Hole in wood diaphram

[00Z]

On a remodel job I recently did the engineering on the homeowner has added a dormer to the existing roof cutting two 2'x4' openings in the roof. The holes are separated by one 2' bay. The dormer was not shown on the plans. Normally I would strap around openings like this. The homeowner already has the roof on. The dormer is lager than the openings. I hate to have him remove a portion of the roofing and possibly part of the dormer to install straps. Then again, I really don't want to provide the stamped detail requested by the inspector. What do you normally do in similar situations? Is there a code exception allowing some minimum opening I am not aware of. I guess I should just do the full analysis to determine the loads around the openings. Thoughts.....

 

[BAretired]

The two foot width between dormers cannot be relied upon to do very much. Perhaps you should consider it a 6' x 4' opening and run your analysis based on that.

BA

 

[msquared48]

I would not expect this to normally be a structural problem in a residentiasl roof diaphragm.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[00Z]

Thanks for the responses.

BAretired - That is a good point. I think your right it should be analyzed as a 6'x4' opening.

msquared - I was surprised the inspector was requiring engineering sign-off on something like this. Unfortunately he is.

IMO this is a situation where technically straps should be installed. However, the chances of this ever causing a problem is very, very unlikely. So now I am in the position of causing him extra expense to cover my a** (and potentially paying for my time/work)for what???

One part of me says it isn't my fault, he could have consulted me before doing the work. The other part says if it was my house and money I wouldn't be removing roofing to install straps.

 

[msquared48]

Yo me, this sounds like the inspector is justifying his job.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

Would the diaphragm have sufficient depth even considering the penetrations which were cut into it?

 

[woodman88]

At a minimum, I would required blocking, between trusses/rafters, glued to the sheathing and strapping applied from below, across each side of the openings.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[FSS]

If you don't think it is a structural problem (and that is entirely up to you and your engineering expertise/judgement), and the inspector is questioning things, then just write a letter saying it is OK. That's why we are engineers.

Not sure about your need for straps on the upper surface of the roof. Why can't you do things from underneath?

 

[00Z]

I don't have a problem with blocking and strapping underneath other than the fact that the sheathing is not nailed to the blocking. I didn't consider glue as an option.

I provided a detail without requiring removing roofing to install straps.