Are cracks in a porch floor a problem?

expert EXPERT April 26, 2007

Cracking of a concrete floor can be due to

• settlement or heaving of columns or walls
• weakening of the slab itself
• rusting of rebar (metal reinforcing bars in the concrete)
• shrinkage of the concrete

Spalling results from water absorption and subsequent freezing. It can also occur if deicing salts are absorbed then crystallized below the surface of the concrete. Spalling can be more pronounced if the concrete mix was poor.

If it is not severe, cracking or spalling doesn't have serious structural implications. The concrete can be patched and monitored. Severe cracking may be indicating ongoing settlement of the structure, or serious weakening of the slab. Major repairs may be needed. Severe spalling usually indicates poor drainage or a poor concrete mix or both. It is unsightly and causes the surface to be uneven, but it rarely poses a serious structural problem.

The concrete floor should slope to the outside edges to promote drainage. Any penetrations into the slab should be well caulked. The edge of the concrete should have a drip edge, and ideally a drip edge flashing.

If an interior space exists below a concrete porch, balcony, or deck, ensure that the slab is watertight, that the drip edge is adequate, and that the side walls are free of open mortar joints or cracks. Make a note to check the interior space for moisture penetration.

Observe the size and location of any cracks in the concrete floor. Cracks the width of a dime or smaller are not immediate concerns, but you should recommend sealing and monitoring. For larger cracks, you may be wise to suggest further investigation. Slab cracking coupled with cracks in support columns or walls suggest movement of the supports. Where the slab shows displacement across a crack, settlement or heaving should be suspected.