouldn't worry about it, they are bound to deflect if you've got all your weight on one joist and there isn't anything in place to stop them moving independently.

 

[bczygan]

Did you size them for a deflection that allows for a ceiling?

 

[gtae07]

Did you size them for a deflection that allows for a ceiling?

Yes, for R30 insulation and 1/2 drywall, and 1/2in OSB decking.

 

[little d]

As you go along and add rafters, I would tie them in like you asked. It will stiffen them up where your working and after, you'll have a stiffer roof/ceiling for very little investment.

 

[gtae07]

Sounds good then, thanks.

Another question: Will installing a pulldown attic ladder before the roof is on cause a problem if it rains? It would make access during construction a lot easier (I'm worried about it falling over as I try to get off the decking; Dad broke a wrist and 3 ribs doing that a few years ago), and would free up my other ladder for use somewhere else without having to drag it back and forth.

 

[bczygan]

Yes, for R30 insulation and 1/2 drywall, and 1/2in OSB decking.

Your weight is a point load. Design is usually for a uniformly distributed load. What do you weigh? Were you at midspan?

Joists are not connected with ceiling material or plywood deck above or blocking, to spread the load and resist sideways movement.

What were your calcs?

Species and size of lumber?
Span?
Spacing?
Deflection limit?
Loads?

 

[DekeT]

I started placing the ceiling joists in this weekend, and noticed that when I hang on them there's noticeable deflection/bounce. They were sized right at the IRC's limit for uninhabited attics without storage, as the overhead clearance will be very low and not worth the trouble for storing much of anything. My concern is that I've decked over the center portion so it can serve as a temporary platform for setting the ridge beam and rafters, and installing the sheathing on the gable ends. Is it going to be a problem? I can limit it to one person on the platform at a time.

Once the rafters are in (they're oversized; hurricane manual sizes them to 2/3 of the 20psf snow load span, and we don't get snow here) I was considering tying them to the joists, creating a truss of sorts, but that doesn't help me before they're installed. I can also put in temporary columns down the center if I have to, but that'll eat a day plus the lumber.

I'm not concerned about it once the roof is in place, just during the construction phase.

Why go all cheap with joists and then spend time and resources expens to beef them up later? Right size them the first time.

 

[gtae07]

Your weight is a point load. Design is usually for a uniformly distributed load. What do you weigh? Were you at midspan?

Joists are not connected with ceiling material or plywood deck above or blocking, to spread the load and resist sideways movement.

What were your calcs?

Species and size of lumber?
Span?
Spacing?
Deflection limit?
Loads?

Span is 23', 2x10 Southern pine #2, 24" OC, L/D=240, 5PSF dead and 10PSF live load. Table R802.4 from the 2012 IRC. I'm 200lb, and was at mid-span. Right now the joists are tied together with 2x4s 6' in from the walls, and OSB decking in the middle 12' span, also tied to the gable wall.

I am familiar with point vs. distributed loads, though I generally am applying them to aircraft and not buildings. I expected noticeable deflection; I'm just wondering in hindsight if it'll be acceptable for short-term construction loads--I didn't at first consider a platform like this as I've never built something this involved before.

 

[gtae07]

Why go all cheap with joists and then spend time and resources expens to beef them up later? Right size them the first time.

By the span tables in the code, I did right size them for the intended use. This won't even really be a storage attic, with only 3' of headroom. No sense making a full-blown floor for that. Maybe it's my fault for not considering the load case of walking around up there during construction, but it's not one that occurred to me till about the time I started working up high and realizing it's a long way down to be working off a ladder...

 

[Beemer533]

Sounds good then, thanks.

Another question: Will installing a pulldown attic ladder before the roof is on cause a problem if it rains? It would make access during construction a lot easier (I'm worried about it falling over as I try to get off the decking; Dad broke a wrist and 3 ribs doing that a few years ago), and would free up my other ladder for use somewhere else without having to drag it back and forth.

I would just tie off the top off the ladder, which is generally a good idea anyways with an extension ladder.

I was up working on a flat roof a couple years ago and one of my guys didn't tie it off (I know, I should have double checked) and the wind blew it over.. We were real lucky it didn't hit anyone or land on a car..

 

[Kevin54]

If I were you, I would step up the decking to 3/4" minimum. This will help carry the load across all of the joist. !/2" will deflect in itself, therefor deflecting a single joist. By installing thicker decking it will not deflect as much on one point and will carry the load over a few joist.

Also how close together are your joist.....16" or 24"?

As far as deflection on one joist, that will be normal until you have them tied in with your decking. But DO NOT tie it to your rafters as that is not necessary. Once you get your decking on, you will notice a huge difference. And also going thicker, you can use T&G decking which will also help to tie things together.

Good luck, and lets see some pics. Rules state that we all like pics and need to see pics.

 

[gtae07]

If I were you, I would step up the decking to 3/4" minimum. This will help carry the load across all of the joist. !/2" will deflect in itself, therefor deflecting a single joist. By installing thicker decking it will not deflect as much on one point and will carry the load over a few joist.

Also how close together are your joist.....16" or 24"?

Too late, the 1/2" was already in by the time I saw this... but it's 24" OC. It does feel a lot stiffer up there already; I didn't go up and walk around but I leaned on it and such from the ladder and it's a lot better than it was yesterday.

Good luck, and lets see some pics. Rules state that we all like pics and need to see pics.

New update posted (see sig)

 

[bczygan]

240 and just meeting the requirements in the table, and not having tied them together yet, with the decking, and you will definitely feel it, when walking on them, but they won't fail unless you give them as sideways load at mid-span sufficient to do it.

You're not hoping to DW the ceiling with l/240 are you?

Another thing to consider is that 1/2" standard drywall is meant to span 16". 5/8" will span 24", but is heavier. You could use a lightweight version of 5/8" to save weight, if is is designed to span it. Or you could use strapping perpendicular to the joists, to get the 16" o.c.

There is also a high strength 1/2" DW made to span 24".

Don't know how the l/240 will work with any of these and how finished of a ceiling you need.

 

[gtae07]

You're not hoping to DW the ceiling with l/240 are you?

I'd planned on drywall.

 

[volleyball]

I would think there is no bridging due to its purpose. Add bridging and it will help with the deflection

 

[little d]

Checked out your build, nice job but, for further references, it is easier to build the walls and attach the gables to that. The way ya did it is ok, just more work. Also, I like building a pocket (like the jig you made) in the gables to hold the ridge board, you can extend it out past the framing for the soffit/fascia (makes it easier to attach the framing) .

 

[gtae07]

Checked out your build, nice job but, for further references, it is easier to build the walls and attach the gables to that. The way ya did it is ok, just more work. Also, I like building a pocket (like the jig you made) in the gables to hold the ridge board, you can extend it out past the framing for the soffit/fascia (makes it easier to attach the framing) .

This arrangement for the gables is preferred by the hurricane build standards and the local inspectors. Without it I'd have a higher parts count and have to deck the underside of the rafters with OSB or ply.

The rafters on the ends (which will sit right on top) will have the pocket for the ridge board.

 

[bczygan]

Worth reading:

http://support.sbcindustry.com/images/technotes/T-GypsumInstallation09.pdf

 

[little d]

This arrangement for the gables is preferred by the hurricane build standards and the local inspectors. Without it I'd have a higher parts count and have to deck the underside of the rafters with OSB or ply.

The rafters on the ends (which will sit right on top) will have the pocket for the ridge board.

AW, ok. I've never built in earthquake or hurricane country so some of your building is different then we do in the midwest.

 

[Flivver250]

Maybe you need to go on a diet.