Poor Handling of Roof Runoff

Posted by in Basements, Case Studies, Roof, Water Sources, Winter Concerns | October 05, 2020

Basement walls not only support the floor framing above, but also retain earth and groundwater from the basement area.  A basement wall commonly retains earth backfill and groundwater by spanning vertically between the basement floor and the wood framing above it.  That is, the backfill soils and the groundwater apply an inward pressure and the basement floor slab and first floor wood framing hold the top and bottom of the wall in place.

The magnitude of the earth pressures generated by the backfill depends on the height of the backfill above the basement floor, the method of placement, and the condition of the soil.  The magnitude of the pressures from the groundwater depends on the depth of the water in the backfill.  This depth is affected by the intensity of rainfall, the effectiveness of surface and roof drainage, the effectiveness of foundation drainage, and the ability of the backfill to allow passage of water to the foundation drains or the water table.

In this case study we are looking at the effects of overflowing gutters.  Note that the basement wall exhibits discoloration, mineral deposits, and efflorescence from repeated infiltration of groundwater (Figure 1). 

Portion of basement wall fallen
Figure 1 Portion of basement wall fallen. Note discoloration, mineral deposits , and efflorescence.

Examination of the gutters disclosed partial plugging from the accumulation of tree debris (featured photograph and Figure 2).   The accumulation restricts flow to the downspouts and allows roof runoff to overflow the gutters and discharge onto the backfill behind the basement wall.  The excessive wetting of the backfill increased the groundwater and the inward pressures from the groundwater. 

Tree debris in gutter
Figure 2 The start of accumulation of tree debris in gutter.

If left uncorrected the result is something akin (my first ever use of the word akin) to Figure 1.     

May this be a reminder to clean those gutters before winter sets in and again in the spring.

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