Why Your Ceiling is Playing Tug-of-War: The Truth About Truss Uplift

Hmmm.

The most staple interjection one can have in the mediation toolkit. In my previous life, I performed structural forensics in the residential industry and often found myself between a stressed builder and a worried homeowner—contentious doesn't cover it. Saying "hmmm" was one of the few ways to, temporarily, appease both sides; and if I had a dollar for every time I said it while examining drywall cracks I'd be doing just fine.

Today, we dive into something that might have vexed you if you own a new home: those pesky nail pops and cracks appearing at the corners of your walls and ceilings. And for those considering buying a new build, heed the following words before your builders start slapping up the drywall. Today, we talk about truss uplift.

Truss Uplift

To grasp truss uplift, it's essential to understand how wood expands and contracts. Wood shrinks in three directions: lengthwise, depthwise, and in thickness. While lengthwise changes are minimal, changes in depth and thickness can be considerable. When wood gets colder, it tends to become wetter. Why? Because wood is hygroscopic—it attracts water. So, when the surrounding air cools, the air’s capacity to hold moisture decreases. This causes hygroscopic materials like wood to absorb moisture as it condenses out of the air. Conversely, warm wood is typically dry because warmer air can hold more moisture and draw moisture from the wood fibers. But why does this matter?

Let’s pick on the wintertime. The top of the truss is situated above all of your insulation. In your ventilated attic space, there is a continuous supply of cold air from the great outdoors. Any moisture in the warmer air that escapes your house into the attic condenses at the top of the truss (known as the top chord) and is absorbed, causing the wood to swell. Meanwhile, the bottom of the truss (bottom chord) is insulated and remains warm, thus staying significantly drier. As a result, the bottom chord contracts. The expanding top chord and the shrinking bottom chord cause the bottom chord to pull upwards. The result? Fasteners tearing through the drywall (nail pops) and separation of materials at the wall-ceiling seams. This phenomenon happens in the summer too but in reverse with the bottom chord getting pushed downward.

The bad news? There ain’t no stopping truss uplift. You’re going to live with it for as long as you have trusses. The good news? What the eyes don’t see, the heart won’t grieve. If the trusses are going to move, at least let them do it in a way that won’t wreck your finishes.

Pink insulation surrounding bottom chord creates a separate climate from that of the top chord. The result is differential movement, which pushes and pulls on the bottom chord. This ends up creating cracked drywall tape and nail pops.

How to Address Truss Uplift

Fasteners:

Glue and screws are your friend. The drywall should be continuously bonded to the bottom chord using adhesive. This allows the drywall to flex along its entire length with the truss instead of creating stress points at each screw location and causing nail pops.

Here’s where you need to be diligent. Home inspectors rarely get to see drywall fasteners. Communicate to your builder you want your ceiling attached with glue and screws!

Mind The Walls:

Do not place glue or fasteners within 18” of partition walls. Where your partition walls abut your ceiling, the drywaller applies paper tape and compound to hide the joint. Ready for some technical prowess? Your ceiling needs to flap at these locations. That’s right, I said it. It needs to flap. What in pray-tell do I mean? You want the last 18” of your ceiling to be ‘loose’. Why? Because when the truss lifts upwards, the drywall at the edges of your room won’t lift with it. This means the paper tape I mentioned earlier won’t pull upwards and sag, and the compound won’t crack.

No glue or fasteners within 18” of the wall!

Make A Gap:

Did you know trusses aren't supposed to touch non-load bearing interior walls? Many trussed roofs don't use interior walls for support and are designed to span forty feet or more. Contractors shouldn't butt walls against the bottom of trusses because it creates a spot where they might nail the truss to the wall. This makes the wall move with the truss, causing drywall issues. Doing so can also cause non-bearing walls to inadvertently bear loads, but this is a topic for another blog.

To avoid this, leave a 3/4" gap between the truss and the wall. Secure the wall with "L" shaped clips with slots, which stabilize the wall and allow the truss to move up and down. This prevents structural issues and keeps the drywall intact. Everyone is happy.

Slotted clip stabilizes the top of wall and has a groove cut into it to allow the truss to move.

Why should you care?

Alright, you’ve stuck with me this far. So, why should you care about truss uplift? Simple: peace of mind and saving money. Understanding these potential issues lets you address them before they become eyesores or costly repairs. A little foresight can go a long way. Don’t wait until nail pops and cracks start driving you crazy. Make sure your new home is thoroughly inspected to avoid unnecessary headaches. Trust me, you’ll thank yourself later.

Ready to take action? Schedule your home inspection today and stay ahead of the game. Book a service and we’ll be in touch as soon as possible!

References

Building Science Corporation. (n.d.-a). Drywall, Wood, and Truss Uplift. Retrieved July 3, 2024, from https://buildingscience.com/documents/reports/rr-0107-drywall-wood-and-truss-uplift

Building Science Corporation. (n.d.-b). Wood Is Good, But Strange. Retrieved July 3, 2024, from https://buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange