DDAC Isn’t a Moss Killer: The Right Way to Treat Mossy Roofs

Mark Cave

Picture a homeowner holding up a quote. It says, in black and white, "chemical moss kill, roof treatment." It looks harmless. Plenty of contractors write it every week. But that one phrase can put you on the wrong side of the regulations, and it usually means the job on the roof is going to be done badly too.

Mark Cave

I'm Mark Cave, from SoftWash UK. Last time, in the Hypo or Biocide episode, we looked at fast acting sodium hypochlorite against slower DDAC biocides. Today I want to deal with a myth that sits right on top of that: the idea that DDAC is a moss killer.

Mark Cave

Let's start with why this matters legally. In the UK, a product that's meant to kill or inhibit plants is treated as a herbicide, or a plant protection product. That sits under the HSE and the Control of Pesticides Regulations. And here's the bit that catches people out. Moss is a plant. Botanically it's a non vascular plant, a bryophyte. So if you market a product to destroy moss, you're making a herbicide claim.

Mark Cave

Now compare that with what DDAC actually is. Soft Wash Pro 50 is a 50% DDAC concentrate, a quaternary ammonium compound, and its HSE number is 10245. It's registered as a surface biocide. That means it's there to control microscopic organisms on hard surfaces, things like algae, fungi, bacteria and lichens. It isn't an approved herbicide. So the label and the registration don't give you permission to sell it as a moss killer.

Mark Cave

That's why the product information says it plainly. Soft Wash Pro 50 should not be described or used as a moss killer. And that applies to what you advertise, what you quote, and what you put on the invoice. If the paperwork says "moss kill," you've made a claim the product isn't authorised to make.

Mark Cave

I know what some of you are thinking. "Mark, I've sprayed it on a mossy roof and the moss has gone brown." Yes, it can. I'm not going to pretend otherwise. DDAC is broad spectrum, so when you apply it to treat algae and lichen, it can also affect moss that's sitting in the same area. The moss can brown and die back.

Mark Cave

But that's an incidental effect. It isn't the authorised use, and it isn't guaranteed. There's a real difference between "this sometimes happens as a side effect" and "this is what I'm selling you." Get that distinction right and you stay on the right side of the line.

Mark Cave

Now let me tell you why the myth also fails on the roof, because the legal problem and the practical problem come from the same mistake.

Mark Cave

Think about a thick cushion of moss on a tile. It behaves like a sponge. If you spray biocide onto it, the moss soaks up a lot of your solution, and it also shields the tile underneath. Spraying doesn't dissolve the moss. So the product gets used up in the cushion, while the algae and lichen spores on the surface below get far less than you think.

Mark Cave

And then there's what happens next. That moss may brown off, but it's still there. It's still heavy and still sitting on the roof. Then the rain arrives, and clumps of rotting moss slide down into the gutters and block the downpipes. The customer rings you a few weeks later, and it isn't to say thank you.

Mark Cave

So the professional approach has two stages. Stage one is physical removal. Heavy moss gets scraped off mechanically, using a profiled roof blade or a stiff nylon brush. Before you start, protect the gutters, and block or bag the downpipes so debris doesn't wash into them. Then collect what comes off.

Mark Cave

Stage two is the chemical treatment. Once the mass of moss is gone, you apply the DDAC at low pressure to treat what's left underneath, the algae and the lichen spores. That also helps slow down recolonisation. This is where the product does the job it's actually registered for.

Mark Cave

Let's talk numbers, because this is where people guess. For Soft Wash Pro 50, the guide is a 1% working solution, fifty to one, for maintenance treatments and lighter contamination. For first time treatments on heavier contamination, a 2% working solution, twenty five to one. Coverage is roughly 4 to 5 square metres per litre of diluted solution, though that depends on porosity, contamination and how you apply it.

Mark Cave

If you're using a Dosatron injector, the concentrate has to be diluted first. One litre of Soft Wash Pro 50 into four litres of water, or five litres into twenty. Then set the injector to four to one for a 2% mix, or nine to one for a 1% maintenance clean. Treat those as guide rates. Always check the label and the safety data sheet, and do a small test patch first.

Mark Cave

Two more points. DDAC is toxic to aquatic life, so keep wash off and overspray away from surface water drains, ponds and watercourses. And apply in dry weather. The biocide needs time to absorb into the tile, and a shower too soon means you've washed your money down the gutter.

Mark Cave

Now, what do you say to the customer? Don't promise instant chemical magic. Tell them the truth in plain words. Heavy moss is removed by hand, because that protects the roof. Then you apply a surface biocide that deals with the microscopic spores and the biological film, and that improvement carries on over weeks and months as the weather does its part. Customers respect honesty far more than a big promise, and it protects you when the roof doesn't look perfect in week one.

Mark Cave

This is what I mean when I say knowledge comes before products. You can buy the best concentrate in the country and still get the job wrong if you don't understand what it's registered for, what it does on the surface, and how to describe it. Podcasts like this are a great start, but if you're serious about offering softwashing professionally, the CPD Certified Softwash Training Course at softwashing.uk goes much deeper. It covers chemical selection, surface identification, application, safety, COSHH and pricing, and you work through it online at your own pace.

Mark Cave

Complete it and you can become an Approved Softwash Applicator, get listed in the directory, and join the National Association of Softwash community. The aim has always been to raise standards in this industry.

Mark Cave

Here's a challenge before the next episode. Pull out your last three quotes and your website. Look for the words "moss kill." If they're there, change them to a clear two stage description. Take care, and I'll speak to you again soon.