wind damage to roof tile

huangyhg

wind damage to roof tile
i live and work in south florida.  as you may all know we have suffered several hurricanes over the past 2 years.  alot of the homes in this area have cement tile roofs.  recently i have received reports from some engineering firms hired by the insurance companies that have been stating in unison that wind does not affect the structural integrity of a cement tile roof.  they claim it can always be tied to poor installation.  my arguement back is the uplift pressures exerted on these roofs during the wind events are enough to cause these tiles to dislodge and release from their cement pads.  i believe deflection of the roof decking has more to do with it than poor installation.  any opinions?  are there any studies that anyone can point me to to get more information on this phenomenon.
friends who rode out andrew's eyewall, and didn't know each other,  described failure of terra cotta tile roofs "bedded" in mortar pretty uniformly in words like this:
>>> the wind was howling and pushing in the garage door (horizontal deflection reported as approximately two feet in the middle of a double-wide door).  the door was holding until the neighbor's roof landed against it.  the entire roof, all at once.  every damn tile.  then the door caved in, and immediately, all the windows on the opposite side of the house blew out.  and our roof exploded and landed on the next house.  <<<
i was nailing tarpaper on salvageable roofs for two weeks afterward, and got to walk and drive through many affected neighborhoods.  there was plenty of broken mortar on the streets and yards to go with the broken tile.  there wasn't a single piece of roofing bigger than a silver dollar anywhere.  
[ no, check that.  the perforated amber plastic strips that are supposed to keep asphalt shingles separated in the bundle but allow bitumen to bleed through in the sun and adhere the shingle tabs were intact and blowing everywhere.  i found no evidence that even a microliter of bitumen had ever bled through the perforations.  but that's a different rant. ]
given the reported violence with which entire roofs of tile were launched and landed on adjacent structures, i'd have to assert that even properly "mudded" tile roofs might succumb to a "zipper" failure across a neighborhood.
but i can't support an assertion of uniformly poor installation in neighborhood after neighborhood of houses erected by different builders at different times over tens of square miles.
all of that is, of course, inexpert opinion and hearsay.
my inexpert opinion also supports your argument.  the very idea that something as brittle as mortar could be an effective adhesive for bonding tile to something as flexible as a wood framed roof structure, even a trussed one, is just, well, silly.
mike halloran
pembroke pines, fl, usa
northern1...mike's observations and comments are excellent and right on target from a forensic viewpoint.  (hey mike...not bad for a non-building guy!)
while roofing in general in south florida and many other places suffers from poor workmanship, it is not always the cause of failures.  this is particularly true of tile roofs.  tile roofing is not done by every guy with a pickup truck who calls himself a roofer.  it has almost always been an exclusive task of those who considered themselves "craftsmen".  while that has changed over the years to more inexperienced roofers applying roof tiles, workmanship on tile roofs is, in general, better than workmanship on other roof covering types.
having said that, i have investigated roof system failures of many different types (i practice primarily in florida).  i have seen poorly installed tile roofs and well-installed tile roofs.  you are correct in your assessment that deflection is a significant contributor to tile roof failures on tile that is "mudded" in place.  most tile is not attached by that procedure (for many years it has been required to be mechanically attached by wiring in place or direct attachment).  mudding is supplementary attachment or "finishing" and reserved for rake, hip ridge, and ridge conditions.
the most common workmanship mistake i see is improper exposure ("stretching" the tile...which increases the moment on the attachment under wind load), and the placement of only one fastener when two are required.  on occasion i see tiles with no fasteners at all.
a proper investigation requires that all of those conditions be considered.  rarely is there a single cause but usually several contributing factors.  as mike noted, during a storm like andrew, everything is getting stressed and the zipper effect will eventually take over.
...and one other thing..
the best motel marquee i've ever seen was on a little motel along us 1 in fort lauderdale right after hurricane andrew.  it said "no pets, no roofers"!
i see there are more early risers than most would like to admit!  thanks for the post.  i battle these problems with insurance engineers all the time.  i seems odd that there could be no other reason for a tile roof to fail than installation. i have one roof that is a nail down install where the nail are uniformly pulled out of the roof decking and protrude about 1/2 inch over the field tiles on the sw slopes of a home in coconut creek and the insurance engineer stated that that was caused by the nail gun loosing air pressure during the install and not fastening properly.  the 110mph +/- winds that lifted all the tiles on one half of the roof had nothing to do with it!  claim denied. time for court!
northern1...you might want to check to see if a nail gun was actually used.  hand nailing is still the method of choice by most tile installers.  too hard to hit that little hole with a nail gun.
with a little forensic prowess, you can prove that the wind pulled those nails out rather than them not being installed all the way to begin with.  also, wind "chatter" can cause that; on flat tiles particularly.
ron,  trust me i get it. these engineering firms appear to say exactly what the insurance company wants to hear.  remarkably all the reports i have seen for hurricane wilma have stated that installation was the problem and not high winds. one report i read today stated that the winds were not strong enough to cause tile to blow off this particular roof and hinted that the tiles most likely were damaged by age and other physical manupulation. whatever that is!? i am trying to gain a concensus of other worldly engineers who would have a broader vision.  i know the effects high winds could have on a roof. i have studied report after report from anywhere i can get them and they all say that high winds can and do cause severe damage to roof structures.  i am amassing as much opinion on this topic i can.    i value all of your opinions and thank all of you for posting replies.  
i forgot to mention that i rode out wilma.  the eyewall passed maybe a mile north of me.  i stepped out the front door to see a _tree_ being blown down the street.  not a branch; the whole tree.  so i turned around and went back inside.  that wind not lift a tile?  feh.
mike halloran
pembroke pines, fl, usa
my point exactly.  i rode through the eye. in boca.  i watched it as my roof blew off and i held the framing of a picture window that was bowing in from the pressure as my wife went into the unlit garage to get more 2x4s to reinforce the frame. (note to self, shutters only protect from impact!)  i think alot of people need to experience these forces for themselves.  then their theories of what could happen would change dramatically.
i am going through the same thing right now battling engineers who work for insurance companies.
right now we are fighting over two houses in the same subdivision.  the same engineer for the insurance company wrote both reports and it was amazing how close the reports resembled each other.
his claim is that the expansion/contraction of the mortar was what caused the damage, not wind (so it's just a very high coincidence that the damage appears right after a major hurricane).  he also makes the claim that is was poor construction practices, but never gives anything to back up this claim...he just says over and over that the damage is consistent with bad construction.
i watched him perform a roof inspection and all he did was walk the roof...he never even touched a tile then stated in his report that he did not find any loose tiles!
any similar damage to other homes in the area and where is most of the damage located... at ridge, eaves, ???
consistent with areas of maximum uplift?  i've often looked at the failure surface to try to determine if damage is fresh or old looking for inclusions, insect stuff, etc.  bad workmanship would likely be reflected in other issues that may pre-date the hurricane...
dik