Moss on the roof (again)

  fullyfitted 21:20 17 Feb 2011

Now that the weather is turning slightly warmer I havea prolific growth of moss and other weeds on my flat extension roof. This is granulated felt covered with a further layer of gravel.

I can’t find an earlier thread on this topic (but I seem to recall it was concerned with pitched concrete tiled roofs) and therefore I would appreciate some advice.

I intend to try and remove as much of the greenery as possible using a plastic rake as I suspect that the roots can be intrusive and may damage the felt. After raking I intend to apply weed killer, any recommendations on types would be appreciated.

Finally I would add that the roof is 30yrs old and this is the first time I’ve had this bother, however I did have a new CH boiler fitted in 2006 and when the wind is in the right direction the steam from the vent goes over the roof. Even though it’s two meters away I suspect it could be providing ideal breeding conditions



  peter99co 21:25 17 Feb 2011

Is it carbon monoxide that it is thriving on?

  fullyfitted 21:32 17 Feb 2011


Better not be, Boiler was only serviced in November by BG


  grey george 21:55 17 Feb 2011

30 years from a flat roof is good going 15-20 is the average. This year seems to be a good one for moss and algae. You need to be sure any chemicals won't effect the felt or enter the water course. Is the moss really a problem? The birds love picking through it.

  fullyfitted 22:09 17 Feb 2011

grey george

Thanks for the response, to be honest I don't know if it's harmful, that why I asked. I've just got a nagging memory from the previous post.

With regard to weed killer the one I use on the patio contains GLYPHOSATE. This comes with dire warnings about not letting it enter the water course,so that one is a non starter

  john bunyan 22:18 17 Feb 2011

I spent a week or two on my concrete tiled roof with a scraper, wire brush and a dustpan and brush. Good views but I am over 70 and my wife was a bit concerned! I think a bare copper wire along the ridge may deter moss...

  Forum Editor 23:48 17 Feb 2011

for two reasons - there's a substrate for it to grow in, and a constant supply of moisture.

Gravel is used on a flat roof for two reasons - it reflects sunlight, and it is not unattractive, at least for a while. Solar heat gain can considerably shorten the life of a flat roof, which is why conventional roofing felt (and the more modern torch-on bituminous polymer membranes) have mineral chippings bonded to the surface - they provide a heat reflective layer.

Gravel on a roof receives a constant rain of fine dust particles from the atmosphere, and these aren't efficiently washed out by rainfall - they accumulate and form a layer of mud at the bottom of the gravel. Airborne seeds germinate, and their roots get onto the mud, forming even more of an obstacle to free drainage, and so end up with your 'prolific' plant growth.

The answer isn't weed killer - it will wash away, and the plants will regrow. The answer is to remove the gravel layer and all the mud and debris, wash the gravel with a hose to clean out all the muck, and relay it on the roof, afte a careful check to make sure that the waterproof covering is sound, and shows no signs of failing. Alternatively you could dispose of the dirty gravel and put new stuff on the roof - it's easy to do.

As grey george has said, the roof owes you nothing - at 30 years old it is definitely well into injury time.

  Forum Editor 23:49 17 Feb 2011

"I think a bare copper wire along the ridge may deter moss..."

It would if there was a ridge - we're discussing a flat roof.

  morddwyd 08:04 18 Feb 2011

Coincidence - I had some dubious character at my door yesterday afternoon saying he was doing some work for a neighbour and offering to remove the moss from my roof. I declined politely (well, politely for me!)

Don't want to hi-jack the thread, but is moss very harmful?

  john 52 09:46 18 Feb 2011

Just a suggestion do you not think walking and and raking the surface may do more harm than good to the 30 year old flat roof .

  carver 09:53 18 Feb 2011

May I suggest that you have the roof re felted, if it is as old as you say then it's well past it's best by date, the cost will be considerably less that water damage caused by a leaking roof.

Some insurance company's are now being a bit awkward when a flat roof is left as long as that before being replaced.

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