Like any job, you will find many businesses have their own way of treating your roof and are dead set on it. Is that a bad thing? Not necessarily. As long is the end result is one that you are happy with and it is not damaging your property or your home, it is all good.
Let's focus now more on the what. In roof cleaning, everyone has a special mix, a special sauce, or a proprietary treatment. Most tend to be very similar to the next guy's blend, and for many the only thing "proprietary" is how heavy-handed their employees are that day when mixing. Others have truly spent a lot of time and effort working out a specific blend based off of personal experience and the experience of others to create something they feel bests their competitors. Still, the reality is that there are always a couple consistent ingredients that tend to be in most, if not all, commercial mixes. Most mixes fall in one of two camps: the "chlorine bleach (sodium hypochlorite)" or "hydrogen peroxide-based" cleaners (there are numerous other forms and additives, but these two are some of the more common in the local, Vancouver region).
First off, ask your contractor what they spray. Anyone with knowledge of their product and their process should never have a problem telling you what they use (this does not mean they will tell you the recipe though!) or at the very least, the active ingredient. More often than not, it will be a concentrated chlorine bleach or a commercially-prepared powdered peroxide mix. There tends to be a heated rivalry between these two types of operators. So let's look at those and what they really are.
Sodium Hypochlorite (Chlorine Bleach)
This is probably the most widely used of any methods across North America. It also happens to be the most thoroughly-researched and tested, having the longest track record. "Wait, but another contractor said bleach is toxic and will ruin my roof and plants". This is what we like to call a sales pitch. There is no question that bleach can damage plants if sprayed heavily on them by inexperienced operators, just like someone throwing a bunch of bleach in their washing machine with a load of colours. Do not let scare tactics and negative marketing talk you out of a roof clean or persuade you from one style to the other. Talk to your contractors, get a quote, get a feeling for who they are, and see if they really know what they are talking about and if they back up their work. The reality is that it is no different than the bleach you use to do laundry, clean your counters and bathrooms, and add to your swimming pool. It is even used to disinfect water supplies and food preparation equipment. Why? Because it works. What makes us choose this for our roof cleaning? Simple, the people that manufacture asphalt shingles have extensively tested and approved this method as the only one to be covered by the manufacturer's warranty. So as a roof cleaner, we really have two jobs. Know our work and protect our client's home and property. That means only using warranty-approved treatments and ones that we can comfortably warranty our own work with, knowing it will last.
So what exactly is it? Simply, it is salt, water, and an electric current that creates sodium hypochlorite (NaOCl). So when it breaks down after being sprayed what happens? Sodium hypochlorite rapidly reverts back to its natural state when left to dry or is easily diluted, breaking down in the environment to water, oxygen, and salt. Hardly the dangerous chemical competitors and negative marketing would lead you to believe.
Let me start with saying peroxide-based cleaners can also produce some great results. "Another contractor said that stuff is just a gimmick and doesn't really work". Again, this is a sales pitch. Peroxide has a great cleaning ability, and can help clean your roof or algae stains. Peroxide is also a bleaching agent and is used in many of the same situations as its chlorine bleach counterpart. Concentrated peroxide can also have many of the same risks on skin and plants and if it will come into contact with plants either by leaky gutters or operator overspray, the same job preparations and procedures should be followed as with bleach, such as proper rinsing. A few drops of either spray are most likely not going to harm anything, but inexperienced and new operators can often spray nearly as much over the roof line as on the roof itself. So again, speak with your prospective contractor, ask questions, and be sure that they can satisfy your concerns. Peroxide can definitely clean a lot of things!
So what is it? H2O2 is the chemical formula, which means it is made of 2 hydrogen molecules and 2 oxygen molecules. So when it breaks down, what happens? Upon breaking down, peroxide will simply turn in to oxygen and water.
So in conclusion, do not let scare tactics sway you one way or the other. Both break down into similar states and both can clean. Talk to the companies you are interested in, make sure they know what they are doing and can help you understand the process too. The reality is if they do not have the answers on the ground, they will not have the answers on the roof. Make sure you feel comfortable with the operator, the company, and their methods.
If a company feels the need to bad mouth another or their process, they are probably a little insecure of their own methods. For us, we choose chlorine because we know the cost of a roof. So like any warranty, whether it be a car or a roof, the manufacturer has guidelines. As a roof cleaner and the people responsible for helping you keep your roof in the best shape possible, it is our job to adhere to those standards.