Los Angeles gets almost no rain for seven months and then gets a season's worth in a handful of storms. That pattern is exactly what catches job sites out: crews dry-in late because it has not rained since spring, and the first atmospheric river finds every open deck, unlapped wall and bare slope at once. The work below is not complicated and none of it is expensive relative to the damage it prevents, but it has a deadline. This is the checklist of what to have on hand, in the order water actually reaches your project: the roof, then the walls, then the ground.
Start at the roof, because that is where the water starts
An open or partially covered roof deck is the most expensive thing on a site when it rains, because water that gets through the deck ruins framing, insulation and finishes below it rather than just the roof. Dry-in is the priority, and dry-in means a continuous underlayment properly fastened, not a tarp weighted with scrap.
Synthetic underlayment is the practical choice for a fall dry-in: it holds up to weeks of UV exposure, it does not wrinkle and tear the way felt does when it gets wet, and it is safer to walk on a sloped deck. Our underlayment comparison covers where felt still makes sense.
Fastening is what decides whether the underlayment survives a wind-driven storm. Plain roofing nails pull through synthetic sheets; cap fasteners do not. 1 in. plastic cap nails are the standard for underlayment and housewrap, with 1-1/2 in. caps where the sheathing is thicker or a second layer is going on. Our roofing fasteners guide covers cap nails, coil nails and the length rules inspectors check, and if you are capping with a gun, the staple crown and leg guide covers the alternative.
If the roof is finished, the pre-storm job is different: clear the valleys and gutters, check that flashings and penetrations are still sealed, and look at the low-slope sections, which is where standing water finds the smallest defect. The roof coatings range covers recoating flat and low-slope areas while the weather still allows it, and our shingles guide covers the steep-slope side.
Walls: barrier, laps and every penetration
Water that gets past cladding is normal and expected; the weather-resistive barrier behind it is what is supposed to catch it and send it back out. On an open wall, that barrier has to be continuous and lapped correctly before the rain, not after.
Asphalt-saturated building paper is the standard behind stucco in Los Angeles, with the 60-minute grade where a heavier paper is specified, and California practice on wood-based sheathing is commonly two layers. Our housewrap guide covers the lap direction and sequencing, and the rule is simple enough to check from the ground: upper sheets always lap over lower sheets, so water runs down and out rather than into a joint.
If the wall is lathed but not yet plastered, the lath and accessory metal are what the storm will test. Screeds and trim set to the finished plane, netting that is not oil-canning, and laps kept out of corners all matter more once the wall is wet. Our stucco systems guide covers the lath and accessory stage in detail.
Penetrations are where most wall leaks actually happen: hose bibs, conduit, vents, light boxes and window heads. Sealant applied to a dry surface now is worth far more than sealant applied to a wet one in December. Match the product to the joint and the substrate rather than defaulting to whatever tube is in the truck, which our sealant comparison lays out, and the full range is in the caulk and sealant collection.
The ground: slopes, flows and the stuff that moves
Roof and wall work protects the building. Yard work protects everything downhill, and in a city built on hillsides that often means a neighbour's property, a street, or a storm drain the city will ask you about.
Bare soil is the problem. Any slope stripped for grading, foundation work or a landscape conversion will move in the first real rain unless it is covered or interrupted. Straw wattles installed on contour slow sheet flow and catch sediment, erosion control blanket covers exposed faces, and woven ground cover fabric pinned with garden staples holds loose areas over the season. Our hillside erosion control guide covers laying these out so they actually work rather than simply being present.
For the water that has to be diverted rather than slowed, water-activated flood barriers and flood bags store flat and dry and expand when they are needed, which makes them far easier to keep on a site than sandbags you have to fill in advance.
Below grade, the fix is drainage rather than barriers. Perforated 6 in. perforated S&D pipe in a rock-filled trench, tied into solid line where the water needs to be carried rather than collected, is the standard answer at a foundation or the base of a wall. Our retaining wall guide covers the drainage and backfill sequence, and the full range is in the drainage collection.
The checklist, in order
Work it in the order water travels, and do it in one pass rather than a piece at a time.
- Roof deck dried in with underlayment, fastened with cap nails, laps running downhill.
- Valleys, gutters and downspouts cleared; low-slope areas checked for standing water.
- Wall barrier continuous and correctly lapped; upper sheets over lower sheets everywhere.
- Every penetration sealed on a dry surface: hose bibs, conduit, vents, window heads.
- Bare slopes covered or interrupted with wattles, blanket or pinned fabric.
- Diversion on hand where flow has to be redirected, stored dry and ready.
- Foundation and wall drains open, outletting somewhere, not buried in fines.
- Stockpiles covered and materials that swell moved off the ground.
That last item is the one people skip. Bagged cement, plaster and mortar are ruined by a single wetting, drywall stored flat on a slab wicks water, and OSB left exposed for weeks swells at the edges in a way it does not recover from.
Ordering ahead of the weather
The awkward part of storm prep is that everyone does it in the same week. Underlayment, cap nails, building paper, wattles and sealant all go short across Los Angeles when a storm shows up in the forecast, and the crews that dry in cleanly are the ones who bought in September rather than the morning the front arrives.
Buy the whole sequence at once: underlayment and caps, paper and sealant, wattles and staples, pipe and rock. We deliver across Los Angeles County; send the roof area, the wall elevations and the slope conditions and we will put the dry-in package together on one truck.
Frequently asked questions
When should I dry in a job site in Los Angeles?
Before October. LA's rain arrives in a small number of large storms rather than steadily, so there is rarely a gentle first rain to warn you. Treat the end of September as the deadline for roof dry-in and wall barrier rather than waiting for the forecast.
Can I use a tarp instead of roof underlayment?
A tarp is emergency cover, not dry-in. It is not fastened to shed water in a coordinated way, it tears at the grommets under wind, and it leaves the deck exposed the moment it shifts. Properly fastened underlayment is the correct material.
Why do I need cap nails instead of regular roofing nails?
The plastic or metal cap spreads the bearing area so the fastener does not tear through the sheet under wind. Synthetic underlayment and housewrap both pull through plain nail heads, which is exactly how a dried-in roof fails in the first storm.
How many layers of building paper go behind stucco?
Two layers over wood-based sheathing is the common California practice, but the requirement depends on the substrate and the system. Confirm it against your approved plans before the lath goes up, since it cannot be corrected afterward.
What is the quickest way to stabilize a bare slope before rain?
Straw wattles installed on contour, trenched in slightly so water cannot run underneath them, with erosion control blanket or pinned fabric on the exposed faces between them. It is fast, it needs no equipment, and it works by slowing flow rather than trying to block it.
Do I need drainage if I just installed a retaining wall?
Yes. Backfill drainage is what keeps water from building hydrostatic pressure behind the wall, and it is the most common reason walls fail. Perforated pipe in clean open rock, outletting to daylight or a drain line, is the standard detail.
