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I dug up my compost pile last weekend. it is made of aerated concrete blocks, just layed in a square, no cement. located on the rooftop of our 4 stories building, located in Hong Kong.

Beside the worms, BSF larvae, flies and other insects, I saw a lot of geckos and a nest of cockroaches.

In that specific environment, should I consider them as pest?

I have none in the house, in Hong Kong, cockroaches are extremely common and they are really located at the compost pile place only.

I believe they are helping the organic matter breaking down process, but is that true? do they come without side impact?

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I am going to answer from a scientific viewpoint, and away from the 'eww' factor that often pervades advice websites.

Many cockroaches are detritivores, which according to the Harvard Library website Cockroaches, they

live on decaying vegetation and the like. They can play an important role in breaking down decomposing organic matter and cleaning the environment.

There are some websites and groups that advocate keeping and even breeding cockroaches for composting, such as the Unconventional Farmer Cockroach Composting page, which state, in terms of compost benefits,

Eliminate waste efficiently (produce few nutrient dense droppings per large food intake)

Also, if they are in your compost they are happy - a warm moist environment with plenty of food. They are very unlikely to venture into the house. According to the World Health Organisation document Cockroaches, even though they are not considered to be a primary source of disease, they can spread intestinal diseases, such as

diarrhoea, dysentery, typhoid fever and cholera.

the WHO also states that the key to control is cleanliness, deny them a food source anywhere but their happy compost home and they would have no reason to venture from the compost.

Whether or not they are a pest depends on their species, according to Johnson Group Pest Control page (the page is included as it includes images as part of the descriptions),

Common types of cockroaches in Hong Kong include Periplaneta americana (American cockroach), Periplaneta australiasiae (Australian cockroach) and Blattella germanica (German cockroach). Of these three species, the one that has the greatest potential for becoming persistent and troublesome is the German cockroach, which prefers indoor locations.

If they are on the roof, it may be an option to have the air ducts etc checked for other nests within the building.

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    that is a damn good answer. I believe our house is clean then, because I saw one only inside over the last 12 months, and no neighbour talked about that neither. They are apparently from the Australian type, though not so sure about that. I'll dig up the pile this weekend to see. Anyway, the answer is complete, they are not pests, might though carry some diseases (let's be fair, like all other animal, being pest or pets). I'll see later whether they can be food for my fishes :)
    – Memes
    Commented Nov 14, 2014 at 10:39

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