Thank you for your post.
I have just checked this and there appears to be an error in the tool. Thank you for bringing this to our attention, I have just passed it onto the development team to look into.
There is a way that you can easily get around it which is to toggle the overhang on/off using the 'Ctrl' button when drawing the roof. This will then either use the measurement entered for the overhang or disregard it and make it 0 depending on whether Ctrl has bee pressed or not (the line when drawing the roof will go bold and the text near the cursor will change to show this). If you do this for all the sides it should fix the issue.
Would you be able to please let me know if this fixes the issue you're having?