How do I re-point the joints in a patio?
Choosing between dry mortar, wet mortar and resin jointing compounds, and the preparation that decides whether the joints last two years or twenty.
- Difficulty
- intermediate
- Time
- 5 hr
- Read
- 4 min
- Safety
- caution
Short answer
Rake the old joints out to at least 20mm deep, brush and wash them clean, and choose a compound to suit: a brush-in resin jointing compound is easiest and most reliable for most patios, wet mortar is strongest for wide joints, and dry mortar mix is quick but least durable.
Joints fail because water gets into the bed underneath, weeds establish in the gap, and frost then breaks up whatever is left. Re-pointing is as much about keeping water out of the bed as it is about appearance.
Safety
What you'll need
- Plugging chisel or joint raker
- Stiff brush
- Bucket and sponge
- Jointing compound, or sharp sand and cement
- Pointing trowel or jointing iron
- Knee pads
Step by step
- Check the slabs are sound first.There is no point pointing around a rocking slab — relay it first. Pointing will not hold a loose slab still.
- Rake the joints out properly.At least 20mm deep, and to the full width of the joint. A thin skim of new mortar over old failing material lifts out in a year.
- Remove every trace of weed and moss.Roots left in the joint push the new pointing out. Scrape, brush and, where necessary, treat the roots before starting.
- Clean and wet the joints.Brush out all dust, then wash through. Dust prevents adhesion, and dry slabs suck water out of fresh mortar before it can cure.
- Choose the method for the joint width.Resin jointing compounds suit joints from about 5mm to 40mm and are the most forgiving. Wet mortar suits wide joints and gives the strongest result. Dry brush-in mortar mixes are fast and least durable.
- Follow the resin compound instructions exactly.Most need the patio damp, not wet, worked in with a squeegee and compacted with a pointing trowel, then cleaned off the surface immediately. The cleaning-off window is short and staining is permanent.
- Mix wet mortar stiff, not sloppy.Around three or four parts sharp sand to one cement, damp enough to hold its shape. A wet sloppy mix stains the slabs and shrinks as it cures.
- Work in small sections.A few square metres at a time so you can finish and clean each area before the material starts to go off.
- Compact and tool the joint.Press the material well down into the joint and finish slightly below the slab surface with a jointing iron, so water runs off the slabs rather than sitting in the joint.
- Clean the slab faces as you go.Mortar left to dry on the slab face is very difficult to remove and often stains permanently. A damp sponge, rinsed often, immediately after finishing each section.
- Protect it while it cures.Keep off it for at least 24 hours, longer in cold weather, and cover it if rain or frost is forecast.
Tips
- Rake out to 20mm. A skim over the old joint always fails.
- The cleaning-off window on resin compounds is short and the staining is permanent — read the instructions before you open the tub.
- Finish joints slightly below the slab face so water sheds rather than sits.
Common mistakes
- Brushing dry sand and cement into a dry joint — It only carbonates properly where it gets moisture, so much of it stays as loose powder and washes out. If using a dry mix, the patio must be damp and it must be watered in as directed.
- Pointing in frosty or very hot weather — Frost during curing permanently weakens mortar, and hot sun dries it before it cures. Aim for mild, still, dry conditions.
If it doesn't work
New pointing crumbling within a year
Cause: Joints not raked deep enough, or mix too dry — Fix: Rake out to at least 20mm, clean thoroughly, and use a resin compound or a properly mixed mortar.
Cement staining on the slab faces
Cause: Material left to dry on the surface — Fix: Patio cleaner or a specialist cement remover, tested in an inconspicuous corner first. Prevention is far easier than cure.
Weeds back within months
Cause: Roots left in the joints — Fix: Rake out fully and remove all root material before re-pointing.
Joints cracking along the edges
Cause: Slabs still moving — Fix: Relay the loose slabs on a full mortar bed before pointing again.
Questions people ask
What is the best patio jointing compound?
For most domestic patios, a brush-in resin compound rated for the joint width you have. It is more forgiving than mortar, flexes slightly, and resists weeds and frost well.
How deep do the joints need to be?
At least 20mm. Anything shallower has too little material to hold together and lifts out within a season or two.
Can I point a patio in winter?
Only in a mild dry spell with no frost forecast for at least 24 hours. Frost during curing permanently damages mortar and most resin compounds have a minimum working temperature too.