Are your gutters overflowing during Seattle downpours even though they don't look completely clogged? Are you worried that a contractor recommending a larger system is trying to sell you an upgrade you don't actually need, and wondering if there’s a better solution?
At RoofSmart, we help hundreds of Seattle-area homeowners every year improve their gutter systems. We've found that choosing the right size isn't just about how much rain falls on your home. Your roof's drainage area, how quickly it sheds water, the number of trees around your property, and even where your downspouts discharge can determine whether a smaller system works or repeatedly backs up and overflows.
In this article, we'll explain how gutter and downspout sizing affects drainage, when upgrading to a larger downspout makes sense, and why Seattle homeowners also need to consider tree debris, gutter guards, and sewer connections before deciding which setup is right for their home.
There are different sizes of gutters and downspouts to choose from. The size you get depends on the layout of your roof. You need to get the right size gutters and downspouts for the amount of space on your roof that you’re trying to drain.
For example, here are two downspout sizes you can choose based on the area you’re draining:
This measurement is based on the amount of water your downspouts are set to handle. But downspouts concentrate debris, so even a small obstruction at the top of the downspout can cause failure.
Maybe your downspout works when there’s only water running through it, but in the Seattle area with all the trees around, that’s not a realistic possibility. If any of the debris gets caught at the top and it chokes off the flow of the water by half (or more), now all of a sudden the downspout that works when it's clear isn't totally clear, and now it fails.
This leaves you with a decision: do I need to upgrade to a bigger downspout? Is that the only solution?
|
Situation |
Smaller Downspout |
Larger Downspout |
Why It Matters |
|
Small roof drainage area |
May be sufficient |
Usually not necessary |
Size should match the amount of roof area feeding the outlet |
|
Large roof drainage area |
Can become overwhelmed |
Better choice |
More water needs a larger outlet |
|
Home surrounded by trees |
More likely to clog |
Handles more debris |
Seattle roofs often collect leaves and pine needles |
|
Frequent gutter overflow |
May indicate insufficient capacity |
Can improve drainage |
Overflow may be caused by the downspout, not the gutter |
|
Downspout drains onto ground |
Larger outlet can flush debris through |
Often beneficial |
Debris can exit without entering a sewer line |
|
Downspout connects to sewer |
Larger outlet may send more debris underground |
Requires caution |
Leaves and decomposed debris can clog the buried drainage system |
|
Gutter guards installed |
Helps keep debris away from outlet |
Downspout size still matters |
Guards reduce clogging but do not fix an undersized system |
|
Standing seam metal roof |
Standard system may struggle |
Larger drainage system often helpful |
Metal sheds water faster than asphalt shingles |
If your roofer has suggested that you need to get larger gutters and downspouts but you aren’t sure if it’s necessary or just an upsell, check if your downspout has constant clogging issues that cause the gutter to overflow.
There are two reasons that you would need a bigger downspout:
If you have a smaller roof area, but your house is surrounded by trees, you should get the bigger downspout because it will need to be able to handle a lot of debris.
It could take as little as one leaf lying across the opening of the downspout to clog it. That's an argument for getting gutter guards, but it's also the argument for a bigger downspout. Those are your two options.
So, if you don't trust gutter guards or don't want to pay for them, then the easier and more affordable option is to get bigger downspouts because you want it all to flush down the downspout.
The bigger the downspout opening, the more debris it will fit, and of course, the more overall water volume it can handle.
This is an important factor to consider because the Seattle area gets a lot of rain, and heavy downpour can start at any time. You want your downspout to be able to handle the amount of water it’s getting, even if there is debris in the gutter.
And if there are trees near or on your property, then you can safely assume that there will be debris in your gutter. If your house is surrounded by multiple trees, then the downspout is at a much higher risk of clogging than normal.
Now, the downside of getting bigger downspouts and sending all the debris down it is if it's plugged into your sewer. If you're flushing a bunch of tree debris down it, then it’s probably going to cause a really expensive backup later. But if your downspout just floods out onto the ground, that’s fine.
If you’re worried about overwhelming the sewer system with a bunch of debris from your downspout, then you should choose gutter guards instead of just getting a bigger downspout.
If you’re in an older home in an older neighborhood, then your downspouts may be directed into the sewer because it used to have a sewer system before it was connected to other systems.
Basically, you need to decide where and how you want to deal with this debris. Do you want it to collect up there on the gutter guards, where you could reach it with a ladder, or do you want to risk the possibility of having to pay to clear out the sewer?
Plus, it’s not just regular debris like leaves or pine needles that can clog your gutters: when this debris sits in the wet gutter and decomposes, it turns into a muck substance. It can look something like this:
This is a worse issue than just having debris because now the debris is getting sticky and not flowing through the downspout or sewer properly.
Choosing the right gutter and downspout size can make a major difference in how well your home handles Seattle's heavy rainfall and constant tree debris. If a smaller downspout can't keep up, or keeps getting clogged by leaves and pine needles, moving to a larger size or adding gutter guards may help.
But bigger gutters and downspouts aren't always the solution. If several sections of your roof send water into one valley, you have a long roof slope that accelerates runoff, or your roofing material sheds water particularly quickly, your roof may be concentrating more water into one section of gutter than even a properly sized system can handle.
Read our article “Is the Layout of My Roof Causing My Gutters to Flood?” to learn how certain roof layouts can overwhelm your gutters, how to recognize when concentrated runoff is the real problem, and what custom drainage solutions can redirect the water before it floods your gutters.