Would putting a sprinkler on my roof help cool my home?

Seems like it would be a bit wasteful and not that efficient.

I'd imagine being on the roof, ambient temperature is much less of an issue then the constant sun exposure just baking the unit. Like when I get in the car here in TX sometimes, ambient temperature is in the 90s but the car baking in the sun says its 110+. If it were parked in the shade, it would read the normal temperature.

So I would think focusing on blocking as much sun as possible would be the best solution.

I'm picturing some sort of open sided mini roof structure, almost like a little car port over the unit or a well ventilated little shack built up around it.

These are just random crazy ideas of my own though, never heard of this issue nor a solution for it.

EDIT
This is under the assumption the AC unit is what you are trying to cool, if it is the temperature of the roof you are wanting to reduce, I'd encourage considering @Vebjorn Ljosa's and @Jeff Widmer's answers over a water cooling solution.

I think you will just waste a lot of water compared to the cooling effect that you get.

A better way to cool the roof is to change its color. A roof with a clean, smooth bright white surface can reflect about 85% of incident sunlight and emit thermal radiation with 90% efficiency. This surface will be only 9°F warmer than the outside air on a typical summer afternoon. For comparison, the surface of a standard gray roof that reflects only about 20% of incident sunlight will be 69°F warmer than the outside air. See http://coolcolors.lbl.gov/.

Some buildings in California are now required to have light-colored roofs in order to reduce energy consumption for cooling.

If this is for a building with windows that open, I'd think instead about sprinklers/lawn/misters at ground level - my house has no A/C but we get a reasonable north-south breeze, and if I turn on the sprinklers on the north side of the house and open the windows, there's a noticable cooling effect.

In high desert country, it's not unusual for restaurants with outdoor seating to install mist delivery systems to cool the outdoor areas during the hot parts of the day.

Ultimately, the value of evaporative cooling is going to depend a lot on the relative humidity in your location - where the air is dry, evaporative cooling works nicely. If you are in a humid location, there's already plenty of moisture in the air, so you're not going to get much evaporation, and correspondingly not much cooling.

I have heard of people using roof-mounted sprinklers as a last line of defense in high fire danger areas - not sure how well it really works.

Related to @Vebjorn Ljosa's answer, you could also install solar panels on your roof. Check out an organization called One Block Off the Grid (http://1bog.org/) if you are interested in putting solar panels on your own roof. 1Bog will help with getting a local contractor and depending on the state you live in, you could end up selling your energy credits back to other businesses for extra money.

Plant an oak tree, wait 10-20 years, shade created, problem solved!

The question was will it cool the house, and yes it will. As for efficiency, cost, etc.... I live on a lake and use a pump.

I live in southern Louisiana. In 2014 on a hot summer day i have checked the temperature in the attic and was 130°. I then applied a continuous sprinkle on the roof and the temperature in the attic was maintained at 85° with a outside temperature of 98°. It works and my AC started working much less.

It works for sure,especially on metal roofs. When continiously sprinkling a metal roof the roof works like a heat exchanger. You are carrying heat away faster than it comes :)

Gotta love armchair quarterbacks. I live in the Caribbean. Houses are constructed of concrete block with stucco/parging covering the block. Roofs are usually terracotta tile or concrete tiles. Very few dwellings have an attic space to separate the exterior roof from the interior roof and insulation is a word that hasn't yet been defined here. Concrete stores solar heat all day and releases it very slowly at night when the outside temperature can be as much as 20 degrees cooler than the interior temp. On their own, temps inside don't start to fall off until 3am and the sun comes back 3.5 hours later. Not much of a respite. Light colored paint helps, but concrete is still concrete and hot concrete stays hot until something cools it off like a good soaking rain.

I've done exactly this. Hooked up a heavy duty impact sprinkler head on the center peak of the roof. Attached a garden hose and a water timer. Adjust sprinkler to end it's arc about three feet from the edge of the roof. Water is sprayed for 10 minutes every hour to thoroughly soak the roof. Excess water into the gutters and into a rain barrel that drip feeds the banana plants that just love lots of water.

Inside of the house is 10 to 15 degrees cooler than before I started spraying. Would have preferred to run the sprinkler every 30 minutes for 5 minutes but could not find a hose compatible timer with that degree of programming precision. I have found it best to keep the roof from getting hot in the first place rather than trying to cool it down after it's hot. Timer starts about 30 minutes after sunrise and shuts off for the night about 1 hour after sunset.

Whole system, costs less than $100.00. Makes a big difference. Water use is high but I'm on a well so I don't care. A $1000+ roof misting system might be more efficient at cooling the air above the roof and use less water, but humidity is often high here in the Caribbean so air cooling via evaporation may be hit and mist (pun intended). Water that is cooler than the roof cools the roof. It's that simple.

I knew of one guy who tried a soaker hose on the roof. Since he turned it on only when the roof was hot, it took grains off the shingles, shortening the life of the roof.

I would want this kind of cooling for only a short burst of heat that overloads my air conditioner. We have maybe a two or three day burst of heat once a year that makes a boost necessary for the air conditioning. Otherwise it is fine. All of the expensive solutions (solar panels, paint the roof, plant big trees, etc) are a waste for this kind of situation.

What about spraying the vinyl siding on the side the sun hits with a mist?

I am a retired engineer in humid Maryland with a house having a flat roof (1/4" per foot slope), a typical gravel built up roofing. Along the high edge, I installed a drip piping system, 1/16" holes on 18" C/C, a few inches above the roof, valved down to a drip flow. It is controlled by a timer which turns on for 15 minutes once every hour or so. In hot temps, near 100 F I run it more. I can see the wet line slowly drain down to the gutter edge. It wets about 80+% of the surface. (It will not work well on sloped roofs, as it needs resident time to evaporate.)

The gravel on my driveway will run 130 F, the minor drip out of the low side down spout runs 8o deg. This is what my 5 ton unit fights. Most of the water evaporates, some runs off. The evaporation works in the boundary layer in which the humidity is far less than what is reported on the weather report.

I do not have enough instrumentation to accurately analyze the system but guesstimate it is a 6 - 20 ton equivalent unit, more in hot days. It costs about 50 cents per day, on the hottest days.

It also eases the thermal stresses on the roofing material in which heat kills the elastomers, and of course drastically reduces the wear and tear on the compressor.

I would patent the idea but it is not mine. My dad, an engineer, used it in WWII, in the military, when only limited refrigeration units were available. He made meat lockers for the troops, beneath a cooled roof.

A soaker hose fed by a small pump (say 0.5 GPM) from a rain barrel under a downspout would work beautifully. Power the pump directly from a solar panel, and VOILA! instant daytime controls.

Since anything that doesn't evaporate returns to the rainwater barrel, and the sun's power is free, I see no need to tend it at all unless there is a dry spell or when winter is on its' way.

As a rule of thumb, splitting the difference helps unpack the concept. 110 deg. F roof contact temperature imparts heat to 70 degree F water. Thus underside of the roof membrane should fall 20 degrees to 90. That's half way to the cooling the roof surface, and possibly a quarter of the entire cooling load.

water evaporation = 2250 kj/kg at best an AC has outpout of 500% => evaporation of water is tens of times cheaper

Yes, water on the roof will help cool it. Cooling with liquid water running off from a sprinkler is not efficient, but evaporative cooling from a small amount of water (like a periodic sprinkle) is very efficient.

1 gallon of water consumes 8000 BTU as it evaporates. If you spread 1 gallon of water on the roof once an hour and it evaporates you've made the same cooling effect as adding an 8000 BTU air conditioning unit. (They are rated in BTU of heat moved per hour.)

That's like a 0.66 ton AC unit dedicated to cooling your roof surface! (In HVAC, a 1 ton means moving enough heat to melt 1 ton of ice in 24 hours, which takes 12,000 BTU per hour. So 8000 BTU / 12000 BTU = 1 ton of cooling).

Add too much water (or too often) and it begins running off, at which point not all of your water is doing evaporative cooling. It's doing regular heat transfer via thermal mass, which has a much smaller impact.

It takes 1 BTU to heat a pound of water by 1 degree. If you put 1 gallon (8.34 pounds) of 70-degree water sealed in plastic on a 160 degree roof, and the water heats up to 130 degrees, you've consumed 500 BTUs of heat from the roof (60 degrees x 8.34 pounds). If it's not sealed in plastic and evaporates, you consume an additional 8000 BTU as that gallon evaporates. Once the evaporation cools the roof to, say, 100 degrees, an additional gallon of 70 degree water can only warm up to at most 100 degrees, a 30 degree increase, which would only consume 30 x 8.34 = 250 BTU.

Simply put, your cooling is very efficient if little or no water runs off. Add as much water as you can without any runoff to get maximum efficiency cooling.

In industrial buildings, this kind of evaporative cooling doesn't just reduce the heat coming in from the roof - it can reverse the heat flow and make the roof a cooling element! Warm air inside a large plant will rise and will often be well above 100 degrees inside the building; an evaporation-cooled roof could bring roof surface temp down from 165 F to 90F degrees and will be carrying heat out of the building.

Some engineering calculations at the industrial level: https://www.plantengineering.com/articles/consider-evaporative-roof-cooling-to-reduce-your-hvac-load/