Collapsing Porch Slabs

Posted by in Buying/Maintaining a Home, Concrete | March 18, 2013


One of the more dramatic types of failures that I have investigated have been the collapse of front porch slabs (see photo above).  These occur when the concrete porch slab drops anywhere from a few inches to quite a few feet into the void under the concrete slab.  Most times no one is injured.  But when someone was standing or sitting on the slab and it dropped a few feet, serious injuries have occurred.  Why does this happen?  Should I be worried about my front porch slab?

First, let’s understand how these concrete slabs are constructed. When a concrete porch slab is placed, it can be placed and supported directly on a soil or gravel backfill or it may be placed over and supported on a form framed with a steel or wood framework.

With the first type of construction, the soil or gravel backfill under the slab often consolidates, that is, it compacts under its own weight, and drops: most people call this settlement, but technically it is not settlement.  Settlement occurs when a heavy object compresses the soil much like a shoe print in a soft soil.  Consolidation is the “dropping down” of some soil or gravel particles “to fill the voids between other soil or gravel particles.  It is similar to the dropping of the level of the grade around the basement of a recently constructed home.  Consolidation is a repacking of the soil or gravel so that it takes up less space.

The dropping of a concrete slab from the consolidation of the soil usually appears as a progressive dropping where the slab “follows” the soil or gravel down.  In some situations the concrete slab is prevented from dropping by its bonding or binding on the wall of the house.  The dropping of  a concrete slab from this condition usually occurs suddenly with a shallow drop measured in inches. The short drop is because the compaction of the soil or gravel usually only creates a shallow void measured in inches.

Some contractors recognize this bonding or binding phenomenon and provide steel rods, wood ledger boards, steel angles, masonry nails, or pockets in the wall for the concrete to flow into and lock the slab into the wall.  Unfortunately, steel corrodes, wood decays, and unreinforced concrete tabs shrink and break away allowing the concrete porch slab to drop into the void.

With the second type of construction, where the concrete porch slab is supported on a framework of steel or wood over an unfilled space (void), the collapse of the slab is usually more dramatic because the depth of the void under the slab is usually measured in feet.  In this case, a framework initially provides an adequate support over a deep space or void, but as the framework corrodes or decays, its strength is lost and when the heavy concrete slab loosens from the wall, it falls into the void space.  To avoid this type of  collapse into a deep space or void, a concrete slab should not be supported on wood framing and when it is supported on steel framing, the steel framing must be protected from corrosion resulting from the prolonged and repeated exposure to excessive amounts of moisture.

Many times a porch slab exhibits warnings before it collapses.  Sounding of the slab with a hammer or wood bat identifies whether or not there is a void.  Examination along the wall may identify whether or not the slab is dropping.  The pattern of cracking in the concrete slab, the vertical offsets across the cracking, the pitches of the sections of concrete slab may also identify movement, the lift of the slab from the perimeter walls, and/or the cracking along the tops of the perimeter walls may also identify whether or not the slab is dropping.  And finally, if a slab shakes or vibrates when walked on, it may be loose and near collapse.

If a porch slab is exhibiting these characteristics, the risk of collapse may be reduced by filling the void under the slab with a grout or urethane fill.  If the void is due to consolidation of the backfill, properly filling it may require repeated re-filling over a number of years until the backfill stops consolidating. Please note that care must be taken so that the perimeter walls of the void are not damaged by the pressure from pumping the grout or urethane.  If the void is very large, a portion of the slab or perimeter wall may need to be removed to allow access to fill the void with a combination of gravel, grout, and/or urethane.  But caution when removing a portion of a slab, because removal of a portion of the slab or wall may cause construction vibrations that could loosen the slab from the wall and allow it fall without warning.  Furthermore, NEVER ENTER A SPACE OR VOID UNDER A PORCH SLAB BECAUSE IT COULD COLLAPSE SUDDENLY AND CAUSE INJURY OR DEATH!  Repair of a loose, poorly supported, or fallen porch slab requires the skills and experience of a qualified and experienced contractor.