Roof Deck Seams Visible - What's Wrong?

any idea of the cost of that product Marcel?

Possibly varies from State to State and regions, so here is the calculator for you to punch in the info to see the difference in cost.

I understand that drying in the home will reduce air flow and roof sheathing drying time. Since panels only expand as additional moisture becomes available for absorption, how does drying in the home provide additional moisture?
It may raise humidity levels in the home interior, since the wet roof sheathing will dry to the inside as well as the and that water vapor has no good way to leave the home interior.

If the roof has underlayment and shingles installed, most drying will take place to the inside, raising interior humidity levels higher than they’d be if the panels were free to dry to the outside. Unless this humid air is evacuated by opening the home, sheathing panels will try to reach equilibrium moisture content with the humid air inside the home. Maybe this is why panels continue to swell.

This doesn’t really make sense either, because panels don’t re-absorb moisture from the home interior, high interior humidity would simply slow the rate at which panels lose already-absorbed moisture. Panels swelling is caused by moisture absorbtion, not a reduction in the rate at which they dry, so why would drying in the home cause panels to swell?

From the American Plywood Association in response to my e-mail…

It may be that the OSB was very dry (fresh from the mill about 3 to 4% moisture content) at the time of installation and that the roofing materials were applied very shortly after the sheathing was finished. If that was the case it is possible that normal acclimation of the panels to equilibrium moisture content (could be as high as 12 to 14%) even without rain exposure led to panel expansion. And, as you suggest, if the roofing was applied immediately after rain exposure the expansion may be somewhat attributable to that.

Here’s the link ]( DIMENSIONAL STABILITY&section=&content=srv_search)to their tech pages.

Try [Minimizing Buckling of Comp Shingles]( a Free PDF Download)

[FONT=Arial][size=3]Wood structural panels are hygroscopic. Panel moisture content is a function of relative
humidity (and temperature to a very slight degree) when not exposed to direct wetting.
In construction applications such as roofs, walls and floors, the panels in service are
protected from wetting so the panel moisture content is primarily a function of humidity.
Panels may be exposed to direct wetting during construction, and during the service life
for some applications. When exposed to direct wetting, the moisture content is
influenced by wetting time and by panel variables that affect capillarity such as veneer
species of plywood and wax additives of OSB.
An APA study evaluated the equilibrium moisture content of structural panels. Results
indicated that the moisture content of plywood and OSB at a given relative humidity is
lower than the published values for solid wood. The APA data below is based on an

absorption cycle at a temperature of approximately 70
[size=3][FONT=Arial]Table 1. Equilibrium moisture content of solid wood and structural panels at 70

[/size][/FONT][size=3]°[/size][FONT=Arial][size=3]F (21[/size][/FONT][size=3]°[/size][FONT=Arial][size=3]C)[/size][/FONT]
Moisture Content (%)
Solid Wood

[/size][/FONT][FONT=Arial][size=1]1 [/size][/FONT][FONT=Arial][size=2]Plywood OSB[/size][/FONT]
[size=2][FONT=Arial]10 2.5 1.2 0.8
20 4.5 2.8 1.0
30 6.2 4.6 2.0
40 7.7 5.8 3.6
50 9.2 7.0 5.2
60 11.0 8.4 6.3
70 13.1 11.1 8.9
80 16.0 15.3 13.1
90 20.5 19.4 17.2

[/size][/FONT][FONT=Arial][size=1]From Wood Handbook by U.S. Forest Products Laboratory[/size][/FONT]
[FONT=Arial]A standardized wetting cycle was developed by APA and has subsequently been
accepted in various performance and manufacturing standards. The method wets the
exposed surface only with a water spray system; the back side is exposed to the
resultant high humidity. The procedure was designed to simulate a panel exposed to
weather such as when wetted during construction. The wetting cycle is used to evaluate
dimensional stability and is used prior to structural tests of sheathing panels. The
following figure relates wetting time and panel moisture content for 7/16-inch OSB and

[/FONT]-[FONT=Arial]inch plywood.[/FONT]
[FONT=Arial][size=1]Form No. TT-028A Page 2 of 5
December 2006
© 2006

[/size][/FONT][FONT=Arial][size=1]APA – The Engineered Wood Association[/size][/FONT]
[FONT=Arial][size=2]Panel Moisture Content During Wetting[/size][/FONT]
0 12 24 48 72
[/size][/FONT][FONT=Arial][size=1]Wetting Time (hours)[/size][/FONT]
[size=1][FONT=Arial]Moisture Content %
[/size][/FONT][FONT=Arial][size=3]DIMENSIONAL STABILITY[/size][/FONT]
[FONT=Arial][size=3]The moisture content of structural panels is generally 2 to 8 percent when manufactured.
When exposed to elevated humidity or wetting after manufacture, the resultant increase
in moisture content leads to dimensional increase in thickness, length and width. The
increase in length or width is reported as linear expansion. The thickness increase is
called thickness swell.
[/size][/FONT][FONT=Arial][size=3]Linear Expansion[/size][/FONT]
[FONT=Arial][size=3]Linear expansion of structural panels is generally evaluated by measuring length
increase due to a change in moisture condition. A steady state moisture condition can
be achieved by oven drying, humidity exposure, or vacuum soaking. Linear expansion
can also be measured following exposure to moisture such as a one-sided water spray
to simulate wetting typical of construction site exposure.
Linear expansion by humidity exposure is typically conducted by using standardized
initial and ending humidities such as 50 and 90 percent humidities. Since humidity
exposure in actual applications can vary, APA undertook a study to evaluate the relative
expansion of structural panels across interim moisture conditions from oven dry to
complete saturation. The relative linear expansion (RLE) and thickness swell was
determined at various equilibrium moisture contents by exposure to various humidities.
The RLE at each moisture content was determined as a percentage of total expansion
from oven dry to complete saturation by a vacuum-soak cycle. The relation is shown
[/size][/FONT][FONT=Arial][size=1]Form No. TT-028A Page 3 of 5
December 2006
© 2006

[/size][/FONT][FONT=Arial][size=1]APA – The Engineered Wood Association[/size][/FONT]
[FONT=Arial][size=2]Linear Expansion
y = 5.125+69.109*log10(x) r^2=.986
Moisture Content (%)
Relative Linear Expansion (%)
0 5 10 15 20 25
[/size][/FONT][FONT=Arial][size=3]The RLE is a function of moisture content as expressed below:
RLE = 5.125 + 69.109 LOG

[/size][/FONT][FONT=Arial][size=1]10 [/size][/FONT][FONT=Arial]size=3[/size][/FONT]
RLE = relative linear expansion at reference moisture content as a
percent of total from oven dry to saturated (%)
MC = reference moisture content (%)
The above relation can be used to estimate actual expansion when the linear expansion
from oven drying to saturation is known. Approximate moisture content of panels after
manufacturing is 2 to 4 percent for OSB and 5 to 8 percent for plywood. Some
acclimation to ambient humidity conditions may occur during transit. The following
provides information on linear expansion from oven dry to saturation (i.e., vacuum soak).
The oven dry/vacuum soak cycle represents the extreme amount of potential expansion
which may occur. It is not representative of expansion which normally occurs in-service.
Table 2. Linear expansion from oven dry to vacuum soak
[/size][/FONT][FONT=Arial][size=2]Panel Type No. Tested Along Direction Across Direction
Avg (%) COV (%) Avg (%) COV (%)
7/16" 734 0.23 25.7 0.38 20.0
23/32" 499 0.22 25.6 0.38 21.1
3/8" - 1/2" 203 NT NT 0.34 44.4
19/32" - 3/4" 187 NT NT 0.31 35.1
[/size][/FONT][FONT=Arial][size=1]NT = not tested
Along direction refers to the strength direction
Across direction refers to the perpendicular-to-strength direction
Form No. TT-028A Page 4 of 5
December 2006
© 2006

[/size][/FONT][FONT=Arial][size=1]APA – The Engineered Wood Association[/size][/FONT]
[FONT=Arial][size=3]The effect of linear expansion of structural panels may lead to buckling of panels after
they are nailed to supports. The buckling potential has been studied by APA and
reported in

[/size][/FONT][FONT=Arial][size=3]Research Report 144, Predicting Buckling Performance of Plywood[/size][/FONT]
[size=3][FONT=Arial]Composite Panels for Roofs and Floors

[/size][/FONT][FONT=Arial][size=3], Form D455, and [/size][/FONT][FONT=Arial][size=3]Research Report 149,[/size][/FONT]
[size=3][FONT=Arial]Dimensional Performance of Wood-Based Siding

[/size][/FONT][FONT=Arial][size=3], Form L310.[/size][/FONT]
[size=3][FONT=Arial]APA’s use recommendations recognize potential for expansion of structural panels.
Gapping of panel edges provides room for panel expansion prior to developing axial
compression which can lead to buckling. Techniques to minimize buckling are
addressed in APA use recommendations and technical notes.
[/size][/FONT][FONT=Arial][size=3]Thickness Swell[/size][/FONT]
[FONT=Arial][size=3]Thickness swell of structural panels is evaluated by techniques similar to those for
testing linear expansion. Thickness swell can be evaluated using steady state humidity
cycles or direct wetting. Unlike linear expansion, thickness swell is sensitive along panel
edges since the end grain of the fiber increases capillarity. For that reason, thickness
swell measurements are typically focused on the swell near the panel edge. In addition
to the above moisture cycles, thickness swell is frequently measured using the 24-hour
soak specified in ASTM Standard D1037.
The thickness swell of plywood is primarily related to the radial expansion of the wood
species with some increase expected from release of compression set that occurs
during pressing. The thickness swell of OSB is generally greater than wood due to
release of compaction stress created during pressing.
APA undertook a study to evaluate the relative thickness swell (RTS) of structural panels
across interim moisture conditions from the oven-dried condition to complete saturation.
The RTS was determined at various equilibrium moisture contents by exposure to
various humidities. Unlike linear expansion, thickness swell increases uniformly as a
function of moisture content. The RTS can be expressed as below:
RTS = 4.0 * MC
RTS = relative thickness swell at reference moisture content as a
percent of total from oven dried to saturated (%)
MC = reference moisture (%)
The above relation can be used to estimate actual thickness swell when the thickness
swell from oven drying to saturation is known. The following table provides information
on thickness swell for structural panels when measured using the oven-dry- to-saturation
[/size][/FONT][FONT=Arial][size=1]Form No. TT-028A
December 2006 Page 5 of 5
© 2006

[/size][/FONT][FONT=Arial][size=1]APA – The Engineered Wood Association[/size][/FONT]
[FONT=Arial][size=3]Table 3. Thickness swell from oven dry to vacuum soak
[/size][/FONT][FONT=Arial][size=2]Panel No. Tested Thickness Swell OD-VS
Average (%) COV (%)

[/size][/FONT]" [FONT=Arial][size=2]- 1/2[/size][/FONT]" [FONT=Arial][size=2]203 9.4 19.3[/size][/FONT]

[/size][/FONT]" [FONT=Arial][size=2]- 3/4[/size][/FONT]" [FONT=Arial][size=2]187 8.8 18.0[/size][/FONT]

[/size][/FONT]" [FONT=Arial][size=2]154 32.9 17.1[/size][/FONT]

[/size][/FONT]" [FONT=Arial][size=2]61 28.9 15.1[/size][/FONT]
[FONT=Arial][size=3]Since thickness swell is especially sensitive to one-sided wetting such as during construction,
other methods such as the 24-hour soak or water spray methods are also used to test thickness
swell as applicable to construction applications. The following figure presents the relation
between wetting time and thickness swell of OSB sheathing panels. Note that the thickness
swell after 24 hours of total water immersion is about the same as three days of one-sided
[/size][/FONT]OSB Sheathing Response to One-Sided Wetting
[/size][/FONT]One-sided Wetting Time
[FONT=Arial][size=2]1 Day 3 Day 7 Day 14 Day
[/size][/FONT][FONT=Arial][size=2]Moisture Content (MC)[/size][/FONT]
[size=2][FONT=Arial](from one-sided wetting)
[/size][/FONT][FONT=Arial][size=1]Edge Swell[/size][/FONT]
[size=1][FONT=Arial](from one-sided wetting)
[/size][/FONT]**[FONT=Arial][size=2]MC using 24 hr soak[/size][/FONT]***
[size=2][FONT=Arial](total immersion)
TS using 24 hr soak
(total immersion)
[/size][/FONT][FONT=Arial][size=1]Moisture Content or Thickness Swell (%)[/size][/FONT]***
[FONT=Arial][size=3]Technical Services Division

[FONT=Arial][size=1]The information[/size][/FONT]

That is a similar response to CertainTeeds manuals.:slight_smile:

If I were the potential Buyer of one of these particular homes, I’d walk from the deal.

If the builder experienced this effect happening in one home, what made him continue the same application in the other homes?

Hello…someone needs a bi+ch-slap.

Here’s a guess, and only a guess. If they used Flake board, flakeboard does not have exterior glue, and will easily absorb moisture from air, condensation, etc… Flakeboard looks simular to oriented strand board, but has different glue. Flake board was around for some time before oriented strand board even existed. I just run accross a house with flakeboard roof decking. All along the eaves, it was swelling up to about 1-1/8 inches. It pushed the bottom course of shingles up so that it acted more like a ski jump at the bottom edge. I don’t know if they even make flake board any more. Haven’t seen any in a while.

cheap plywood or osb…swelled when it got wet.

Bears repetition, wrong guage product for application this gentleman identified the problem. The moisture condition exacerbated it too. That was likely during, before or after installation( the moisture problem) could have been inattention to storage, installation during rain or a failure to install waterproof membrane in a timely fashion or on top of wetted materials. The problem is multiple fold, starting with improper construction technique and material.

Bears repetition, wrong guage product for application this gentleman identified the problem. The moisture condition exacerbated it too. That was likely during, before or after installation( the moisture problem) could have been inattention to storage, installation during rain or a failure to install waterproof membrane in a timely fashion or on top of wetted materials. The problem is multiple fold, starting with improper construction technique and material.

In regards to zipboard… I have used it and do not like it. It is more expensive but the problem I have with it is no different then flashing issues that we all see at times. Anyone who thinks that adhesive tapes applied on wood are permanent are crazy. Water is a natural soluble,…given enough moisture in the air, it will break down. Their warranty by the way does not cover the tape! explain to me what they are covering.

If putting flashing over tyvek (not allowing the tyvek to run over the flashing) is wrong (which it is) explain to me the difference in this principle…putting tape over these joints not expecting any different results.
Are they trying to make the home air tight or moisture tight…there is a difference…besides, homes should not be air tight…we are not living in submarines (nothing personal to you navy boys).
A builder puts zipboard on and fiberglass insulation in the walls…you now have a two moisture barrier system in place…the 6 mil poly for the insulation and the zipboard on the exterior… you have effectively created another problem. You see where I am going with this…one has to be careful about using various products in conjunction with other systems. One has to look at the building science of the whole house systems in relation to one another…this is what is being missed on many homes today…which helps keeps us in business (lol)… that is if you undestand the building concept behind a home…for those who are looking at getting into the HI business with little to no knowlege of such they are flying by the seat of their pants.

In regards to installation…most framers miss studs when nailing… so does one go back and spot take all these holes. (Im anal… I make my framers go back and pull out nails where they missed studs / joists)

When I installed it a few years ago, the green barrier was suseptible to damage… it is only 4 perms anyway…that is a joke.

There were more labor cost associated with the product simply because you had to be more careful with scaring it up as well as taping all joints. Anyone who has done quite a bit of framing knows it easiter to install tyvek before standing the walls up…no ladderers to contend with other then the lapping when connecting walls. I pay one guy $15.00 to run around the house stapling the lap joints when all is said and done…how long do you think it will take to tape all those joints on a 2 story home.

I have built and framed hundreds of homes…I have installed zip board on only 1 of them…for the above reasons is why I will not install it on any other without carefully informing my clients as to the downside of the product and having them understand that certain products should not be used inconjuction with this product.

I applaud huber for being innovative however there have been all sorts of innovative products that consumers simply did not want…how long was the 8 track tape around (laugh).


  1. It’s wrong. swelling of the roof deck.
  2. 7/16 on 24" OC is just plain wrong as well (and I don’t care what the local codes say).
  3. looks like crap.

Run away.

I like the technical discussion, but for the purposes of a hime inspection, it’s just plain messed up, non-professional construction. Tell the client that and let them make up their own mind.

Simple, really.

All the new homes here in CA are 7/16 osb 24" center and I would say 99.99% of them are jsut fine if installed right…we have used osb in our reroofs for over 15 years now and I have nerver had one issue with it…my 2 cents

Brian, that is because of

Maybe I am suffering from regional bias. We have rain and snow, here. such a roof would have collapsed, around here, long ago.

Hope this helps;

Does plywood sheathing also suffer from this “edge swell” problem?

The roof deck is probably stable, but the shingles need replacement. The warranty is void and they will continue to buckle although the severity will vary seasonally. Bad contractor! No beer!