I created a yeallow rectangle as the background, put a gray one over it and then a yellow one over that. I lined the top yellow rectangle up with the gray one. Despite the fact that i lined them up the line appeared, but it disappears when i zoom in. I tried to clip the objekt, but the problem stayed. 

Thanks for any help.

Please consider another approach to avoid anti aliasing issues: Draw a rectangle for the door over the house shape and go Path->Difference - adjust fill+stroke color:

thx for your fast answer, just saw it now