Roof Geometry: Hip Roof vs Other (Gable)

This topic covers section 5 of the wind mitigation form. It covers questions about Roof Geometry for structures built in compliance with the Florida Building Code, South Florida Building Code, and previous model building codes that were in force at the time of construction. It also provides information on retrofit requirements and upgrade connections that relate to roof geometry such as gable end bracing, roof deck attachment, and roof-to-wall connections. All questions and answers provided in this section are geared towards helping inspectors and home owners better understand the use and completion of the form.
User avatar
Robert R Sheppard
Site Admin
Posts: 0
Joined: December 10th, 2016, 11:51 am

Roof Geometry: Hip Roof vs Other (Gable)

Postby Robert R Sheppard » March 14th, 2017, 6:04 am

Why is the inspector measuring the perimeter of my home?

It’s a good question, why is the inspector measuring the perimeter of my home and for what purpose? The truth be told, even the inspector may not fully understand why they are doing this other than to provide measurements on a form. It has been stated by construction industry professionals (including Engineers, Architects, and Insurers) that a hip (hipped) roof structure will resist wind damages better than a gable roof structure simply due to the shape of the roof.

The thought process goes something like this: The slope of the roof structure down to the exterior supporting wall (hip roof) creates a less resistive path for high winds (lateral loads) to be more efficiently redirected around the structure. Where a gable roof structure (gable roof, see diagram below) creates flat vertical profiles at the end walls that forces wind to abruptly change direction creating higher wind pressures at the exterior walls, corners, and surface area of the roof structure.

Image

So, what’s the issue?

Increased wind pressure does not necessarily mean your homes roof structure will be damaged during a storm. Your home may be designed and constructed to offset any inefficiency in roof shape that produces increased loads. Through the design and construction of the roof structure, it absorbs additional pressures/loads and redistributes those forces throughout various points within the framework.

There is absolutely zero evidence to support the conclusion that the “shape” of your roof structure alone will affect it’s potential to resist wind damages. ZERO evidence!

This is especially true if your home is constructed in compliance with the Florida Building Code, or your existing home has been reinforced with newer and approved mitigation techniques. You heard that right, the shape of your homes roof structure will play little to no part in its’ ability to resist wind damages during the next wind storm, it’s method of design and construction will.

With that being said, an existing homes’ roof structure that is built to a sub-standard building code, is damaged in any way, or was not adequately reinforced (retrofitted) or constructed, will definitely have an effect on its’ potential to resist wind damages (this also applies to newer construction). The only evidence that exists to support any conclusion otherwise is that certain shapes of roof structure may experience higher or lower wind pressures at specific locations over other shapes. That’s it! Higher or lower wind pressures, not eminent damage.

There can only be one conclusion, it is roof design and construction, not roof shape, that determine a roof structures ability to potentially resist wind damages. The overall shape of your homes roof structure plays little role in how it may potentially mitigate wind forces. Degree (both low and high) of slope, external mechanical forces (such as impact from flying debris), pre-existing damage to the roof structure or roof system, or substandard method of design and construction will play a much larger role in a roof structures ability to resist wind forces than shape ever could.

Are there certain roof shapes that redirect wind forces more efficiently than other? Absolutely. Will the shape of your homes roof structure affect the wind pressures placed on the roof structure? Yes. Does this in any way give credence to the fact that certain roof shapes have a greater potential to fail during a wind storm? Absolutely not!

The shape of your homes roof structure will determine how “efficiently” wind will be redirected around its’ exterior, of that there is no doubt. But basing a roof structures’ potential to fail due to an increase in wind pressure experienced at the surface or exterior walls is unfounded and lacks even the most basic understanding of how we design and construct homes today.

Comparing a hip roof geometry to a gable (Other) roof geometry is like comparing a Formula 1 (F1) car to a Funny Car. While the F1 car may be aerodynamically superior in every way, the Funny Car (which is about as aerodynamic as a couch) produces much higher speeds than the F1 car and has a much quicker acceleration rate. The difference between the two? The Funny Car offsets any loss in aerodynamic efficiency with an abundance of power (strength).

Image

A homes' roof structure functions in the exact same fashion, any inefficiency in aerodynamic shape can be offset via reinforcing of the structure (strength) at various points within the roof construction.

The wind (loads) pressures a roof structure may experience are carefully and meticulously calculated during the design phase of a home. This means that the increase in wind pressure experienced due to the shape of a roof structure has been accounted for, that construction of the roof structure includes specific built-in mitigation techniques to resist those forces on a newer or properly updated existing home. The “shape” of the roof is therefore irrelevant on a newer or properly reinforced existing roof structure, especially those constructed or updated in compliance with the Florida Building Code.

If it wasn’t, why would we still be building roof shapes that have the potential to fail during high wind events?

It has been stated in numerous studies that a gable roof geometry will experience upwards of 40% more uplift forces than a hip roof geometry. Ok, but newer homes are typically over-engineered at 80-100%+ above what the building code requires for uplift resistance.

As an example, the Florida Building Code requires a minimum uplift resistance at the roof-to-wall attachment (integral in a roof structures ability to resist wind loads) of 900 foot pounds in west Broward County. The roof-to-wall connections I see most often used in this location are rated for over 2,100+ foot pounds of uplift resistance. The 40% increase in wind pressure due to roof shape doesn’t seem so worrisome now, does it?

Image

Other factors include the direction in which the wind approaches your home, if your home is struck by debris, if the other components of your roof structure have been updated (roof-to-wall connections, roof deck attachment, roof covering), if your roof structure is damaged in any way, or even how well your neighbors’ home is constructed. Most damages to newer roof coverings were a direct result of neighboring home debris from sub-standard roof coverings (Source: FBC Triage Team).

In fact, the damage probability for newer homes built in compliance with the Florida Building Code typically inverts itself when it comes to roof shape. Numerous Florida Building Commission studies have shown hip and ridge tiles receive the most damages on newer structures during a wind storm. Guess which type of roof geometry has the most number of hip and ridge tiles? You guessed it, a hip roof geometry.

But to understand why it has been falsely interpreted that shape alone may determine a roof structures potential to resist wind forces, you must first understand what issues gable roof structures face.

Gable end walls are exactly what the term says they are, a wall. Any wall, regardless of where it is situated on the exterior of a structure, will fail if it is not properly constructed, damaged, or inadequately reinforced. In the past, most gable end walls were constructed of the roof framing (trusses). This means that the last truss at the exterior end wall is used to continue the lower wall up to the roof line or bottom cord of the roof rafter.

But regardless of the method in which a wall is constructed, a gable end wall is still a WALL. Nothing more, nothing less. The shape of a roof structure is meaningless if the wall portion of the roof structure is deficient. It could potentially fail even without experiencing high winds.

Implying, or even attempting to conclude, that walls fail simply due to an increase in wind pressures that are at or below their design level is not only absurd but reckless. This implies that no matter how the wall is reinforced, it is deficient and will fail. When a gable roof structure is retrofitted (addition of bracing) to resist wind loads, it is not "reshaped", it is reinforced.

Gable end walls do not fail because of their shape, they fail because they are insufficiently reinforced at the truss system (when the truss system is used to frame the wall). But not all gable end walls are constructed of the roof framing. In many situations, the gable end wall is constructed of the exterior wall assembly extending down to the foundation. This can be wood frame or reinforced concrete construction and is the ideal manner in which to properly construct a gable end wall for reinforcement.

This means that the lower supporting exterior wall (below the truss system) extends up and continues to the roof line. This type of wall will not react in the same manner as a truss framed gable end wall that has not been properly constructed. Any wall built to a substandard building code, or not adequately engineered and constructed, has the highest potential to fail at the point where it is the weakest, the connection to the supporting structure (foundation or lower wall)

The following video shows two homes that are built identical to one another in exterior “shape” and roof geometry. Both have a Gable roof geometry (Other on the wind mitigation form). The difference between the two is the home on the left, looking at them from the front, has not been reinforced at various points within the framework connections (load paths). In the first test, the home fails at the foundation/exterior wall attachment, allowing the home to actually slide off its’ foundation and be destroyed.

https://www.youtube.com/watch?v=RD8ZZjeQmC4

Now, do we say the first home failed because of the “shape”, or do we say it failed because the walls were inadequately reinforced/secured? Remember, a wall is a wall regardless of where it is situated on the exterior of the home. The second test shows the roof structure blowing off the supporting wall structure while the reinforced home with the exact same Gable roof geometry stays intact.

Now, do you think the difference between the two roof structures (which are identical) was "shape" or "method of construction"? Doesn't get much simpler than that folks!

All evidence supports the conclusion that roof shape plays a very small role, if any, in a roof structures ability to resist wind damages. So, why is the Inspector measuring the exterior of my home? The current wind mitigation form uses “roof shape” to assess a home’s potential to resist wind damages. But, roof geometry does include mitigating factors for the complete classification of the roof structure.

Other sections of the form will contribute to your home receiving the full premium credit for a hip roof geometry, should your home qualify. If your homes roof-to-wall connections have not been updated, or your homes’ roof system (roof covering + roof deck attachment) has not been updated, you will not receive full credit for a hip roof geometry on your insurance premium. You will receive a percentage of the full discount for hip roof geometry only.

So how can you be sure your home is properly inspected and the wind mitigation form accurately completed to represent your homes current condition? I would recommend hiring a licensed Engineer, Architect, or Building Inspector to assure that your home is properly inspected and assessed against wind damages.

To finish, if your home was constructed “in compliance with” (emphasis added) the 2001 Florida Building Code, or the 1994 South Florida Building Code, your homes roof geometry is irrelevant. The inspector should leave the section of the form that covers “Roof Geometry” blank. The inspector may need to supply additional information with the form to support this conclusion, but either way you should be receiving a qualifying hip roof geometry for your home as all aspects of the roof structure were designed, engineered, and constructed specifically to resist wind loads.
Licensed Home Inspector #3289
ICC Cert. Residential Building Inspector B1 #8325007
ICC Cert. Residential Electrical Inspector E1 #8325007
ICC Cert. Residential Mechanical Inspector M1 #8325007
ICC Cert. Residential Plumbing Inspector P1 #8325007

User avatar
Robert R Sheppard
Site Admin
Posts: 0
Joined: December 10th, 2016, 11:51 am

Re: Roof Geometry: Hip Roof vs Other (Gable)

Postby Robert R Sheppard » March 14th, 2017, 11:03 am

As with all my articles, I like to give verifiable sources for the information I provide. This prevents any discrepancy between hearsay and verifiable fact. The following links are provided by the Florida Building Commission Building Code Triage Team. These studies are actual field inspection verification of post-windstorm damages experienced over the 2004-2005 Hurricane Season.

I will post the link and information referenced in each study that is relevant to the above article.


Hurricane Charlie: http://www.floridabuilding.org/fbc/publ ... HARLEY.pdf

FLORIDA BUILDING CODE (FBC) AND CODE COMPLIANCE: Residences designed to the Florida Building Code (FBC) and built since March 01, 2001, proved to be very adequate, which was repeatedly demonstrated in newly constructed homes that sustained only cosmetic damage (soffits) compared to older homes in the same neighborhood that had significant roof damage (failed barrel tiles, shingles, doors and windows). Measured wind gusts to 150 mph were recorded in Charlotte County, which exceeds ASCE wind design standards contained in the Florida Building Code.

The Triage Team examined over 30 individual damaged homes and an estimated 100 area surveyed [undamaged] homes in Port Charlotte, Punta Gorda and Punta Gorda Isle that were subjected to an estimated 145 mph winds, with 150 mph or greater gusts. Some of these homes had been in the eye of this storm and subjected to winds from both easterly and westerly directions. In one upscale neighborhood in Punta Gorda Isle, which required barrel tile roofs under a county ordinance, numerous older homes had sustained significant roof damage from 40% or greater in the ridge and field areas of the roof. The newer homes built to the FBC sustained 10% or less roof damage in those same areas, but this damage was directly attributable to the windborne debris from failure of the older roofs. Had not new homes in this same area sustained damage from the failed tiles of the older homes, they would have remained unscathed due to improved methods of installation.

On this metal building, we found significant indications of improper wall-to-foundation structural rebar ties, which most assuredly accounted for the failure of the east wall, which took the brunt of the wind force. The roof lacked proper bracing, as determined by a retired building official accompanying the Triage Team. Pictorial evidence was gathered at the site to substantiate these claims. The plans were not reviewed, nor building inspected by the County Building Department; deferring instead to the engineer for inspection of this threshold building. The building permit was issued based on an affidavit from the Design Architect. According to the local building official, the breakdown apparently occurred by the contractor and threshold building inspector. Subsequent investigation by the Triage Team revealed that initial approval by DCA of this building as a designated hurricane shelter was based on acceptance of affidavit of the design architect, attesting to the adequacy of the design as qualifying for the shelter program.

No significant structural damage was observed on any homes built to the FBC (since March 2001), except for some minor ridge shingle failure due to the roofer not using a longer roofing nail. The longer nail is needed for full penetration of 2-3 layers of shingles, underlayment and sheathing on the ridge. Code compliance within Polk County and the cities of Bartow and Lake Wales is vigorously being enforced, which accounts for the survivability of these homes and the adequacy of the FBC





Hurricane Dennis: http://www.floridabuilding.org/fbc/publ ... ennis2.pdf

As far as roofing is concerned, the tile roofs continue to be a large problem. The ridge and hip attachments are the largest part of the problem. These tiles break away and fly into the field of the roof breaking up the tiles in the field of the roof. Then all of it peels away creating large areas of windborne debris that destroys the house adjacent to them. Several houses might have been fine with minimal damage, but due to the broken tiles from the house next door their siding and windows were compromised causing massive water infiltration.

The newer shingles seemed to be doing well. The edges are the biggest failure point on this type of roof system. The eave drips are not getting enough nails and are pulling away. Properly installed new shingles held up surprisingly well.

The products in the field are not being installed as the products were tested. There are many failures that did not have to occur but due to substandard installation practices and poor inspection procedures there is a lot getting through a big crack. We can build great structures that withstand some of the toughest storms but the weak link can ruin the whole house. After a storm more improper material installation violations should be handed out. Not necessarily to hurt the contractors but to send the message that installers will be held accountable for proper installation methods.





Hurricane Ivan: http://www.floridabuilding.org/fbc/publ ... E_IVAN.pdf

FLORIDA BUILDING CODE (FBC) AND CODE COMPLIANCE: Code compliance is being vigorously enforced in Santa Rosa County, which accounts for the minimal damage to homes built to the FBC (since March 01, 2001) and the adequacy of the FBC, except for tree damaged roofs. Substantial roof damage was observed in this county to both pre-FBC built residences and older mobile homes.




Hurricane Jeanne: http://www.floridabuilding.org/fbc/publ ... E_JEAN.pdf

FLORIDA BUILDING CODE (FBC) AND CODE COMPLIANCE: Residences built to the FBC (since March 2001) survived the storm with little or no damage, attesting to the vigorous code enforcement by the County Building Official. This included roofs with three tab composite asphalt-fiberglass shingles, which, when properly installed, did indeed survive major hurricane force winds. Few barrel tile roofs were observed in St. Lucie County in the residences that the team inspected. Horizontally blown rain penetrated some CBS construction and window installations and may be an area for further study by the Commission.





Hurricane Wilma: http://www.floridabuilding.org/fbc/publ ... _WILMA.pdf

PALM BEACH & BROWARD COUNTIES: Widespread power outages were observed, causing major interruptions in services to residents and businesses, including food, and gasoline. Water was not interrupted in these counties, but was a problem for portions of Miami-Dade County. Some intersections had traffic signals controlled by emergency generators, with the rest becoming four way stops or police controlled. Trees downed by the high winds caused major damage to the power grid. Some older residences, subjected to the stronger winds, did not fair well, as expected. Numerous privacy fences and pool enclosures, came down in the winds, creating a potential danger to small children in the neighborhood. Some tree-damaged homes were observed, and older pre-2001 FBC construction suffered wind damage. Leading edge and ridge barrel tiles on residential construction continue to be a problem, either from poor installation or inadequate code/manufacturer’s requirements on both pre and post 2001 FBC construction.
Licensed Home Inspector #3289
ICC Cert. Residential Building Inspector B1 #8325007
ICC Cert. Residential Electrical Inspector E1 #8325007
ICC Cert. Residential Mechanical Inspector M1 #8325007
ICC Cert. Residential Plumbing Inspector P1 #8325007


Return to “Roof Geometry”

Who is online

Users browsing this forum: No registered users and 1 guest