i wouldnt put window cleaning in the same category as tiling because, one is a skilled trade and the other is a low skilled cleaning operative and i dont mean that in any way derogatory....
what ever way you tot up the sums, it normally equates to the same .......
x amount of meters in a day = y amount of money
or
x amount of houses windows cleaned = y amount of money
its your choice, if you can make more money running a window cleaning round then tiling then thats fair enough...
if you have done the sums and you think its a winner then go for it
it wont take long to train somebody and if you can generate the work load for the lads then you'll have no worries will you?
i have friends who have car valeting business's and window cleaning business's but as with doms mate, its usually the property and other sidelines which bring the big money in......i just think you would make more money from tiling and restoration work imo because thats your niche
anyway, go for it, hope it works for you mate