A Baffling Issue (at 1st) with a Raised Patio Deck

Posted by in Case Studies, Soil Issues, Winter Concerns, Wood | June 15, 2020

When we get reports of a raised patio deck pitching down, sliding glass doors in the patio walls binding, and doors not latching anymore, we begin by measuring the pitch of the floor and walls to help determine what is dropping or what is lifting.  In most cases, we find that one or more of the perimeter walls has dropped unevenly due to some type of earth movement, such as consolidation of fill material, shrinkage of soils from moisture loss, landslide, etc.  However, in one particular case it was proposed that the pitching deck and the poor operation of the doors were due to excessive weight of ice and snow that overloaded the patio foundations.

My measurement of the floor pitch, the wall pitch, and the wall lean with a four foot level indicated that rather than dropping, the perimeter of the patio deck were actually lifted.  My measurements showed that the perimeter of the patio deck had even lifted relative to the dwelling.  And what baffled me at first was that the perimeter of the patio deck had also lifted relative to the deck posts and foundation (Figure 1). 

Figure 1 Level shows that right end of top of sliding glass door opening is lifted relative to the left.

This was possible because the floor deck framing was cantilevered over a beam that was set on the posts (Figure 2).  In fact, the joists had lifted off the top of the beam exposing relatively clean nail shanks (Figure 3) and fractures at the grain line where ties had been used to fasten the joists to the beam (Figure 2).   

Floor joists for patio deck cantilever over top of beam on posts
Figure 2 Floor joists for patio deck cantilever over top of beam on posts.
Relatively clean nail shank exposed where joist lifted off beam
Figure 3 Relatively clean nail shank exposed where joist lifted off beam.

I asked myself, “What could have possibly lifted the edges of the patio deck framing?” 

As I walked around the raised patio I noticed that some of the lattice work under the patio deck was buckled and some was even fractured (Figures 4 and 5).  Ahh, the lattice work is lifting the perimeter of the patio deck.  But how?

Lattice buckled
Figure 4 Lattice buckled.
Lattice fractured
Figure 5 Lattice fractured.

Then I considered the significance of the very wet condition of the soil and the poor surface grading that allowed surface runoff to pond in the yard all around the patio deck (Figure 6).   I examined the lattice work further and noticed that it tight to the grade (Figure 6).  Again, Ah ha!  The grade had heaved lifting the lattice work and the perimeter of the deck.

Grade extremely wet  Lattice set tight to grade
Figure 6 Grade extremely wet. Lattice set tight to grade.

Further discussion with the owner disclosed that he thought the patio deck was more level than it had been a few weeks ago and he was now able to latch some of the sliding glass doors. 

Additionally, it had been an extremely cold winter, much colder than usual, and the owner reported that the posts and footings for the patio deck were set more than three feet below grade, that is, below the accepted frost line. 

Now it all made sense.  The grade frozen deep enough (frost heave) to lift the lattice enough to lift the perimeter of the patio deck, but the frost did not extend deep enough to lift the posts and their footings. 

As Sargent Preston of the Yukon would say, “This case is closed. King.”