How to add downspout extensions
The cheapest fix for a damp basement, and the one most often skipped. No ladder, no skill, about an hour.
- Doing it yourself
- $20–$120
- Typical contractor price
- $150–$400
Tools
- •Hacksaw or tin snips
- •Cordless drill
- •Tape measure
- •Spirit level
Materials
- •Downspout extensions or a roll-out sleeve
- •Self-tapping sheet metal screws
- •A splash block for each outlet
Step by step
Work out how far the water has to go
Six feet from the foundation is the usual minimum, ten on clay soil or where the ground is flat. The test is simple: run the hose and see where the water actually goes.
Check which way the ground slopes
An extension discharging onto ground that slopes back toward the house achieves nothing. Put a level on a board across the grade — the fall should be about an inch per foot for the first six feet.
Watch out: This is the step that decides whether the job works. Regrading a low spot matters more than the extension itself.
Cut and fit the extension
Slip the extension over the existing downspout outlet, not inside it, so water cannot escape at the join. Two self-tapping screws hold it.
Put a splash block at the end
Without one the discharge digs a hole, and the hole fills and holds water against exactly the ground you were trying to keep dry.
Test it in real rain
Watch during the next proper downpour. A hose is a fraction of what a roof delivers, and problems only show at full flow.
Stop and call someone if
- You need water taken away underground, or the grade slopes toward the house and cannot be fixed with a shovel.
- There is standing water against the foundation after every rain — that is a drainage assessment, not an extension.
Or read more about downspouts & drainage.
Common questions
- Are the roll-out sleeves any good?
- They are fine as a cheap seasonal fix and they get out of the way of the mower. They perish in UV within a few seasons and they do not survive being driven over.
- Can I bury the extension?
- Yes, into a solid (not perforated) pipe running to daylight or a dry well. It has to stay below the frost line or it freezes solid and backs up in exactly the weather you need it working.
Last reviewed September 19, 2026. General guidance only — your house, your roof and your local building code are what actually govern the work.