Myron Ferguson | April 01, 2016


Article // Framing, Drywall, Tile, Flooring

Q: Is Truss Uplift a Carpentry Problem or a Drywall Problem?

 

A: Neither. It is a physics problem. The solution lies in carpenters and drywall installers working together

 

Trusses are a fast way to get a roof framed. They are often a less expensive than stick-framed roofs because they use smaller dimension lumber and they go up faster. And, trusses are flexible in accommodating different design needs, so they tend to be the darlings of designers.

Trusses have been around for a long time and so has their primary drawback: truss uplift.

 

Truss uplift is not a structural problem: it's a thermodynamic one

Roof truss in normal position.

When a newer house shows signs of interior cracking at ceiling of the top floor in the winter, this can normally be associated with truss uplift. Roof truss uplift occurs when the bottom chord of the truss is exposed to significantly different moisture and/or temperature conditions than the rest of the roof truss. Because the bottom chord of the truss is buried in heavy insulation, it experiences different conditions than the top chord. 

Dry wood shrinks, moist wood expands.

When moist air from the inside escapes into a cold attic, the moisture condenses on the trusses and roof deck. If the temperature is below 32, it also freezes. When the temperature rises above 32F, the moisture soaks into the wood. The dampness causes the top chords to expand relative to the bottom chord, which was warm and dry all winter.

This differential movement between top and bottom chords causes an arch in the center. Because the truss ends are secured to exterior walls, (a location that resists outward thrust), as the bottom chord expands, the force pushes it upward into the attic space.

When the trusses arch, they pull away from the top plates at the interior partition walls running at right angles to the trusses. 

Figure 3 – Roof truss in “uplift” position. NOTE: The walls that are prone to the problem are the interior walls that run at right angles to the trusses, but walls that run parallel to trusses especially those close to trusses are also in danger of being effected by truss uplift.

This movement can cause a simple hairline crack or it can create large gaps along the corners, but it is a cosmetic problem mostly occurring in homes in cold climates.

The ceiling/wall cracks typically open in the winter and close in the summer. As a result the problem is often an ongoing one. The building contractor is held responsible, who calls the drywall contractor to re-tape the corners and un-pop the popped screws.

And then the same thing happens next year.

 

You can't stop truss uplift, but you can stop the cracks that result

Truss uplift is not truss uplift if the homeowner can’t see it. I think the best approach is to stop the truss uplift from having an effect on the drywall joints—let the trusses move. If the truss can move and the can drywall bend, no crack will result, and the homeowner will be happy.

Most truss movement occurs over a period of time, so to prevent cracks, you must allow the drywall to flex near the corners. 

 

How to prevent the problem

When building a new house the framing contractor should secure the interior partitions to the trusses with truss clips. The clips attach to the tops of the interior wall partitions and are then fastened through the slots to the trusses.

Photo_Crack

The fasteners are not driven tight, which allows the truss to move without pulling on the wall. This keeps the partition wall stable and plumb and secures the rafters at the proper on-center spacing.

The drywall installer must then install the drywall in such a way as to prevent cracking. A common method of doing this is to float the corners between ceiling and wall. Do not screw or nail the drywall to the trusses within 16 inches of an interior wall and within 8 inches of the ceilings on the exterior walls.

The ceiling drywall panel is fastened to blocks of wood nailed between the trusses to the top plates or to metal clips or continuous angles that are attached along the top of the interior walls.

The key to eliminating truss uplift cracks and screw pops is avoiding nailing or screwing the ceiling drywall to the bottom of the trusses at or near the intersection. The movement is gradual so the drywall can flex slowly. As the rafter moves up, the drywall along the wall/ceiling intersection stays in place.

 

 Four Ways to fix the problem:

1. The solution to an existing condition: The tops of the interior walls should be unnailed from the trusses.
  • Remove or cut free the nails from inside the attic.
  • Remove all of the ceiling screws within 16 inches of the corners at the center partitions and all the wall screws within 8 inches of the ceiling along the wall.
  • The holes in the drywall should then be patched with dry wall joint compound and the walls and ceilings repainted.

This solution will be difficult to accomplish and will be costly.

 

2. Install crown molding around all the second floor ceilings, nailing the trim only to the ceilings.

This procedure would cover the cracks and maintain a good looking ceiling corner. Note:  When installing the crown molding in this manner, remember to paint the trim and drywall in the winter months so that there is no paint stripe at the bottom of the trim when the trusses lift next winter.

 

3. Change the way the attic is insulated—another costly and invasive approach 

If the insulation is moved up to the roof deck, a conditioned attic will be created, so the temperature and humidity will be consistent. In homes where the attic is part of the conditioned space,—typically with spray foam against the roof sheathing—truss uplift problems are much less likely.

 

4. Use a Truss backing angle

Trim-Tex has a product called a Truss Backing Angle which is installed before the drywall is hung which helps prevent truss uplift.

Trim_Tex Solution_Truss UpliftTruss Photo_03

The idea is not really new; Trim-Tex is just trying to offer a better alternative. The backing angle is made out of vinyl, which is strong enough to hold the drywall edge in place along the top edge of the wall. It is attached the entire length of the walls being treated.

This means that the drywall is backed for its entire length and can easily be fastened anywhere along the edge as long as the fasteners don’t go through the angle and into the truss. The screws start very easily in the vinyl engineered extrusions which speeds up production.

 

—This article first appeared on ThisIsDrywall and is by Myron Ferguson, aka That Drywall Guy, who points out on his website that over 80 percent of the visible interior of a home is covered with drywall, and "The bitterness of poor quality remains long after the sweetness of low price is forgotten."
Amen, brother. 

© Myron R. Ferguson 2014 

 

Comments

"Because the bottom chord of the truss is buried in heavy insulation, it stays warm and dry in winter. Warm, dry wood shrinks."

"Because the truss ends are secured to exterior walls, (a location that resists outward thrust), as the bottom chord expands, the force pushes it upward into the attic space"

This appears to me to be contradictory statements with the bottom chord.

Bottom chord expansion would curl the ends of the truss up, because the top chords are holding onto the ends, and the exterior walls don't have enough strength to hold back an expanding bottom chord or ceiling joists, they can be easily thrust out.

I have read eight articles on truss uplift, and not one has conclusively explained the cause, and many such as this one, are contradictory.  If the bottom chord is perpetually warm and dry, it does not expand.  If the top chord moisture level  rises every winter and expands, will more ventilation moderate expansion or exacerbate it?  I have read that the moisture comes from within the house and migrates through the ceiling into the attic--this is the reason we ventilate attics over conditioned rooms.  If, on the other hand, the moisture source is from outdoors, then ventilation is the culprit, not the solution.
I have used vapor barriers on the ceiling to reduce water vapor intrusion into the attic and I have eliminated the vapor barriers, and either way I have experienced truss uplift.  I have added extra attic ventilation and still had uplift.  What I have learned is trusses in my region (Pacific NW) will lift regardless of what you do, so apply the drywall with the assumption that every truss over 12' will lift, and expect uplift to become noticable about 5' away from the bearing point.

I learned about truss lift the hard way 35 years ago on a "super-insulated" house I was building at the time. I can't believe that after all this time truss lift is still considered a problem, still the finger pointing. "Who should take responsibility for the effect on the structure it has?" New products and simple solutions are sometimes difficult to put in place with complicated truss systems today. I think networking between the framer and drywall crews are essential.

The moisture issues in this article are backwards.  Wood expands when it is wet; it contracts as it dries.  It expands with heat (not much) and contracts with cold.  Winter air in cold climates ( I live in Minnesota) is extremely dry, so to imply that the top chord of the truss is getting wet in the winter is just plain bad physics.  The top chord and the webbing dry and cool and contract lifting the bottom chord, which stays warmer and doesn't dry quite as much.  I have repaired several homes with truss lift issues and the solutions offered in this article are pretty much on, however, the work is much more difficult to achieve than to describe.  Finding and removing fasteners from finished drywall, installing blocking in attics with limited headroom and insulation, this is challenging work and expensive.  My repairs typically run 10k - 30k.  Sometimes what appears to be truss lift is actually subsidence caused by load path issues, but that is another discussion.

Daniel Morrison's picture

Because the interior air is warmer than the exterior air, it has a lot more moisture in it. Cold attic air is less able to hold moisture, so when warm indoor air leaks into the attic, moisture condenses and freezes until the temperature warms above 32F.

The bottom chord, buried in insulation, is living a very different life than the top chord, which is freezing, thawing, soaking, expanding, drying, contracting, and freezing again.

—Dan

I am a professional engineer living in Northern Michigan with plenty of COLD weather.  I can tell you that a properly ventilated attic space is the same (or very close) to the outside air temperature and humidity at any given time.  Winter air in cold climates is Extremely dry.  If condensation is present in the attic it is conclusive proof that the ventilation system is not properly sized or not properly installed.  Either way, it should not be allowed to continue or long term problems caused by the moisture will occur. I have seen trusses lift in cold weather with what appeared to be a properly ventilated attic which lead me to conclude the driving force was temperature related. If it is a humidity issue, it is more likely caused by the lower chord remaining at a warmer temperature due to the surrounding insulation, and could therefore also be holding slightly higher humidity levels..  The cause may be difficult to determine but I do agree with the means of prevention.

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