The visible roof is a shield, and the layer that truly keeps water out is the underlayment underneath it.
At a glance
- Deck, underlayment, flashing and covering do four different jobs.
- Water gets in at edges and holes, not through the middle of a field.
- Ventilation is part of the roof, not an extra.
- Every layer has a life span, and they are not the same.
The deck
The deck is the wood surface nailed across the rafters. On homes built after about 1960 it is plywood or oriented strand board. On older Los Angeles bungalows it is one by six boards with gaps between them, which roofers call skip sheathing.
The deck carries the load and gives the nails something to bite. Soft or delaminated decking has to be replaced before anything goes over it, and no roofer can tell how much is bad until the old roof comes off.
The underlayment
Underlayment is the sheet that runs across the whole deck under the roof covering. It is the last line of defense and, on a tile roof, it is the real waterproof layer.
The old standard was asphalt saturated felt in thirty pound weight. Modern jobs use synthetic sheets or a self adhered membrane at the eaves and valleys. Synthetic lays flatter, tears less, and holds up longer in heat.
The flashing
Flashing is sheet metal shaped to bridge a joint. It goes where the roof meets a wall, around a chimney, in a valley, at a skylight curb, and around every pipe.
Almost every leak a roofer chases starts at flashing. Metal moves with heat, sealant dries out, and a joint that was never layered right will let water in the first time rain drives sideways.
The covering
The covering is what you see. Shingles, tile, metal panel or membrane. It takes the sun, the wind and the foot traffic so the layers below do not have to.
It also sets the look and the fire rating of the assembly. In Los Angeles the fire rating is not a small detail, because much of the county sits in or near a high fire severity zone.
The air
A roof needs to breathe. Cool air enters low at the eave through soffit or bird block vents and leaves high at the ridge or through a static vent. That flow carries out the heat that would otherwise cook the shingles and the moisture that would otherwise sit on the sheathing.
A blocked soffit is the quiet killer of roofs in this city. Insulation gets pushed into the eave during an energy upgrade, the intake closes, and the attic turns into an oven.
Common questions
Which layer fails first
Almost always the flashing or the underlayment, not the shingles or the tile you can see.
Can a roof be waterproof without underlayment
No. Even a metal roof needs it, because condensation forms under the panels.
Does skip sheathing have to be replaced
Not always. Many tile jobs go back over it, and shingle jobs usually need solid sheathing over the top.