Tornados of the South - Wood Works Engineering,...

A p r i l 2 0 1 1

Tornados of the South

By

Bryan T. Readling, P.E. – APA Field Services Division

StructurAl performAnce of newly conStructed HomeS in nortH cArolinA, AlAbAmA, And GeorGiA

Tornadoes of the South – April, 2011

Form No. SP-1154 ■ © 2011 APA – The Engineered Wood Association ■ www.apawood.org

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bAckGround

Damage observations were conducted by APA after the April 16, 2011 tornados in Eastern North Carolina and

the subsequent events scattered across the South on April 27, 2011 including Tuscaloosa, Alabama and Griffin,

Georgia. Even in these powerful storms, investigations found that structural failure was often due to a lack of ade-

quate connections.

Building scientists and engineers use the term “load path” to describe how forces flow through a structure and connec-

tions and how the structure can be designed to resist wind loads. Many homes failed as a result of an interruption in

continuity along the load-path to the foundation. If poorly made, critical connections along the uplift load-path are lost

when the dead weight of the building is overcome by the wind force. A continuous load-path from the roofing and siding

(cladding), to the framing, and to the foundation must be provided for reliable building performance.

This study focused on the performance of homes constructed approximately within the last 10 years. There were

many similarities in observations with past APA Damage Assessments but the EF-4 and EF-5 tornados of April 27, 2011

were exceptional in their size, severity and impact on populations.

obServAtionS

Structural failure along the critical load-path was often located at the roof-to-wall connection. Most of these con-

nections were made using toe-nails through the roof framing and into the top plate of the exterior walls. Toe-nail

connections are weak because they rely upon the withdrawal capacity of nails, which is limited. Light-gauge metal

connectors provide good performance in wood framing because load is applied perpendicular to the nail shank,

instead of pulling the nail straight out. Toe-nail rafter connections are still prescriptively allowed in most non-

hurricane areas by modern building codes. It is generally recognized that these connections may not provide the

capacity to resist some high wind pressure requirements of the International Building Code.

Metal roof-to-wall connectors should be installed in-line with the load-path. Failure was observed when metal roof

framing connectors were installed on the inside face of the wall top plate instead of on the exterior of the wall and in-

line with the load-path in the structural wall sheathing.

A common observation, especially in hardest hit areas, was the loss of the exterior walls due to poor attachment of the

wall framing to the foundation. In many cases hand-driven cut masonry nails were used to attach the bottom of sup-

port walls to the concrete or masonry foundations, but in a few locations the nails were observed to be pneumatically

driven framing nails. Modern building codes generally require anchor-bolts to be embedded into concrete or grouted

concrete masonry unit foundations.

Breaches or openings in the building envelope, and the resulting pressurization of the building interior, caused cata-

strophic failure of homes in many examples. Openings in walls due to failure of doors, windows, and non-structural

cladding systems were common. These often resulted in heavy damage to home interiors due to rainwater infiltration

and flying debris penetration. Larger breaches from loss of weak garage doors and exterior cladding systems often

acted to initiate catastrophic failure and exacerbated deficiencies along the aforementioned load-path.

In homes with gable roofs, failures were most notable at the bottom of the gable-end roof and ceiling framing where

they are connected to the top of the gable-end wall below. This joint is often not well connected laterally to the rest of

the building and is weak to resist negative pressure on the exterior gable-end surfaces.



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The gable-end is also a vulnerable location for non-structural cladding systems because the walls within the roof

cavity are not backed by drywall like the exterior walls within the living space. Material failure was commonly

observed when foam wall sheathing was used in conjunction with low-strength siding. Besides being vulnerable to

wind pressure, these products provide little resistance to the flying debris.

In most observations where buildings were at least partially intact, wood wall and roof sheathing loss could be

attributed to improper attachment. When nails were used as prescribed in the building codes good performance was

observed. Staples performed poorly since they generally offer less resistance to pull-out than nails and must be used

in greater quantity. Greater structural failure often resulted from poor roof sheathing attachment to the last rafter or

gable-end truss.

buildinG for GreAter tornAdo or HurricAne reSiStAnce

Tracking tornado paths in North Carolina, Alabama, and Georgia, APA focused on the performance of homes con-

structed within the last ten years. In many cases engineers can point to one of several common weak links as the

cause of catastrophic failure in homes that were on the periphery of these powerful tornados. It is possible to do a far

better job of protecting a home’s occupants and their possessions. Already commonplace in hurricane prone areas,

wind-resistant shell designs are simple and represent a relatively small increase in construction cost over houses built

to code minimums. The following APA recommendations, some of which exceed the minimum requirements of the

current codes, address the common weak links and provide tips for building a wind-resistant shell:

1. Nail wall sheathing with 8d common (0.131 in. x 2-1/2 in.) nails at 4 inches on center at end and edges of wood

structural panels and 6 inches on center in the intermediate framing. This enhanced nailing will improve the resis-

tance of the wall sheathing panels to negative wind pressure. The use of deformed shank nails will even further

improve the performance of the sheathing at a minimal cost. Staples offer less resistance to blow-off than nails and

so a greater number of them are required to achieve the same level of resistance.

2. Sheath gable end walls with wood structural panels, such as plywood or oriented strand board (OSB). In the 2011

tornados, gable end wall failures were frequently observed when non-structural sheathing was used.

3. Tie gable end walls back to the structure. One of the weakest links in residential structures during high wind

events is the lateral connection between the bottom of the gable-end and the top of the wall below.

4. For the roof framing to wall connection, use an H1 or equivalent connector attached on the exterior (sheathing

side) of the exterior walls. The roof to wall connection under high wind loads is subject to both uplift and shear

due to positive or negative wind pressure on the walls below.

5. Nail roof sheathing with 8d ring shank or deformed shank (0.131 in. x 2-1/2 in.) nails at 4 inches on center along

the edges of the wood structural panel wall sheathing and 6 inches on center along the intermediate framing.

6. Nail upper story sheathing and lower story sheathing into common wood structural panel Rim Board®. The most

effective way to provide lateral and in some cases uplift load continuity is to attach adjacent wall sheathing panels

to one another over common framing.



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7. Continuously sheath all walls with wood structural panels including areas around openings for windows and doors.

8. Extend wood structural panel sheathing to lap the sill plate. The connection of the wall sheathing panel to the

sill plate is important because this is where uplift forces are transferred into the sill plate and into the foundation

through the anchor bolts.

9. Space ½ in. anchor bolts 32 inches to 48 inches on center with 0.229 in. x 3 in. x 3 in. square plate washers with

slotted holes.

More information and illustrations of these construction details can be found in the APA guide: Simplified Tips for

Improving Tornado or Hurricane Resistance of Light-Frame Wood Construction, Form M310, available for download at

www.apawood.org.

SummAry

The concept of building a storm-resistant shell or building envelope is nothing new but the concept has seen greater

interest since recent severe weather. In the past, misperceptions about tornados were often used to justify a lack of

attention to structural details. A common myth remains that all tornados are so strong that structural failure is immi-

nent no matter how well a building is constructed. This rationale results in homes that lack adequate attention to a

few important details.

Weaker tornados rated as EF-0, EF-1 and EF-2 by the National Weather Service statistically comprise 95 percent of all

tornados. These smaller less-violent tornados produce winds which a carefully constructed home can be expected to

withstand.

Stronger tornados rated EF-3, EF-4 and EF-5 are statistically much rarer. They are harder for homes to survive but

improvements in design can still help. This might be the case when a building is located along the outer reaches of

the area influenced by a strong vortex.

Individual components (exterior walls, roofs, floors) are often strongly built. This is especially true when framing is

sheathed with wood structural panels, but the assemblies must still be tied together to complete the load path. A few

thoughtfully constructed connections can mean the difference between homes that survive the more common and

less powerful tornados, and those that don’t.

Homeowners deserve better, and by following a few simple guidelines, the chances of a home surviving a tornado

intact improve dramatically. First, securely attach OSB or plywood sheathing to integrate the framing. Thoughtful

joint location and blocking of horizontal joints in the wall sheathing can eliminate common discontinuities in wall

framing and at the Rim Board. Next, connect roof framing to the walls using metal connectors instead of just toenails.

Lastly, to complete the load-path, use anchor bolts into the foundation using large plate washers atop the bottom plate

of the wall. This type of construction is already commonplace in earthquake and hurricane prone areas and can be

used to improve peace-of-mind and safety for occupants in areas susceptible to tornados.

About the author:

Bryan Readling is a registered professional engineer. He received a master’s degree in Civil Engineering from Clemson

University, where his graduate research included wind tunnel studies of low-rise structures. Before joining APA in

1990, Bryan worked as a consulting structural engineer in Atlanta.



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FigurE 1:

Extensive damage to the mostly brick homes

in a subdivision in Fayetteville, NC, included

this home where damage likely initiated from

the failure of garage doors. A subsequent pres-

surization of the garage portion of the home

caused walls on the gable-end and rear of the

garage to be pushed out.

FigurE 2:

The gable-end of the home in Figure 1 is

shown here. A hinge has formed at the second

floor level in the gable-end wall. Minimal fas-

tening was observed at the connection of the

ceiling and second floor sheathing to the top of

the gable-end wall and the bottom of the gable-

end truss.

FigurE 3:

The interior side of the gable-end truss is

shown here. The truss was fabricated using

a standard attic truss as was used over the

garage to frame the bonus room. Minimal fas-

tening was observed at the 2x10 bottom chord

member to the drywall ceiling and OSB floor

sheathing.



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FigurE 6:

Poorly attached brick veneer was lost on the

rear of this home in the Fayetteville subdivi-

sion. Despite this, the OSB wall sheathing has

remained intact.

FigurE 5:

Another home within the Fayetteville subdi-

vision exhibited weakness at the gable ends.

This home lost gable-end attic trusses at two

locations on the roof. Garage doors on the

left side of this home were observed to be

breached and the front gable end wall above

the garage has failed.

FigurE 4:

This is the rear of the home shown in Figure

1. Numerous examples of isolated roof sheath-

ing loss at step-down trusses on hip roofs were

observed. It is likely that the complexity of the

connection between the roof sheathing and the

step-down trusses has resulted in poor resis-

tance to negative pressure in these cases.



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FigurE 9:

Almost complete collapse of this home in the

Fayetteville subdivision was observed. Garage

walls were pushed out of the left side and rear

of the home, suggesting that the failure of this

home was likely initiated due to internal pres-

sure after the envelope was breached at the

garage doors.

FigurE 8:

A section of roof that was blown off within the

Fayetteville subdivision was connected to the

top plate of the wall using metal connectors.

The connectors were installed on the inside

face of the wall which may have resulted in

weakness due to misalignment with the load-

path within the exterior wall sheathing.

FigurE 7:

These two homes within the Fayetteville sub-

division lost significant amounts of the fiber-

cement lap-siding. Despite this, no breaches

were observed in the OSB wall sheathing.



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FigurE 12:

Another home with near total collapse of

above-grade wood walls was fully sheathed

with OSB wall sheathing. The wall sheathing

lapped the rim board and sill plate but was

fastened to the rim board only with 16d nails

at approximately 16" on center. No nails were

observed from the bottom of the sheathing

into the sill plate.

FigurE 11:

Roof trusses from the home in Figure 9 con-

tained two 16d toe-nails used for the connec-

tion to the top of the wall.

FigurE 10:

Detail from Figure 9 of the bottom of the

left-side garage wall shows that the OSB wall

sheathing was poorly attached to the bottom

plate using 8d nails at approximately 16" on

center. The minimum fastening recommenda-

tion for this application is 6" on center along

panel ends and edges.

Anchor bolts with minimally-sized round

washers were observed in this location at 48"

on center.



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FigurE 15:

Built between 2004 and 2009, these homes

are in Wilson, NC. The author observes tor-

nado damage at the end of a cul-de-sac. The

tornados in this location were not nearly as

strong as those observed the following week

in Alabama.

FigurE 14:

This image shows the tornados that struck

the Eastern part of North Carolina on

April 16, 2011.

FigurE 13:

This home in the Fayetteville subdivision was

partially stripped of the fiber-cement siding

fastened to the framing at 16" on center. Most

of the siding nails remained embedded in the

wall sheathing with small pieces of the sid-

ing retained below the head of the fastener in

most cases.

Source: National Weather Service



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FigurE 18:

Shown here is a detail of the end wall in Figure

17. Non-structural foam panels did little to

strengthen this wall. Note the separation

between the foam wall panels and the top

plate.

FigurE 17:

This Wilson, NC home was almost completely

destroyed. Shown here is the left end-wall

that was braced at the corners using OSB.

Extruded polystyrene foam panels were used

to infill between the OSB corner bracing.

FigurE 16:

This figure shows two of the homes in Wilson,

NC that were almost completely destroyed.

Others in the vicinity lost substantial amounts

of wall coverings.



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FigurE 21:

A close up of this left gable end wall shows

that the foam wall panels were likely rein-

forced by the presence of the interior drywall.

Where no drywall existed at the gable-end

framing, the siding and foam wall sheathing

were almost completely stripped.

FigurE 20:

Several homes in Wilson, NC lost the vinyl

siding and extruded polystyrene foam panels

covering both gable ends.

FigurE 19:

This photo shows the foam insulation panel

connection to the bottom plate of the wall.

Note the small bits of the wall sheathing still

attached to the bottom plate. Structural panel

wall sheathing lapped over this joint can be

used to add significant strength to the connec-

tion between the wall studs and the bottom

plate.



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FigurE 24:

The right rear portion of the home in Figure

23 is still supported by the section of the wall

reinforced by the OSB corner bracing. The

corner bracing from the right front corner of

the home is still intact and lying in the yard,

visible in the left-most portion of this photo.

FigurE 23:

The exterior walls of this Wilson, NC home

were mostly destroyed. Despite this, the roof

remained in place over the foundation, a testa-

ment to the relatively light tornado forces and

the weakness of the foam wall panels in pro-

tecting the building envelope.

FigurE 22:

The right side gable end wall of the home

shown in Figure 21 shows a similar failure

of the foam and vinyl materials covering the

gable-end framing.



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FigurE 27:

Foam panels at the gable-end are almost com-

pletely stripped from the framing. The rel-

atively weak foam sheathed portion of the

end wall contained a window that was lost

between the OSB corner bracing and the util-

ity closet due to pressurization.

FigurE 26:

This photo shows the rear of the home in

Figure 23. Most of the rear wall is severely

damaged with the exception of the OSB cor-

ner bracing at the left-rear corner. A mirror

still hangs on the interior wall of the bath-

room at the left-rear corner of the home.

FigurE 25:

A close-up of the corner bracing seen in Figure

24 shows that the bottom plate remained

attached to the slab foundation along the side-

wall but the connection of the bottom plate

along the front wall failed.



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FigurE 30:

This two-story fiberboard-sheathed home col-

lapsed after racking failure of the first floor

walls occurred.

FigurE 29:

The left-front portion of the home shown in

Figure 28 shows loss of vinyl siding at the

gable ends and instability at the top of the

gable-end wall.

FigurE 28:

Homes in this part of suburban Northeast

Raleigh, NC were heavily damaged. Inter me-

diate fiberboard wall sheathing with vinyl

siding covered the exterior of most of these

homes. Homes in this area were constructed

between 2001 and 2004.



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FigurE 33:

A close-up of the foundation shows negligible

connection of the sill plate to the foundation.

The builder attempted to use nails to connect

the sill plate to the foundation. These nails

were mostly bent at the tip and did not sig-

nificantly penetrate the masonry foundation

walls.

FigurE 32:

This home also shown in Figure 31 was

sheathed primarily with fiberboard and used

OSB corner bracing.

FigurE 31:

This home was moved off of its foundation

and displaced approximately 6 feet toward the

rear. The attached garage totally collapsed.



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FigurE 36:

This failure in Alberta City combines two

vulnerabilities, the gable end and the garage.

Loss of the garage door likely resulted in pres-

surization of the garage and failure of the

gable-end wall.

FigurE 35:

These homes were in the same area and built

during the same time period as those in

Figure 34. Similar failures of roof coverings

and wall claddings and good performance of

OSB sheathing were observed.

FigurE 34:

These homes in Alberta City in Northeast

Tuscaloosa were built in 1999 and 2000.

Although stripped of much of their roof and

wall coverings, the OSB sheathing was well

attached and largely remained intact.



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FigurE 39:

The right side of the homes shown in Figure 38

show gable-end failures likely associated with

breaches in the front walls.

FigurE 38:

Although gable-ends were a vulnerability in

these two heavily impacted homes in Alberta

City, these OSB sheathed homes remain

standing and offered some protection to

inhabitants.

FigurE 37:

As seen in much of the homes destroyed in

this area, nails were used to attach the bot-

tom plate of the gable-end wall of the home in

Figure 36 to the foundation.



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FigurE 42:

A gable end failure occurred on the front and

right side of this home in Coaling, AL and

the garage door was lost on the left side of

the home. The front wall of the garage was

pushed-in apparently by wind pressure

as there are no indications that an impact

occurred in this location.

FigurE 41:

Although many of the homes in this Coaling,

AL subdivision were equipped with storm

shelters, according to residents, none of them

were occupied during the severe weather

event, partly because an alarm system

that had operated in the past did not wake

residents. This storm shelter was part of a

detached garage that was totally destroyed.

The main portion of the home remained intact

with moderate structural damage.

FigurE 40:

Homes in this subdivision in Coaling, AL were

built between 1999 and 2004. Most of the

homes within the subdivision were heavily

damaged by the tornado that passed through

this neighborhood at 5:27 a.m. All of the homes

were fully sheathed with OSB but connection

continuity and lateral support for gable-end-

walls were found to be lacking.



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FigurE 45:

The safe-room on the front of the home in

Figure 42 was formerly accessed through the

garage and may have acted to strengthen the

home against the tornado forces.

FigurE 44:

This photo shows the roof truss connection to

the top plate using four toenails. The toenails

were placed in opposing pairs and resulted in

splitting of the truss material.

FigurE 43:

The right rear of the home in Figure 42 shows

the loss of the gable end and adjacent roof area

over the children’s bedrooms. This home is

fully sheathed with OSB with pink housewrap

under vinyl siding.



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FigurE 48:

The right side of the home in Figure 47.

FigurE 47:

This home in Coaling, AL lost all of its exterior

walls.

FigurE 46:

This photo shows the edge of the slab where

the garage wall was blown-in on the home in

Figure 42. Non-galvanized spiral shank nails

were used to attach the bottom plate to the

foundation. This photo shows very little pen-

etration of the nail into the concrete slab and

also the gouge made by the nail as the wall

moved inwards.



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FigurE 51:

This home in Coaling, AL was completely

swept from the foundation. Remarkably the

family members inside the home were swept

away from the building footprint by the

tornado but were not seriously injured.

FigurE 50:

Non-galvanized spiral shank nails 24" – 48"

on center were observed within the piece of

bottom plate material. Most of the nail tips

were bent and did not significantly penetrate

the slab foundation.

FigurE 49:

Close-up of a bottom plate nail that remained

embedded in the slab of the home in Figure

47 after the exterior wall was demolished.



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FigurE 54:

Attachment of the gable-end for the home in

Figure 53 used 16d toenails 29" on center from

the bottom of the gable-end truss into top plate

of end- wall.

FigurE 53:

This home located on the periphery of the

tornado in Coaling, AL lost the gable-end

framing.

FigurE 52:

This piece of bottom plate material was the

only piece of the exterior walls that remained

of the home in Figure 51. Cut-nails with a rect-

angular cross-section were used to attach the

bottom plate to the slab.



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FigurE 57:

The condition of the left rear of the home

in Figure 56 shows that an impact occurred

between the two windows. Soffits were blown

away from most of the home’s perimeter,

which likely led to loss of roof sheathing along

the eaves.

FigurE 56:

This home in Coaling, AL was heavily

impacted by the tornado, but fared better than

adjacent homes. This is possibly due to superior

structural performance inherent in hip roof

construction as compared to the gable roofs

more typically found in this neighborhood.

FigurE 55:

For the home in Figure 53, nails into the top

chord were measured to be 48" on center

with staples located 12" on center between the

nails. The nails were placed at the top of roof

sheathing panels apparently in an effort to

hold the panels in place until the staples could

be placed within the field of the panels.



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FigurE 60:

The same location shown in Figure 59 shows

roof sheathing loss corresponding to loss

of lightweight vinyl soffit and porch ceiling

materials.

FigurE 59:

This photo of the left front portion of the

home in Figure 56 shows loss of soffit material

and porch ceiling materials.

FigurE 58:

Inside the home shown in Figure 56, where

the impact occurred, shows that the wall

studs separated from the top plate. OSB wall

sheathing likely saved this wall from complete

failure at the impact location.



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FigurE 63:

The rear of the home in Figure 62 is shown

in this photo. Based on the failure of a few

small trees in the backyard of this home, the

winds were mostly straight-line at this loca-

tion. The homeowner was dismayed that the

home suffered such severe damage despite the

fact that 2 of the 3 geese decoys in the back-

yard remained in-place.

FigurE 62:

This newly constructed home in Griffin, GA

lost vinyl siding and foam wall sheathing at

the gable-end trusses on both the front and

rear of the home. Despite this, no damage was

observed to a nearby detached lightweight

metal carport that is anchored with metal

stakes.

FigurE 61:

This image shows the tornados impacting

the state of Alabama on April 27, 2011. The

Alabama images contained in this list of

photos all came from the tornado marked in

blue and were taken between Tuscaloosa and

Birmingham, AL.

Source: National Weather Service



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FigurE 65:

This is the damage to the foam wall sheathing at

the rear gable-end of the home in Figures 62, 63

and 64 before the blue tarps were installed.

This photo was supplied by the homeowner.

FigurE 64:

This is the damage to the foam wall sheathing

at the front gable-end of the home in Figures 62

and 63 before the blue tarps were installed.

This photo was supplied by the homeowner.

Tornados of the SouthWe have field representatives in many major u.S. cities and in Canada who can help answer questions involving APA trademarked products.

For additional assistance in specifying engineered wood products, contact us:

APA – The engineered Wood AssociATion

Headquarters7011 So. 19th St. ■ Tacoma, Washington 98466 ■ (253) 565-6600 ■ Fax: (253) 565-7265

Product suPPort HeLP desk(253) 620-7400 ■ E-mail Address: [email protected]

discLaimerThe information contained herein is based on APA – The Engineered Wood Association’s continu-ing programs of laboratory testing, product research, and comprehensive field experience. Neither APA, nor its members make any warranty, expressed or implied, or assume any legal liability or responsibility for the use, application of, and/or reference to opinions, findings, conclusions, or recommendations included in this publication. Consult your local jurisdiction or design professional to assure compliance with code, construction, and performance requirements. Because APA has no control over quality of workmanship or the conditions under which engineered wood products are used, it cannot accept responsibility for product performance or designs as actually constructed.

Form No. SP-1154/issued August 2011

http://www.apawood.org

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