Phoenix gutters pull away from the fascia because aluminum expands and contracts across a daily summer temperature swing of 50 degrees or more, and that constant movement works the fasteners loose from the wood behind them. It is a fastener and thermal cycling problem, not a weight problem...
**Phoenix gutters pull away from the fascia because aluminum expands and contracts across a daily summer temperature swing of 50 degrees or more, and that constant movement works the fasteners loose from the wood behind them. It is a fastener and thermal cycling problem, not a weight problem, which is why it happens in the dry heat of June as often as it does during a monsoon.**
What is actually moving
Aluminum has a high coefficient of thermal expansion. A long gutter run on a Phoenix roofline is measurably longer at 4pm in July than it was at 5am the same morning, because rooftop surface temperatures here routinely exceed 160 degrees while the overnight low is far cooler. That run has to grow and shrink somewhere. If it is fastened rigidly at many points, the expansion is absorbed by the fasteners and by the fascia wood, and each cycle enlarges the hole slightly. Repeat that daily for a few hundred summer days and the fastener no longer has a tight grip on anything.
Why spike and ferrule systems fail first
Older Phoenix homes and a lot of builder grade installs use a spike and ferrule, which is a long nail driven through the gutter face, through a sleeve, and into the fascia. There are two problems with that in our climate. A nail resists pull out purely by friction against the wood, and thermal cycling plus dry Arizona wood shrinkage reduces that friction. Worse, the spike is driven through the gutter face, so the gutter itself has to slide along the spike as it expands, grinding the hole in the aluminum larger. Once that hole is oversized, the gutter can move even when the spike has not. Hidden hangers that clip inside the trough and screw into the fascia solve both problems, because a screw holds by thread rather than friction and the hanger does not pierce the gutter face.
How dry fascia wood makes it worse
Arizona fascia boards live at very low humidity. Wood shrinks as it dries, and a fastener set into fully dried fascia sits in a hole that is relatively larger than the one it was driven into. This is the reason a gutter on a twenty year old Phoenix house can detach from a fascia board that looks perfectly sound from the ground. The board is not rotten, it is dry and the fastener hole is worn.
What this looks like from the ground
A visible gap between the back of the gutter and the fascia. A gutter that is level in one section and dipping in another. Staining on the fascia directly behind the trough, which means water is running behind the gutter rather than through it. Fasteners whose heads stand proud of the gutter face. A gutter that rattles or shifts in wind.
Can it just be reattached
Sometimes, and sometimes not. If the trough is straight, the fascia is solid, and only a few fasteners have backed out, reattaching with hidden hangers set into fresh wood is a legitimate repair and the right call. If the gutter face is torn at the old spike holes, if the trough has a permanent dip, or if the fascia has crumbled where the fasteners used to grip, then reattaching puts a new fastener into compromised material and the same failure returns. The honest answer requires getting up to the roofline and testing the fascia, not guessing from the driveway.
The permanent fix
Three things, in order. Replace spike and ferrule with hidden hangers, spaced closer than code minimum for long Phoenix runs. Set those hangers into sound fascia, replacing any section that no longer holds a screw. Allow for thermal movement at the ends of long runs rather than pinning a run rigidly at both ends. Done correctly, the system accommodates the summer expansion instead of fighting it.
When to deal with it
Now, in October, is the right time in Phoenix. The monsoon has finished loading the system and the summer heat cycling has finished working the fasteners for the year, so whatever came loose is loose and visible. Fixing it before winter rain means the system is sound before it is next asked to carry water.