Why does water come in under my front door?

An external threshold is not a barrier — it is a drainage detail. Water is expected to get past the door in driving rain, and the threshold's job is to catch it and send it back outside.

What you'll need

• A torch
• A thin probe or a piece of wire
• A hosepipe or a watering can
• Exterior sealant
• A weatherbar, if one is missing

Step by step

1. Watch it happen.Pour water over the threshold from outside with a watering can and lie down inside with a torch. Where it appears tells you far more than any inspection.
2. Check the drain slots.Most aluminium thresholds have slots through the front face, often behind small covers. Blocked with grit, leaves or paint, the chamber fills and overflows inwards.
3. Look at the ground level outside.New paving, decking or a raised path that sits level with or above the threshold means water pools against it. This is the single most common cause after blocked slots.
4. Check the internal floor level.A new floor laid up to or above the top of the threshold upstand gives water a level path inwards. It happens routinely when tiles or laminate are fitted.
5. Test the door seal.Close the door on a strip of paper along the bottom. If it pulls out easily, the bottom seal is not compressing on the upstand and rain is being driven straight over it.
6. Look for a weatherbar.On a timber door, a weatherbar or storm rail on the face of the door throws run-off clear of the joint. Many doors never had one, and many lost theirs to a repaint.
7. Check the drip detail.The bottom of the door should have a drip bar or a groove so water cannot track back underneath by surface tension. A perfectly flat, square bottom edge wicks water inwards.
8. Reseal the threshold to the floor.The joint between the threshold and the substrate beneath it should be bedded in sealant. Lift a loose threshold, clean it, and re-bed it rather than sealing around the edge.
9. Do not seal the drain slots.It is a common instinct and it makes everything worse. The slots are how the water leaves; sealed, the chamber fills and comes inside instead.
10. Consider a surface water solution.Where the ground cannot be lowered, a channel drain or a gravel margin in front of the door removes the standing water that is causing the problem.

Tips

• Clear the threshold slots twice a year with a piece of stiff wire. It takes a minute and prevents most of these problems.
• A porch or canopy that keeps direct rain off the door is worth more than any amount of sealant.
• Water appearing several feet inside is usually running along the threshold and down at one end. Follow it back rather than assuming the leak is where the puddle is.

Common mistakes

• Sealing the outside of the threshold with silicone — It traps the water that is meant to drain out, so the chamber inside the threshold fills up and discharges inwards. The leak often gets dramatically worse immediately after this fix.
• Fitting a new floor level with the threshold — The threshold upstand is a deliberate height difference. A floor brought up to it removes the step that was keeping water out, and no amount of sealant afterwards replaces it.

If it doesn't work

Questions people ask