Why does water come in under my sliding patio door?
Why a sliding threshold is designed to let water in and drain it out again, how to find and clear the drainage that has stopped working, and the four other routes water takes.
- Difficulty
- beginner
- Time
- 45 min
- Read
- 3 min
- Safety
- caution
Short answer
A sliding door threshold is not a dam — it collects water in a channel and drains it out through weep slots at the front. Almost every case is those slots blocked. Find them along the outer face of the threshold, clear them with a thin plastic sliver, and flush the channel with water.
Sliding patio doors are drained systems rather than sealed ones. Wind-driven rain gets past the outer seal by design, runs into a channel in the threshold, and leaves through small slots to the outside. When those slots block, the channel fills and the water goes the only other way it can, which is inwards over the inner upstand and onto your floor.
Safety
Step by step
- Find the drainage channel.Slide the door open and look along the threshold. Between the outer track and the inner edge there is a channel, sometimes covered by a slotted trim. That is the drain. It should be dry a few hours after rain.
- Find the weep slots.On the outside face of the threshold, at the very front, there will be small slots or holes — often two or three along the length, often covered by small hinged flaps or plastic caps. Get down to ground level outside to find them.
- Clear them without damaging them.A thin sliver of plastic, a cable tie or a stiff bristle. Never a screwdriver or a drill bit, which widen and distort the slot so it then admits wind-driven water directly.
- Flush the channel.Pour half a litre of water slowly into the channel from inside. It should appear at the weep slots within seconds. If it pools, there is a blockage further along or the channel has settled and no longer falls towards the slots.
- Check the outside ground level.If the paving, decking or a new patio outside has been built up to or above the threshold, water cannot leave the weep slots at all and will back up. This is an extremely common cause after landscaping and it cannot be fixed at the door.
- Check the outer seal and the brush pile.The sliding sash runs against a brush or fin seal. A flattened, missing or dirt-clogged brush lets far more water into the channel than the drainage was designed to handle. Replace it if it no longer stands up.
- Check the sash height.A sash adjusted too high leaves a gap at the bottom that admits driving rain in quantity. A sash too low sits in the drainage channel and blocks it. Both show up as water inside.
- Check the corners and the frame joints.Water can enter at the frame's mitred corners and at the junction between the threshold and the jambs. Look for staining or salt marks inside those corners specifically. This is a sealing job, from outside, in dry conditions.
- Check the outside sealant and the sill below.The joint between the frame and the sill or the wall below it should be sealed and the sill should throw water outwards with a fall. A flat or back-falling sill delivers water to the base of the frame continuously.
- Test with a hose after each fix.A gentle hose from a low angle, mimicking driving rain, on one part of the door at a time. Have someone inside watching. This isolates the route far faster than waiting for weather.
If it doesn't work
Water appears only in driving rain from one direction
Cause: Wind-driven rain overwhelming the outer seal, or an open joint on the windward side — Fix: Replace the brush seal and check the frame corner joints on that side. Consider whether the opening is genuinely exposed enough to need additional shelter.
Water appears hours after rain stops
Cause: The channel is full and slowly overtopping, or holding water that wicks in — Fix: Blocked weep slots or a channel that no longer falls. Clear and flush; if it still holds water, the threshold has settled and needs looking at properly.
Started after a new patio was laid
Cause: Ground level built up over the weep slots — Fix: The paving has to be lowered locally or a drainage channel formed in front of the door. Nothing at the door itself will fix it.
Water at one end of the threshold only
Cause: A failed corner joint or a jamb-to-threshold junction — Fix: Clean and reseal that junction from outside with a good exterior sealant, in dry conditions, having cut out the old sealant completely.