Truss heave


Can APX 1/4” cracks in the drywall where the ceiling meets the wall be caused by moisture in the attic space?  The cracks are forming on almost all of the walls.  The house was built in the mid 60’s and is a 1 story ranch with a simple truss bearing on east and west masonry walls.  The problem just began about 3 years ago.  Nothing has been done to the house other than AC repair and paint.  The AC is on the roof, and the air handler is in the attic space.  My best guess is that moisture has either been introduced to the attic space, or removed causing the truss to expand or contract.  Don’t see any leaks on the roof, and the house is in the desert where we get very little rain, but we do get seasonal downpours, so there could be a leak somewhere that I missed.  Anyone hear of anything like this happening?

Jul 15, 19 12:54 pm

The scariest cause I've seen for that was on a commercial project that
had had a few different ill-advised additions.  When one of the new
wings were added it blocked off one the gable vents at one end of the
building - which was enough that the attic temperature increased
dramatically, causing the trusses to warp severely - to the point where
some of the truss plates pulled out completely.  In that case the
problem wasn't noticed until gyp board started falling from the
ceiling.  Have you been in the attic? Are you sure the air handler is operating properly?  Is there adequate venting?  Has it become blocked somehow?

Jul 15, 19 1:26 pm

I peeked up there. It’s my bothers house. I suspected that it could be caused by condensation from the unit and poor ventilation causing high humidity. With this heat it doesn't take much moisture to jack the humidity up. It’s about 110 here now, and the attic space feels like 150. I literally couldn’t be up there for more than a min. It so so fucking hot. I’ll check it out at night when hopefully it’s a little cooler. The AC was serviced right before the problem began, so could be from that. Just odd that such an old home all of a sudden starts having these issues.


I’ll definitely check vents. Thanks!

Wood Guy

High heat will actually drive the relative humidity down. But you probably have cool, clear nights, with night sky radiation cooling the surface of the roof below the air temperature, leading to condensation on framing members.

It is strange that it's only happened in recent years, though. It's hard to imagine how having the A/C serviced would cause a problem like this. Has anything else changed? Is the gap consistent or does it vary with location? 



The 1/4" gap is okay. The house expands and contracts. The old puddy probably is gone now. Did the building once have moulding strips up there? If so, that is why the moulding was there. Install some moulding with coped corners not mitered. You may have other issues with the house.


Need more roof vents.....differential pressure.

Jul 15, 19 1:52 pm

Any wall cracks?  These exterior or interior walls?  I've seen interior wall partitions built without a slip joint and slab on grade.  So, the slab heaves, the walls become structural, and lift the trusses from the stable walls on foundations.  This is the most common one I see.  

There's also the opposite; the foundation is unstable and sinks or lifts which causes mayhem.

Other; you sure it's trussed?  A lot in that time period are stick framed.  The ceiling joists tie it together (collar tie).  If they aren't tied together properly, the loads can push down and spread out the walls.  I've also seen nails back out over time.

Another I've seen;  Large eave?  60's doesn't have hurricane clips.. So the roof can pull out the toe nails and lift up during wind events.

I've also seen overloaded framing.  Tile replacement sorts of things, so the trusses sag and push down.  

Other oddball ones; Wood rot at the sill.  Basically, the sill plate deteriorates in areas dropping the wall and making other things support the roof framing.  I've also seen this below interior walls if there is a crawlspace or foundation crib walls.

Moisture shouldn't be causing that much growth.  60's would normally be old growth wood; that old stuff is tons better than the harvest stuff of today and doesn't warp as bad.  And with today's wood, you'll see about 1/16" per plate for shrinkage.  Usually doesn't become an issue until you get over 2 stories.  

Jul 15, 19 3:01 pm

I didn’t see any cracks on the walls. These are interior walls. All of the cracks are in the corners where the ceiling and wall meet. They look like separations more than cracks as they are 1/4” thick apx. My first inclination was that the foundation was heaving, as we do have expansive clay soil here. But, is that likely to just start causing issues after all of these years? Also, it’s slab on grade, and didn’t see any cracks in the floor tile. No vertical wall cracks either.


I'm still guessing heave. Hard to say without knowing how it's framed. So it's worst at the interior wall to exterior wall? Run a level at the floor along that wall... it might indicate the center bowing up on the slab. Another place is a level at the ceiling... If the trusses are parallel to that wall, the ceiling might shoot up to the top of wall from the bottom chord. If perpendicular, the ceiling would dive to the exterior wall if that truss is lifted. Materials flex more than most give them credit for before cracking... L/150 for deflection means stuff normally bows and bends.


Oh, I've seen old buildings start moving for a variety of reasons.. usually its a change in the subgrade moisture. That can be neighborhood development in the area along with irrigation. Irrigation is another along with 'bodies of water' like koi ponds, water features, etc.. And sewer leaks below the slab aren't uncommon as old cast iron or clay fails. Even remodels can change loading paths to marginal foundations that once were fine, and now can't handle it.


Yeah, definitely possible that the sewer pipe can be leaking. The neighborhood is pretty old...


Ya.. it can be hard to source the problem. On sewer, you could do a leak down test, but I'd start by scoping the line and cleaning it. Hopefully, the source can be identified and corrected and the soils stabilize; mud will harden sort of thing. If not, and a structural concern isn't identified, I recommend something like a crown molding detail and/or control joints that allow for it to move. Works well for those predictable, recurring cracks and a ton more cost effective versus a new foundation and structural slab for old places.


Thanks! I’m going to have him get a plumber to scope the line and see if that’s the proble


Is the condensate line draining freely? As wood guy said a volume of heated air can hold more moisture than the same volume of cooler air so the hot air in the attic should be at a lower relative humidity than the outside air - unless the air conditioning drain line is inadvertently putting moisture into the attic. The high heat and the high moisture may be allowing the roof structure to shrink and expand over a 24 hour cycle more than it would normally do absent the high humidity? Just a guess. Builders around here, and roofers replacing roofs, are going to a long continuous vent along the entire length of the roof ridgeline to improve ventilation and reduce attic temperatures. 

Jul 15, 19 5:58 pm

Ya, I’m not really sure if the line is draining correctly. I’ll have to get up there and take a closer look.

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