I have a mezuzah where the top part of each letter is written on the sirtut. In other words, if the height of the top part of the letter is 1 kolmus, the sirtut runs exactly in the middle, with roughly half over and half under the sirtut.
Is this problematic? I though that the top edge of the letter should touch the sirtut.
How much of the letter needs to be over the sirtut to make it pasul?