Here are my ideas for the web site improvements:

• Layout will not be changed but few items needs to be added -
• Logo on the black to the top header 
• Add some welcome text, portfolio and about text with some image.
• Main page navigation may benefit from adding categories - with example of images of Industrial, Residential or some other category. 
• Main page will benefit from slogan or short mission statement.
• Visual improvements:
• Emphasize slide-show images, make larger thumbnails and put them inside a frame or something like a inside computer display or laptop or iPad.
• Add some shadows (like to the gray box with slide show), gradients to make overall look more 3d than flat.
•  Metaphors
• Menu on the top - make selection visible like inverse block
• Each page needs an icon next to the page title, like "Portfolio", "About" 
• Bottom footer - add contact icons  - RSS,Twitter, Facebook, G+, Email