Need advice from an artist

Below is a screen shot of a map I have begun.  When I was checking passability I noticed a significant problem.



I used the sloping roof as a base to make autotiles for a straight roof and to make the flat roof.  I have made them so that all can be walked behind and as you can just see from the house bottom right, that is really important so that the player can get to the door in the middle of the house.  It also looks better, imo.

However, when testing by walking behind the roofs I found that the original sloping roof is slightly transparent, meaning that you can just detect the sprites behind the roof - this, naturally, looks ridiculous.

My question is: how do I make the tiles completely opaque without mucking up the colour/hue?

Thanks.

Im not sure if i can help or not, but what graphics editing program did you use? 

GIMP.  It's what I use for all editing/making tiles.

Alright, this is a stretch, but do you still have the original "gimp" file that you edited the tile in? No the exported png image, the actual gimp file.  

All done in one layer as it's so simple to do, so I have the original from which I took the sloping roof, the tile sheet with the flat roof on, and the auto tile for the straight roof.  All as .png files.  

I wonder if you added a layer behind it, painted it white, selected the transparent part of the top layer and deleted that same area on the bottom layer, then merged them, would it make the roof fully opaque without changing the colour?


I really know very little about making art, so it's just a guess.

Oh well, thats to bad then, dont think i can help, but what shaz just said just might. :)

@ Shaz

I tried what you suggested, but unfortunately it didn't seem to have much effect.

Then I thought of what should have been obvious from the beginning.  I took a copy of one of the offending pieces, and added it on top of the original.  That seems to have worked.  Now to do the same for all the other bits and pieces.  Tedious, but at least my sprites aren't showing up like ghosts under the roof.

Many thanks to you both for your time and trouble.