Marking and layout is the third step in building your deck, and is critical for a well laid out deck. Marking and laying will indicate where your deck connects to the house and the location of the support structures. The placement of these footings is critical to the location of the support posts, which will ultimately make your deck a safe and enjoyable place to enjoy the outdoors, and keep it from falling down! Even though there is room for error in your measurements, you should strive for accuracy to the 1/8th of an inch so that the next steps proceed without a hitch. However, the layout step is relatively simple and can easily be completed by any level of DIYer.