Green side up

A couple who want to buy a new house go to the building site. They meet the site manager in the office and he agrees to let them look round the show house.
He opens the door to let them inside and stops and shouts “Green side up” then follows them inside. He shows them round the lounge and halfway through describing all the wonderful features he opens the window and shouts out “Green side up”
They go into the kitchen and the same thing happens. Halfway through he opens the window and shouts out “Green side up” Upstairs they tour through all the bedrooms and the bathroom and in every room he opens the window and shouts out “Green side up”
Finally he takes them down to the hall and asks if they have any questions. The husband says he does and asks. “Why do you keep opening the windows and yell ‘Green side up’?”

The site manager says “Oh that. I have a gang of Irish labourers in the garden laying turf”