Learning Center · Diagnosis
What Is Truss Uplift?
WHEN EACH KIND OF GAP IS USUALLY WIDEST
Truss uplift at the ceiling line
Opens with the first cold fronts, widest in midwinter, closed by early summer.
Foundation cracks on clay
Open as the clay dries out, widest in late summer, ease after the fall rains.
The usual pattern on Central Texas clay. A wet summer or a dry winter shifts it.
What is truss uplift? Truss uplift is roof trusses arching upward in the cool months and lifting the ceiling off the tops of interior walls. That opens a straight, even gap where the ceiling meets the wall, usually on walls near the middle of the house, and the gap closes again by summer. It is the roof framing moving, not the foundation, and the calendar is the tell. Foundation cracks on Central Texas clay run widest in late summer, while truss uplift opens in winter. If your gap keeps that winter schedule and nothing else in the house is moving, it is a carpentry problem with a carpentry fix.
How truss uplift works
Many houses built around San Antonio and Austin in the last few decades have roofs framed with trusses, the factory-built triangles you see when you climb into the attic. Each truss has top chords, the sloped boards under the roof deck, and a bottom chord, the flat board the ceiling drywall is screwed to. The truss spans from one outside wall to the other. Interior walls sit under the bottom chords, but they are not there to hold the trusses up.
In winter those chords live in different conditions. The bottom chord is buried in attic insulation, right above the heated house, so it stays warm and dry. The top chords sit just under the roof deck in cold, damp attic air. Wood takes on moisture from the air around it and gives it back, and its size changes as it does. The USDA Forest Products Laboratory's Wood Handbook lays out how lumber swells as its moisture content rises and shrinks as it falls. Along the length of a board the change is small, but a truss is long. The top chords grow a little and the bottom chord shrinks a little, and a triangle with longer sides and a shorter base has only one way to go. The middle of the bottom chord bows upward and takes the ceiling with it.
Building scientists have studied this for a long time. Research from Building Science Corporation on drywall and truss uplift traces it to the moisture difference between a cold upper chord and a warm lower chord, and treats that difference as inevitable in an insulated attic. You will also hear it called truss arching. When spring comes, the top chords dry out, the truss flattens, and the ceiling settles back onto the wall.
Where the gap shows up
Truss uplift has a recognizable shape. The gap opens along the tops of interior walls, the partitions between rooms and down hallways, and it is widest on walls near the middle of the span, where the bottom chord bows the most. It runs straight. On a wall that runs across the trusses it is about the same width along the whole run, and on a wall that runs the same direction as the trusses it tapers, widest toward the middle of the house. Often the tape in the corner tears in one clean line, or the ceiling texture separates from the wall paint like a zipper. Exterior walls rarely show it, because the truss ends rest on them and do not lift.
You may also see a row of screw or nail pops a foot or so out from the wall, where the ceiling drywall bends as it rises. They are part of the same movement, and the other things that push fasteners out are covered in what causes nail pops in drywall. If you have read our comparison of wall cracks and ceiling cracks, this is the ceiling half of that story. A wall stands on the foundation and reports what it does. A ceiling hangs from the roof framing and mostly reports the attic.
The calendar test
This is the most useful thing a homeowner can do, and it costs a pencil. Our clay is expansive. It shrinks when it dries and swells when it gets wet, so foundation movement here follows the rain. In a long dry summer the soil at the edge of the slab pulls down and away, the perimeter drops, and cracks from foundation movement are usually widest in August and September. Fall rains swell the clay back, and many of those cracks close up again. We explain that cycle in why drought causes foundation damage.
Truss uplift runs on the opposite schedule. The gap opens after the first real cold fronts, runs widest in the coldest, dampest stretch of winter, and closes as spring warms the attic. By June it has usually disappeared, and the corner looks almost normal. Central Texas often sends its first winter fronts right after a dry summer, so a house can be doing both things in the same year. A dated mark is how you separate them.
Put a pencil tick at each end of the gap, write the date beside it, and measure the width once a month with a ruler or a feeler gauge. A gap that opens every winter, closes every summer, and comes back to about the same width each year is behaving like framing. A gap that gets wider from one year to the next, whatever the season, is not. Our guide to monitoring a foundation crack walks through the full routine and what to write down.
The calendar is strong evidence, but it is not proof. A wet winter after a drought swells the clay and can push a slab edge up in the same months truss uplift is active. That upward movement is called heave, and settlement vs heave covers how the two look different on a floor survey. That is why the calendar gets paired with the signs in the next section.
Signs that point back to the foundation
Truss uplift is a ceiling-line problem. Foundation movement shows up lower in the house and in more places at once. These are the signs that send us to the floor instead of the attic.
Diagonal cracks that start at the corners of doors and windows come first. When a slab moves, it racks the wall frame, and the weak points are the corners of openings, so the cracks angle out from them. We cover how to read those in drywall cracks above doors and windows. Doors that start sticking, swing open on their own, or stop latching are next, because a truss lifting the ceiling rarely twists a door frame, and why doors stick covers the other causes. Then floors you can feel sloping and stair-step cracks in the brick outside. A ceiling gap that tapers is not proof either way, because uplift tapers too on a wall that runs the same direction as the trusses. A taper that grows a little more every year is the one to have measured.
Last, look at where the damage sits across the house. Truss uplift shows on interior walls near the middle. Foundation damage tends to collect at one corner or along one side, and it is worst nearest the part that moved, which is the pattern described in one corner of the house sinking. Any one of those signs alongside a ceiling gap changes the question. If you want the full scale for grading a crack by width, direction and growth, use our guide on how to tell if foundation cracks are serious.
Why truss uplift is milder here than up north
Truss uplift is a cold-climate problem first. It is worst where winters are long, cold and damp and attics carry deep insulation, because the top chords sit cold and wet for months while the bottom chords stay warm. Central Texas winters are mild. Hard freezes are rare and usually short, and the attic warms and dries out between fronts. So the gaps we see here tend to be smaller, and they come and go with the colder, wetter winters instead of showing up like clockwork every year.
It still happens. Houses with long-span trusses and thick blown-in insulation over the ceiling are the most likely to show it, and an unusual winter can bring it out in a house that never had it. February 2021 was that winter. The National Weather Service office for Austin and San Antonio recorded San Antonio International Airport below freezing for four and a half days during Winter Storm Uri, and its summary lists burst water pipes among the impacts. If a gap appeared after a hard freeze, look for a second cause before you blame the trusses. A pipe that split in the cold can keep leaking out of sight, and if that water reaches the clay under the slab it swells the soil and can lift part of the floor. Our piece on freeze damage to Texas foundations walks through that chain.
What does not happen much here is frost heave, the northern problem where frozen ground lifts footings. Frost depth in Central Texas is shallow, and the ground rarely freezes deep enough to move a foundation. A winter crack in this part of Texas is far more likely to be the roof framing or the clay than frozen soil.
How truss uplift gets fixed or hidden
Truss uplift is not a structural defect, and it does not call for a structural repair. The trusses are doing what wood does. The fix is about letting the ceiling move without tearing the finish, and that is carpentry and drywall work. We are a foundation company. Our work is piers, beams, shims and leveling, and none of it changes a seasonal ceiling gap. A contractor who quotes foundation work for one has not done the calendar test.
The most common fix is trim. Crown molding fastened to the ceiling only, never to the wall, rides up with the ceiling in winter and back down in summer. Its bottom edge lifts a little off the wall paint instead of a caulk line cracking open, and from the floor it is much harder to notice than a torn corner. The fuller fix is a floating corner. The ceiling drywall is left unfastened for the first foot and a half or so out from the wall, and its edge is held by drywall clips on the top of the wall, so the sheet flexes as the truss rises instead of tearing at the corner. The Building Science Corporation research recommends keeping ceiling fasteners and adhesive at least 18 inches back from interior walls for exactly this reason. That usually means opening the corner, so most people do it when the ceiling is being redone anyway.
In the attic, the trusses should be tied to interior walls with slotted clips that keep the wall straight side to side but let the truss slide up and down. Do not nail or screw the trusses down to the wall to stop the movement. A truss is designed to span from outside wall to outside wall, and pinning it to a partition can load the wall and the truss in ways nobody designed for. If you want the attic connections checked, ask a framing contractor or the truss manufacturer.
Patching the gap with caulk or fresh tape in summer, while it is closed, buys a few clean months and tears again at the first cold snap. Patching it in winter, while it is open, tends to buckle or crack when the ceiling settles back down. Either way, the finisher or trim carpenter is the right call for a confirmed truss uplift gap.
Getting the gap measured
If the gap keeps the winter calendar and nothing else in the house is moving, you can watch it and call a drywall finisher when you are ready. If it is growing year over year, if it showed up with any of the foundation signs above, or if you simply want to know, have the floor measured. A ceiling gap cannot tell you whether the slab is level. A floor elevation survey can.
On a free inspection we take floor elevation readings across the house, read in tenths of an inch, map and photograph the cracks, and walk the drainage outside. The survey shows whether the floor is out of level and, if it is, which part of the house dropped or rose. Our guide to the foundation elevation survey explains how to read one. About a third of our inspections end with no repair needed, and a winter ceiling gap over a level floor is one of the ways to land there. Our inspections are led by ICC-certified inspectors, and when a house needs an engineer's report we work alongside independent engineers.
We inspect homes in Bexar, Travis, Williamson, Hays, Comal, Guadalupe, Kendall, Medina, Bastrop and Caldwell counties, and in that area the inspection is free. Book a free foundation inspection or call (210) 816-0034. If you live somewhere else, the calendar test and the signs above still apply to your house, and for a confirmed truss uplift gap the trade you want is a drywall finisher or a trim carpenter.