Downspout Theory and Capacity
A rain gutter is only as useful as the size and number of downspouts associated with it. Standard sized downspouts (2″ round or 1 3/4″ x 2 3/4″ rectangular) are connected to a 2″ (diameter) round drop outlet (metal sleeve) that is inserted and sealed into the gutter. The area of a standard 2″ round hole is 3.14 square inches. Considering the average rainfall in Northern California, the industry rule of thumb is one downspout for every 35-40 linear feet of rain gutter, on an average sized roof, with little-associated leaf debris. If there is a larger than average roof area or a significant amount of leaf debris, there should be more standard downspouts installed. In high-leaf debris areas, having an effective leaf protection product installed on the rain gutters will dramatically reduce the number, and size of downspouts necessary to effectively drain the roof water. With standard rectangular downspouts, the standard 2″ round drop outlets can be upgraded to rectangular outlets (1 1/2″ x 2 1/2″), which have an area of 3.75 square inches, and produce a 19% increase in water flow.
We suggest oversized downspouts (3″ round or 2″ x 3″ rectangular) in conditions where there is a large surface area of roof, a heavy amount of leaf debris, or if fewer standard downspouts are desired. 2″ x 3″ rectangular downspouts come with an oversized rectangular drop outlet (1 3/4″ X 2 3/4″) that has an area of 4.81 square inches, which results in a 53% increase in water flow. The 3″ round downspouts come with an oversized 3″ round drop outlet, which has an area of 7.07 square inches, which results in 125% more water flow than the standard drop outlets. 3″ round downspouts are standard with the 6″ half round gutter, and can also be used with the 5″ ogee gutter at full capacity. 3″ round downspouts can also be used with the 5 1/2″ fascia gutter; however, the bottom of this gutter is only 2 1/2″ wide. In this application, the 3″ round outlets have to be oval to fit in the narrower gutter base, which slightly reduces the water flow capacity, but is still a significant increase over the standard downspouts.
Round downspouts come in two sizes: 2″ and 3″ (diameter). Round downspouts are only available in unpainted galvanized steel. Pre-fabricated elbows are used for all the turns. The elbows create nice, smooth turns, which is why these downspouts are usually the quietest, and leaf debris tends to flow through easier without getting backed up. 2″ round downspouts can handle the least amount of water, while the 3″ offers the maximum water flow capacity. In some people’s opinions, round downspouts do not look good on every architectural style of home. However, there is no match to the amount of water and leaf debris that can flow through a 3″ round downspout.
Pre-painted Galvanized Rectangular
Pre-painted galvanized downspouts only come in the standard rectangular size (1 3/4″ x 2 3/4″). They are available in all of the Custom-Bilt Metal pre-painted steel colors. We cannot solder a pre-painted metal, so these downspouts have to be riveted and sealed with a clear sealant at every miter (turn). As one could imagine, rivets and sealant are not nearly as strong and nice looking like a painted solder. We are limited to how custom we can make these downspouts because they cannot be soldered.
Pre-painted standard rectangular downspouts perform and cost the same as the unpainted do. However, they are a thinner gauge metal, so they are not nearly as durable or quiet. We often get complaints from our customers that they can hear the water run through these downspouts as compared to their old soldered galvanized ones. We end up installing a lot of pre-painted downspouts because many folks do not want to do any painting along with their gutter system replacement, but we always inform our customers of the pros and cons.
Unpainted Galvanized Rectangular
Unpainted galvanized rectangular downspouts come in standard and over-sized sizes. They are the highest quality steel rectangular downspouts we install.
The oversized rectangular downspouts (2″ x 3″) only come in unpainted galvanized steel. Unpainted galvanized downspouts are the highest quality because they are made out of a thicker gauge metal and can be soldered together. They are incredibly durable, welded downspouts. Being able to solder allows us to make very custom downspouts that we are not able to do otherwise. Because of the metal thickness, these downspouts tend to be much quieter than the standard pre-painted downspouts when water runs through them, and are much less likely to make the dreaded dripping noise. Many of our competitors do not offer these type of downspouts because soldering is a skill that not all installers posses. All of our installers are highly trained sheet metal craftsman and are capable of fabricating and installing the most complicated custom downspouts.
Much of our work consists of installing seamless gutter systems on brand new homes. Every new home we do gets unpainted galvanized downspouts because almost all builders recognize their unmatched quality. It looks fantastic when a smooth soldered downspout gets painted the same color as the siding. The downspouts disappear more and blend into the house better. There is no need to paint unpainted galvanized downspouts. They could be left unpainted forever, but most folks prefer them painted.