Eruv walls must total more than 50\% of the eruv border on each side, so that most of the expanse of the eruv is enclosed, either by an actual wall or by the form of a doorway (tzurat ha'petach), with no gate or gap over 10 amot (about 16' 8”, or 5 m) wide.
Note  In order to count as part of a border for a private domain, any doorway, gateway, archway, etc., must be intended to be a doorway, gateway, archway, etc.--and not there merely for decoration or function (as in supporting something above).
 

In general, porches and awnings on the outside of a house will not qualify as being part of the house for the halachot of carrying on Shabbat if there is no eruv. The porch or awning must be in the structure of a shape of a doorway (tzurat ha'petach): vertical poles and cross beams must be on TOP of the vertical poles in order to be considered a halachically enclosed area.
Note Since these are not intended to be a doorway, no mezuza is required on them.

An eruv forms a boundary around an area of land in order to create a private domain (reshut ha'yachid).  Carrying items within that domain is permitted on Shabbat.  The eruv boundary may include a variety of structures such as:

• Real physical structures—whether natural (such as tree trunks, bushes) or man-made (buildings, fences, cars);
• Natural topographic features (such as slopes); and/or
• Presumptive doorways (often made of poles and wires or strong string).

Solid or Halachically Solid
Two structures (regardless of how thick or wide they are) within 10.5 inches of each other are considered to be halachically solid and constitute a single structure; this is called lavud.
NoteA halachically solid wall may have gaps of more than 3 tefachim (10.5 in.) high or wide (i.e., in either dimension) as long as the other dimension is less than 3 tefachim wide.

Examples

A halachically solid wall can be made of a:

• Wide mesh of ropes or strings; the cross strings are very far apart, as long as the vertical strings are within 10.5 inches of each other.

• Picket fence; each vertical slat must be within 10.5 inches of the adjacent slat OR each horizontal piece that connects the vertical slats must be within 10.5 inches of the adjacent horizontal piece.

• Chain-link fence.

Height
All vertical eruv components must be at least 40 inches high. There is no maximum height for the eruv if it is a halachic doorway (tzurat ha'petach--two uprights and a horizontal bar above and connecting the two).

Width

A solid component (for example, a board, wall, house, etc.) must be at least 12 inches from side to side. 

Non-solid components (for example, a series of narrow bushes, a series of trees with trunks less than 12 inches across, various types of fences, etc.) must be within 10.5 inches of each other and of the ground, both horizontally and vertically, for the entire distance between adjacent trees/bushes. They must be at least 40 inches high or wide.

 

Connectors

Vertical components, such as poles, that are connected above or below in the following ways are also halachic walls, regardless of how far apart they are:

• Connected above, such as with a board or string that rests across the tops of vertical poles, and which are at least 40 inches above the ground at all points along its course, or
• Connected below within 10.5 inches of the ground, such as bushes or small trees with branches that come within 10.5 inches of the ground at all points (even at the attachment point to the trunk).  Components must reach up to at least 40 inches above the ground.