Building a deck over a driveway on a basement house with a garage door that is positioned beneath it presents some problems not associated with ordinary decks. The most obvious problem is spacing the posts far enough apart so you can drive between them. By spacing these posts further apart additional load is placed on them. This creates a need for a larger post and a larger footing poured around them.
How to Span a Deck Over a Driveway
The easiest way to span across a driveway with a deck is to orient the flooring boards so they are perpendicular to the house. This aligns the floor joists parallel to the house. In other words, you will be spanning the driveway with the floor joists instead of a beam supporting the load of floor joists. Not only will the beams be smaller, the flooring boards will not have any joints as long as the deck is less than 16′ wide. The following table is based on the 2012 IRC span requirements for joists made from number 2 southern pine.
|Spacing||2 x 8||2 x 10||2 x 12|
|12″ oc||14′ 2″||18′||21′ 9″|
|16″ oc||12′ 10″||16′ 1″||18′ 10″|
|19.2″ oc||12′ 1″||14′ 8″||17′ 2″|
|24″ oc||11′||13′ 1″||15′ 5″|
Since the size of beams on a deck, or the floor joist spans for that matter,are not generally inspected, the spans in these tables are for the inside of a home with the closest loads.
If for whatever reason the flooring boards must be oriented parallel to the house, then a beam will have to used. The wider the deck is from the house and the wider the driveway beneath it is, the stronger the beam will have to be. These 2012 IRC spans for a header in an interior wall supporting the weight of a single floor with no walls or roof are as follows
|Number of plies in ( )||(2)2 x 8||(2)2 x 10||(2)2 x 12||(3)2 x 10||(3)2 x 12||(4)2 x 12|
|One floor only (20′ wide)||5′ 9″||7′ 0″||8′ 1″||8′ 9″||10′ 2″||11′ 9″|
Posts and Footings
If the deck is oriented in the fashion with the joists spanning the driveway instead of a beam, then the post will be no different than the posts on any other elevated deck. This also applies to the footings. As rule of thumb, use 6 x 6 treated pine for the post as long as they are no more than 14′ out of the ground. An 18″ diameter post hole that is 36″ deep should hold the load fine for the footings. Click here for the post hole volume calculator.
Once again, it is different story if the deck is oriented with a beam spanning the driveway. The longer the beam, the more of a load will need to be transferred to the soil. This can mean larger posts and larger footings. It may be necessary to consult an architect, engineer, or your local building inspector for advice on this in your area.
The placement of the posts may not be as cut and dried as lining them up with the edges of the garage door. It may be advisable to actually drive through the garage door a couple times and note the path of the wheels. There may not be enough room to line up whatever will be travelling beneath the deck without some adjustments to the placements of the posts.