I am curious to know how SIP's used on sloping roofs compare with conventional rafters in terms of resisting falling tree limbs/trees.

I plan to renovate/add to a small building in a wooded area with minimal disturbance to existing tall trees. When a rare large limb comes down, is it safer to have SIP's or 2x10's on 16" o.c.? Is data available anywhere that shows a comparison?

Thanks-

I have no data, other than a little common sense. A rafter is 1.5" wide out of 16". What are the odds that the point load of a falling limb lands directly on the rafter. I don't know whether the rafter or SIP would be stronger, I guarantee that the SIP will be stronger than the single layer of sheathing between the rafters.

mf071609;

Last July a 70 foot tree fell on the back of my own home, did not penetrate the skin, sustained only shingle damage and according to the tree remover it was bearing the full weight of the (upper portion)  tree

Holy sh*t! What kind of tree is that? The first year that I owned my house in Northern California, a single branch of one of the 14 80' tall Redwood trees fell onto my utility wires. That one limb, about 12 feet long, was sufficient to snap off my utility mast. My next door neighbor wasn't so lucky, one limb fell through her roof. Made about a foot wide hole in her living room. And then down the street, a tree up the mountain from another house fell, and demolished one half of the house - fortunately no one was at home at the time.

Dick;

That is a ChinaBerry tree, on my office addition, my wife and I heard it as it start ed to come down, sounded like elephants on the roof.

Well, this is a good question.  And I am sad to report that SIPs constructed from OSB do not offer more resistance to punching shear than conventional frame construction.  This is not completely unexpected since the materials of construction (OSB sheathing) is the same for both methods.

In test we have conducted in compliance with ASTM E1886 (large missile impact) the resistance to missile penetration of SIPs made from OSB skins was statistical similar to 2X on 16" centers.

If you would like to discuss this issue off list please contact me at:

SIP Engineering & Testing, LLC

Matt B. Phelps, P.E.

[email protected]

 

If I had to choose between OSB SIPs or lumber rafters at 16" o.c to resist a falling tree limb, I would choose OSB SIPs. The OSB SIPs have a foam core that is continuous and backs up the two continuous OSB skins with no voids. I think the OSB SIPs would act like a net and obsorb the impact of a falling limb. The top skin may get punctured, but the roof will flex and distribute the load of the impact and the bottom skin may just delaminate without puncturing and catch the limb. With lumber rafters you still have voids between the rafters where the sheathing is unsupported. So the closer spacing of the rafters is holding the sheathing even more rigid making it easier for a limb to punch through the unsupported area of the sheathing.

Now if you were using metal skinned SIPs for this roof, I could almost garentee that the panels would act like a 'net' and no limb would pentrate the roof.

I'm not sure how the 'large missile impact' tests were constructed or quantified. But I will assure you that in the real world, a typical falling tree limb will cause more damage on a raftered roof than it will on a SIP roof.

Ah, Mr. Shelby again,

The large missile impact test is performed in compliance with ASTM E1886 which includes the methods of the test, statistical analysis of the data and data reporting protocol. Your intuition may be right about the SIP being a more resistant to a falling tree limb, a little hard to quantify wouldn't you agree?

I can report that the testing we have done pursuant to ASTM E72 that OSB skin SIPs are much more resistant to shear forces than stud framed assemblies. I am not sure that the force the tree limb striking the roof could be classified as a shear force, but may be better described as a transverse load. This would also be good news for a homeowner who's home was struck by a falling limb as SIPs will also out perform stud frame assemblies for transverse loads; however, that was not the original question to which I responded.

Matt B. Phelps, P.E.
SIP Engineering & Testing, LLC
[email protected]

SIP achieves some of this robustness through a design principle to fail to a known-safe condition. The idea is that any change to the system has to be maintained through periodic refreshment. If the authority for a change isn't available to maintain the change, then it's not around to correct the change when new conditions cause it to be wrong.

polljimmy,

Wow man, you are on another level of understanding!

polljimmy,

Could you please explain what you mean so that a college professor can understand it.  I am lost.

HUH?