How Do I Fill Large Gaps in My Patio? (Quick Read)

Filling large gaps in a patio is a time consuming job that just has to be done, so what’s the best way of going about it?

When you’re laying a new patio, you can fill large gaps between patio pavers with either regular sand, a DIY mix of concrete and sand, or polymeric sand. These can be applied using a trowel and packed down with a screwdriver, then finished by brushing over the gaps with a broom.

In this article, we’re going to look in more detail at what to fill large patio gaps with, how to do it – and answer some of your popular questions about the process…

How Do I Fill Large Gaps in My Patio?

How Do You Fill Large Gaps in Between Patio Slabs?

You can fill large gaps in between patio pavers (aka pointing) with regular sand, a DIY mix of concrete and sand or polymeric sand.

To make a DIY mix of sand and concrete, start on a sunny day with more sunny days forecast. The slabs should be dry. Scrape out the old cement or dirt or leaves. Blow off the patio or use a shop vac to vacuum up the debris.

Mix the sand and cement. Cement can burn your skin so wear gloves. Most experts recommend using 4 parts sand to 1 part cement. No water! Use a fine-grained sand. Mix the solids together well in a bucket or wheelbarrow. The mix should clump together if you grab a handful and squeeze.

Then carefully use a trowel and a piece of cardboard to spoon and pack the mixture into the paver gaps. Yup, it’s tedious work but get some tunes going and enjoy the beautiful day. 

Pack the mix into the gaps using a thin tool, like a screwdriver. Tap the pavers with the wooden end of the trowel. Carefully brush any excess off the slabs and into the gap. Use a tool or your fingers to smooth the mix in the gap.

Once all the sand has been packed into the gaps and the surface of the pavers are free of sand, lightly water the patio. Then stay off the patio while it cures – up to two days.

Here’s a video on a DIY mix of sand and cement.

How Big a Gap Can Polymeric Sand Fill?

Polymeric sand works best to fill gaps between 1/4 inch and 1.5 inches wide. Wider joints will require more time to harden. Some pavers are designed to fit together like puzzle pieces – if the gap is too small, polymeric sand won’t help.

For those of you who aren’t aware, polymeric sand is very fine-grained sand that has just enough cement added that, with water, it sets up to stay put.

No more weeds and ants between your slabs. No more washed away sand. 

Here’s a great reference on polymeric sand and pavers.

How Long Does it Take For Polymeric Sand To Set?

This depends on the weather a little. If it’s hot and sunny, most polymeric sands will set up between 24 and 72 hours.

Experts say that the most critical part of adding polymeric sand to a slab patio is the watering part. You must water enough for all the sand from the bottom of the crack to the top to be watered. If you water too much, then you’ll see a whitish film (which is the cement additive) floating on the water.

If you water too little, the job won’t last. That’s because once you add water, the cement additives will start to harden. If you add too little water, then only a small rim at the top of the sand will harden. 

Once the rim hardens, no more water can penetrate. The sand underneath the rim will be just loose sand and no amount of watering will penetrate then. 

Experts recommend starting with small areas first. Say, 10 feet by 10 feet. Use the “shower” setting on a hose nozzle. The goal is to saturate the joint filled with sand. Spray a joint and observe. If the water seeps down into the sand, then it’s not saturated. 

When a joint is saturated, then water will stop seeping down and begin a small pool on top. That’s when you stop.

What Happens if Polymeric Sand Gets Rained On?

Once the polymeric sand has set up, heavy rain won’t hurt it. But if you have just finished applying polymeric sand and it hasn’t yet set up – like in the first 24 to 48 hours – then you’ll need to protect your patio from a downpour. Use a tarp or plastic sheet.

Polymeric sand should always be installed on a bone-dry patio, so make sure you pick a stretch of good weather. If you leave polymeric sand on top of slabs (instead of in between them), then once it rains you may have sand permanently stuck to the top of the pavers.

How Long Does Polymeric Sand Last? (Can it Be Reapplied?)

Experts say that, if the installation went well, polymeric sand will last up to 10 years.

What is The Difference Between Jointing Sand & Polymeric Sand?

The naming can be confusing because sometimes polymeric sand can be called jointing sand. But for builders, jointing sand is regular sand – no cement added.

Patio polymeric sand

How Do You Fill The Gap Between a Concrete Patio and The House?

Builders call that gap an expansion joint. When the house was built and the slab patio poured, there was no gap. Cement expands and contracts with temperature. The house slab cement is shaded and stable, while the patio cement will expand and contract with the seasons.

The problem is that the two cement blocks – the house and the patio – aren’t expanding and contracting together, so they crack apart in an unsightly way.

The two cement slabs will always be expanding and contracting independently, so you need to fix it with something that is a little flexible – not cement based.

Pick a warm, sunny day after several sunny days and with up to 5 more sunny days in the forecast. The concrete should be bone dry. Then, excavate the crack to about 3 inches deep. If there are ridges and humps in the concrete along the crack, try to level them out with a chisel or grinder. Blow out any sand or pieces of cement. 

If the gap is over 1/4 inch wide or more than 1 inch deep, you’ll need some foam backer rod. Backer rod comes in rolls, in many diameters. Don’t worry about the color, as you’ll be covering it with sealant. If the crack is too deep, first fill it in with some sand. Here’s a good video to watch.

Unroll the backer rod and push it firmly into the crack with a screwdriver, putty knife, or painter’s 5 in 1 tool. Make sure that you leave room for the sealant. Read the sealant’s instructions about the ideal thickness of your particular sealant. Most sealants shouldn’t be more than 1/2 inch thick.

Then fill the crack with a special hybrid urethane sealant that has silane and end-capped polymer technology. Builders use it on commercial buildings, so you may not find it in standard big box hardware stores. Look online and check out the colors to match your cement.

Be sure to watch any instructional videos and read the printed instructions before using this caulk as you can’t go back. It will set up a very strong bond between the slabs.

Set up a protective system so kids and dogs don’t step on the crack for 24 hours. Most sealants completely cure in 3 to 5 days.

Should You Fill Concrete Expansion Joints?

Yes. You should always fill expansion joints. Expansion joints account for cement’s natural properties of expansion in the heat and contraction in the cold. Be sure to choose a flexible sealant.

When your patio or driveway was poured, chances are that the professionals installed a joint expansion system. Many times, an asphalt-saturated fiberboard or felt was used. But that material decomposes.

When expansion joints no longer have any sealing materials, water can leak through, and water under a concrete slab can cause big problems. Water can leak into basements and sunrooms. Water can erode the soil underlying a driveway or patio slab and the cement can collapse into the void.

Here’s a reference for repairing small cracks in cement.

Conclusion

Filling gaps between patio pavers is one of those jobs you can just keep putting off – but we all know it needs to be done.

I still remember to this day cementing between the cracks of a crazy paving patio with my old man when I was a kid. It seemed to take forever!

However, the results were amazing and the patio looked great – so it’s always well worth the effort. 🙂