Roof Sheathing 7

[Dave169]

A contractor in my area recently sheathed a roof with 7/16" 24/16 OSB on 24" O.C. joists. Unfortunately for him, the system he installed is rated at 40 psf allowable live load which is lower than the snow load requirement of 55 psf.

If he adds another layer of sheathing to the roof will the allowable live load double?

Perhaps someone can think of another economical way to bring his roof to code.

Thanks in advance for any help.


David

 

[Lutfi]

I do not think your roof load carrying capacity will double by merely adding another layer! Basically what you will get is twice the dead load on the roof.

To gain more capacity, you need to integrate the two layers by an acceptable means so that they act as one homogenous system.

I have not had personal experience in doing that. However, I would contact some of the industry associations and see if they can point you in the right direction. Plywood is layered and glued to achieve its strength. I am not sure about OSB.

Another option, make him replace it with the right product.

I never been a fan of OSB and will never be.

Good luck


Regards,
Lutfi

 

[37ed]

Adding layers will only add insult to injury.

As stated above, now we have changed the designed of the dead load.

Also this question my seem off hand,but what is the exposure of the OSB? anyway.

Adding layers also does not give the truss engineers a chance to change there design if nessary.

I can not think of a way
to let the added plywood expand,and to secure it to your existing ply wood at the same time.I have tried before.

In other words, the top sheets would need to be secured on top of the lower sheet with no over laps.As your roof heats, and cools there will be contraction, and expansion.If secured wrong the top sheet will try to pull the bottom sheet free from the truss.
This would not be noticeable for awhile, but it happen to me on a 26,000 sf church.
What a mess that was!
TEAR IT OFF.
Good Luck
Ed

 

[boo1]

Higher live loads can be achieved by placing the supports closer then the maximum span indicated on the APA Rated Sheathing trademark. Consider adding blocking at 16" oc or less to reduce the effective span. That would be the most cost effective method to achive the design set points. Also ensure the proper nailing for the blocking and the sheathing is achived.

See lit. at:

http://www.apawood.org/level_c.cfm?content=pub_osb_libmain

 

[DaveAtkins]

First of all, are you sure 55 psf is the ACTUAL snow load, and not the base snow load? 55 psf sounds heavy to me.
Second of all, yes two layers of 7/16" OSB will be twice as strong as one layer, without being bonded together. But you are adding dead load, so that needs to be considered.

DaveAtkins

 

[miecz]

Agree with Dave. Seems like it should be twice as strong.

 

[Dave169]

Thanks for the replies,

It seems there is a difference of opinion whether two layers is twice as strong if they are unbonded. It does add more dead weight but only 1.5 psf. Intuitively, I think two layers unbonded would be stronger but how much, and how can I model this?

Since 37Ed, had a bad experience with two layers due to expansion and contraction I will not recommend this solution. Boo1’s recommendation of additional blocking seems to be the best solution for this case since snow loads are heavy.

Thanks again for all the help.


David

 

[boo1]

Two layer act as a composite section with proper nailing, therefore increase the load rating. But, there may be a moisture diffential problem to consider.

Blocking would be the cheapest route for the contractor to meet the sheathing load ratings. (Win - Win for both of you) With residential construction "Engineering Solutions" should not bankrupt the builder, just enshure the design meets the requirements.

Also recommend checking the truss sheets for the loading cases to insure the 55 PSF was used for the design.

Cheers
Boo1

 

[pba]

The capacity of your sheathing to resist snow load is rated at 40 psf - I would assume that this would already account for the self weight of the sheathing.

If you overlay with a second layer, the capacity should double to 80 psf, assuming the two layers are in direct contact with each other. Load applied to the outer sheet will cause both sheets to deflect an identical amount so the load would be carried equally by the sheets.

The support structure must be capable of supporting the additional load form the extra layer.

Blocking as described in an earlier post would be lighter and cheaper in terms of materials but not necessarily in material cost - That is one for the contractor to consider.

Now if you could glue the two layers together simplistically you could take four times the load...

 

[boo1]

You have too block all our hips any way to have the correct span. We use the culled studs and scrap cut offs, less in the dumpster too.

With the escalation cost of sheathing products, blocking would be 5-10% of the cost of resheathing.

 

[miecz]

If you add blocking to reduce your span, then you'll need to ensure that the plywood can carry the load in its weak direction. The "recommended uniform live loads for sheathing with the long dimension perpendicular to supports" no longer applies, because now your outer ply is parallel to your supports.

 

[boo1]

not plywood, its OSB.

 

[miecz]

Sorry, missed that

 

[37ed]

Blocking is going to transfer loads on the trusses that are not meant to be.

The bottom cord on the church trusses I was talking of before tried to snake out under the new loads.the span was only 30ft.and yes there was bracing.

More is not always better.More OSB,and blocking may be the cheapiest fix,but I think a"do over"is a better long term solution.

Sometimes we get in to these situations where we are trying to fix a mistake.Even costly ones.

In my situation the contractor and I agreed that the best fix was to start over,and it was costly.

But all said and done we had a happy customer,we sleep well,and we both got more work from that customer because we did the right thing in his eyes.thats what counts.

 

[boo1]

In an earlier post I recommended the trusses be reviewed to ensure the 55 PSF Snow Load was used in the design. If the truss loads are wrong then I would agree with tear out and rework. (typically I find input truss data correct, but find they use totally enclosed coef as default)

My experience (20 years residential construction) has found truss blocking does not create problems in transfering loading. I guess the truss company could be asked that question. These loads would probabbly only bee seen after the roof is completed in winter.

37Ed the bottom cord deflection you describe apper to be a gross contruction overload. It appears what you had is a truss design issue, not a sheathing loading.

 

[akastud]

Note of caution: 2x6 blocking at 16" O.C. Also adds 1.5 PSF of Dead Load to the system.

akastud

 

[boo1]

Was the truss system 2x4 or 2x6?

 

[Dave169]

Hi boo1,

The truss system is 2x6 and was designed for the 55 psf snow load.

Thanks for your help.

David