How to fix 4cm gap between wall and concrete slab?

lixiang4hr

Hi Bunnings team,

I recently moved into this house and noticed some gaps between the concrete slab and the wall. It also looks like the slab have sunk a lot, based on the marks visible on the wall.

I’ve already filled some of the smaller gaps using backer rod and sealant, but one section is much wider (around 4 cm). I assume it is because that the downspout here had become disconnected from the drain below, so rainwater may have been running down along the wall and washing out the soil underneath for quite some time. I have reconnect the downspout and filled the gap with some sands.

I was hoping to get your advice on a couple of things:

1. Is this a significant concern if water can get into the gap?
2. What would be the best way to repair it?

I’ve read about using patching cement or mortar, but I’m wondering if that might be too rigid for this type of joint. From what I understand, the joint may need to remain somewhat flexible to allow for movement and expansion? Maybe I can add something between the patching cement and the wall?

Many thanks in advance for your advice.

Kind regards,
Sandy

EricL

Hello @lixiang4hr

From the photo, it appears the concrete path slab has settled away from the house wall, which has opened up the joint between the slab and the brickwork. This type of gap is quite common around external paths and patios, particularly if water has been washing soil away underneath for some time. Your observation about the disconnected downpipe is very plausible, as prolonged water flow in that area can gradually erode the supporting soil and allow the slab to sink slightly.

In most situations this is not a structural concern for the house itself. External paths are usually poured separately from the house foundation, so movement in the path does not typically affect the structure of the building. The main issue is allowing water to enter the gap, which can continue washing out soil beneath the slab and potentially lead to further settling. It can also allow moisture to sit against the wall. Since you have already reconnected the downspout, you have likely addressed the main cause of the problem. Sealing the joint will mainly help prevent water from entering and keep debris or pests out.

You are also correct in thinking that rigid materials such as mortar or patching cement are generally not ideal for this type of joint. The space between the slab and the wall acts as an expansion joint and needs to accommodate a small amount of movement caused by temperature changes or minor ground movement. Rigid products tend to crack or separate fairly quickly in these situations.

A more suitable approach is to treat the gap as a flexible expansion joint. Start by cleaning out the joint and removing any loose sand or debris so the surfaces are reasonably clean. Because the gap is quite wide, you can fill most of the depth with a compressible material such as Sika 10 x 50mm 6m Abelflex Expansion Joint Filler Foam which can be cut or folded, leaving roughly 10–15 millimetres of space at the top. Over this, apply a flexible polyurethane concrete sealant such as Sika 310ml Grey Concrete Sikaflex Pro+ Polyurethane Sealant. This sealant is designed for joints between concrete and masonry, bond well to both surfaces, and remain flexible so they can handle movement without cracking.

Once the sealant is applied, it can be tooled smooth so it forms a neat finish and slightly sheds water away from the wall. With wider joints like the section you mentioned, filling most of the depth with backer foam first helps support the sealant and prevents it from sinking too far into the gap.

One additional detail to keep in mind is the presence of weep holes in the brickwork. These are the small openings in the mortar near the bottom of the wall that allow moisture inside the cavity to drain out. When sealing the joint, it is important not to block these openings. The sealant should only bridge the joint between the slab and the wall itself, leaving the weep holes clear.

Overall, the situation you are seeing is fairly typical for external concrete paths. Reconnecting the downspout was an important step in preventing further soil erosion, and installing a flexible sealed joint should help keep water out and slow any additional movement.

Please remember to wear personal protection such as gloves and goggles when working on your repairs.

If you need further assistance, please let us know.

Eric