I have seen people fill the bottom of a wall cavity with pea gravel. Why is this done? When this is done, should two levels of weep holes be used?
The Brick Institute of America (BIA) in Technical Notes 7 recommends considering the use of a gravel bed on top of the flashing to help keep mortar droppings from clogging weep holes. If the pea gravel is present, any mortar droppings would occur above the level of the weep holes. This method, however, should never be used in place of good workmanship. With the gravel bed at the base of the cavity, excessive mortar droppings would simply be shifted upward some 4 inches. Although the mortar droppings would not clog weep holes in this case, they would still allow water to bridge the cavity and allow pockets of water to collect near the top of the flashing. Using one row of wicks or weep holes at the bottom of the gravel bed and one row at the top of the gravel bed will not substantially improve the performance. Once the water reaches the top of the gravel bed, it will simply travel to the flashing. The top row of wicks and weep holes will see very little water, unless the entire gravel bed is filled with standing water. In general, I do not recommend using gravel beds. The gravel may, in fact, puncture the flashing. BIA recommends placing mortar beneath the flashing to help support it. If this is not done properly, however, localized areas may still be a problem. BIA also cautions against using this method with bolted shelf angles. The weight of the gravel may cause flashing to tear at the bolt.