#trussuplift #ceilingcornercracks #wallcracksatceiling

What’s up with those ceiling-to-wall cracks that appear every winter? Caulk is just a temporary solution. What can you do? DIrtFarmerJay explains what is going on and what to do about it.

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We’ve noticed there are a couple of spots in our home that we can count on a gap forming every year when winter comes on. It only happens on a few wall-to-ceiling joints (also known as the “ceiling line”) on the top floor of our home. Caulk only provides a temporary cosmetic fix, then they’re back the following year.

What is going on? Truss uplift, that’s what! This is a common problem for homes situated in areas where cold winters with snowfall and late fall and early spring rains occur. The available moisture in the air will ultimately create uplift in the trusses that are used in modern framing approaches for dwellings. More on that in a bit.

It used to be that roofs were framed using a center ridge (supported at the gable ends) with rafters “leaned” against them, with separate ceiling rafters installed to resist spread on the outside walls, as well as to create the plane of framing members to affix the ceiling to.

Nowadays, these elements are tied into a component. A common triangular truss unit is set to a certain “pitch”, called out by rise and run. In the U.S., “rise” is how high the top chord rises in 12” of horizontal distance, known as “run.” In our example, a 5/12 truss will rise 5” for every 12” of horizontal “run.”

All the elements (top chord, bottom cord, webs, and so forth) are affixed with nailing plates. The truss is set on the perimeter walls of the structure and affixed, typically in distances between 16” and 24”.

One of the most used shapes for trusses is triangular. As you know, a triangle cannot change without either un-attaching one of the corners, breaking one of the members, or distorting the shape.

And, the last one is precisely what is happening when truss lift occurs. The bottom chord is typically buried in the insulation that is found at the bottom of the attic. So, that portion of the truss is dry, stable, and warmer than other portions of the truss. The webs and top chords are not insulated and readily absorb available humidity/moisture - so they grow slightly in length.

Because the truss is affixed at the outside bearing points on top of the perimeter wall, any growth in the length of the sections of the truss only has one place to go - to the apex of the truss. When this happens, the webs pull up the bottom chord, creating truss uplift.

This effect can be minimized during construction by not affixing the gypsum board to any truss bottom chord within 12” of any interior wall that connects to the ceiling. Also, using overhanging blocking on the top plate of the interior wall, and affixing the gypsum board there, will assure there is no gap that forms in the drywall at the corner.

If you are working to retrofit an already completed home, it’s a bit more complex, and you’ll need to determine if the annual gaps that form in the winter are worth the investment to fix. Simply filling the gap with caulk won’t really fix it, but it may make YOU feel better!

If you have a smooth or lightly finished “skip-trowel” ceiling, you can try locating the bottom chords/ceiling joists that are within 12” of the offending wall. Once you locate the screws or nails that are holding the drywall to the ceiling, you can remove them and patch and paint. This will allow the gypsum board to “float” more and minimize cracking. It’s not perfect, and it won’t work in every case, but if the annual truss uplift cracks are driving you crazy, then it could be worth a try!

There are not many people that know about truss uplift, but now YOU do. And when it comes to either framing correctly for new construction or a retro fix, you can JUST DO IT YOURSELF!

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