Monsoon and your roof

A Phoenix roof has an unusual job description. It never carries snow, it sheds 7.22 inches of rain in a whole normal year, and its worst twenty minutes are usually not the twenty minutes of heaviest rain. They are the twenty minutes before it, when the outflow of a monsoon storm arrives ahead of the rain shaft with nothing falling out of it yet.

So a monsoon roof checklist is not an ordinary maintenance schedule. There is little to do about the water, which is scarce. Nearly everything on it is about three other things: drainage clear enough for the rate the rain actually arrives at, wind that cannot find an edge to lift, and dust out afterwards so the next storm drains as well as the last.

What a roof is actually up against

Three things, and only the first of them is rain.

Rain, in short and heavy bursts. The counts above are the ones that matter, not the totals. A normal August wets the roof on 4.6 days and delivers 0.93 inches across them. The exception is the day that reaches an inch, and there is roughly one such day in the whole season. On a flat or low-slope roof that is the entire test: not how much falls in the season, but whether the scuppers can move what falls in twenty minutes.

Wind, and it arrives before the rain does. The Phoenix forecast office’s own figures are that downburst gusts commonly exceed 40 mph and can top 100 mph, and that the severe thunderstorm warning criterion is 60 mph or greater, possibly with hail an inch or larger. Wind is the reason roof cover fails in a storm that drops almost no water: the outflow spreads outward from the storm and reaches a house minutes before the rain shaft does. The mechanism is set out in Microbursts and the wind that comes first.

Dust. The same outflow that lifts the dust deposits it, into the scuppers and the gutters, and into the drywells at ground level that are the far end of the same system. A roof that drained in June may not drain in August, and the silt is the reason.

The season is fixed, opening on 15 June, which is what makes the pre-season work datable rather than a matter of judgement. Everything that follows is written for dry weather.

The pre-season checklist

Five items, all of them requiring dry weather, which is why they belong in May and early June rather than in the week a storm is forecast.

• Roof drains and scuppers. The city’s instruction is a single sentence: “Clean out roof drains or scuppers to prevent accumulation of rain water on the roof.” Everything that ponds on a flat or low-slope roof gets in through a blocked drain, and a scupper that has silted quietly over three years gives no sign that it has stopped working.
• The drywells, which are the same job further down. A drywell is a pit that holds storm water until the ground takes it, and the city names clearing them alongside the roof work. A silted drywell holds nothing and sends what it should have taken somewhere else: usually toward the slab.
• The trees. The city’s word is thin, not shorten: “Thin out trees and trim down overgrown vegetation.” A thinned canopy lets wind pass through it. A dense one catches the wind and turns the trunk into a fulcrum, which is how a limb ends up on the roof rather than beside it. The federal thunderstorm guidance names tree trimming before storm season as well, and that is the whole of the reason.
• The ground, so water leaves. The city’s line is to “repair landscaping to ensure water will run away from your building”. This is the cheap version of the drainage work, and it decides where the water goes when the scuppers are overwhelmed.
• Anything with a large surface area and no weight. The utility’s list of things that get swept up and end up in power lines (umbrellas, kiddie pools, trampolines) is a list of objects that are big, light and unanchored. That principle covers the shade sail and the loose patio cover as well, and it belongs to the pre-season list rather than the storm-day one, because moving it is the job you want already done.

The item the official lists leave out

One gap is worth stating, because a reader who has been through a hailstorm will look for it. Neither the city’s pre-season list nor the county’s mentions hail, and on the forecast office’s monsoon page the only hail figure is the one-inch criterion inside the severe thunderstorm warning: a threshold for issuing a warning, not a maintenance instruction.

That is not a claim that hail does not happen here; it is a statement about the lists this site could open. If you have had hail damage, that belongs to your insurer and a contractor rather than to this checklist.

The storm-day checklist

Nothing here is a task. All four are things not to do, or things to do before the wind arrives.

• Bring the loose objects in. The last moment this can be done; once the outflow is on you, crossing the yard is the hazard.
• Do not go up on the roof. Not to clear a drain you can hear backing up, and not in the flat calm after a gust front: that calm is the gap before the rain, not the end of the storm. No guidance describes walking a roof in a monsoon, and a fall is the injury this article is most concerned to avoid.
• Take the interior room when the wind arrives. A downburst arrives ahead of the rain and can take roof cover off a house that is barely getting wet. The city’s high-wind guidance is a central interior room without windows, and in a mobile home it is to leave rather than ride it out.
• Stay off the roof for thirty minutes after the last thunder. Both the forecast office and the federal guidance give the same clock, and it means the last rumble rather than the last of the rain.

For the order of the wider storm-day work (the patio, the power banks, the pets) the Monsoon Prep Checklist Builder prints the sourced version for a home, a car and pets, and keeps the claims with no official source in their own block so you can see which is which. Anything that involves a monsoon outage and the air conditioning that goes with it sits on the same calendar as the Home Cooling Task Scheduler.

The after-the-storm checklist

Four items, and the county’s ordering principle applies to all of them: document before you clean.

• Photograph before anything moves. Both the city and the county name this, and both put it ahead of the cleanup. Wide shots of each room, then close shots of what the water touched, taken before the wet ceiling tiles reach the kerb.
• Call the insurance company first. It is the first of the county’s five steps after a flood, ahead of removing the water, for the same reason: a claim is easier to make while the evidence is still on the floor.
• Clear the drains again. The storm has just put everything the wind picked up into the same scuppers you cleared in May. Doing it now is what makes the next storm drain properly.
• Wear gloves and boots, and look before you reach. Floodwater in a desert carries silt, sewage and sharp debris, and the federal guidance is to keep your skin out of it while you work. It also warns that animals may have moved in where the water was: in a desert city that applies to the yard and the garage as much as the house.

What this checklist cannot do

Three limits, stated plainly.

It is not an inspection. Nothing here tells you whether your roof is sound, and none of the agencies named above publishes a per-roof inspection schedule. A judgement about the covering, the flashing or the ponding is a question for somebody qualified to give one.

The rain-day counts come from Phoenix Sky Harbor, one station at one point in the metro area. A storm that soaked your street may never appear in them, which is the argument for keeping your own record: see Keeping a rain log worth keeping.

And a checklist is not a preparation. The five pre-season items are the whole of what the guidance says to do about a roof, and four of them are an afternoon with a rake, a hose and a pair of loppers. The wider pre-season picture puts them in the order the city and the county write them in: building work first, and flood insurance, the one deadline that cannot move, a month before the season opens.