How do I fill a large hole in a brick wall?
The difference between a hole that needs mortar and one that needs a brick, cutting back to a sound square opening, and getting a patch that does not read as a patch.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 3 min
- Safety
- caution
Short answer
Anything bigger than a fist wants a brick, not mortar — a large mass of mortar shrinks, cracks and falls out. Cut the opening back square to whole brick dimensions, wet it, bed cut bricks in a mortar matched to the wall, and point the joints to match. Fill small holes with a mortar matched to the joints, in layers.
The hole is usually left by a removed pipe, an old extractor, a boiler flue, a satellite bracket or an air vent. What most people do is fill it with a lump of mortar or a squirt of filler, and what happens next is that it shrinks, cracks around the edge, lets water in and becomes more visible than the hole was. Any repair of size in masonry is done with masonry.
Safety
Before filling any hole through a wall, establish what it was for. Holes left by removed flues, boiler flues, extractor ducts and air vents were often serving something that needs ventilation, and sealing one up can leave an appliance, a void or a chimney without the airflow it requires. A hole above an opening may be a weep hole. If the hole is large or sits over an opening, check that nothing structural has been removed.
Step by step
- Find out what the hole was for.A former flue or vent may have been ventilating something that still needs it — a chimney void, an under-floor space, a boiler location, an appliance. Sealing a redundant flue at both ends without ventilation is a classic cause of damp in a chimney breast.
- Check it goes only where you think it goes.Probe the depth. A hole through a cavity wall goes through the outer leaf into a cavity, not straight into the room, and filling only the outer face leaves debris in the cavity that can bridge it.
- Decide between mortar and brick by size.Up to roughly the size of a fist, a mortar fill built up in layers is fine. Larger than that and it needs cut brick, because a big mass of mortar shrinks as it cures, cracks away from the edges and eventually drops out.
- Cut the opening back square.Enlarge to whole or half brick dimensions with clean vertical and horizontal faces, following the existing bed and perp joints. Ragged edges make it impossible to bed a brick properly and guarantee a visible patch.
- Remove all loose material and dust.Rake out the surrounding joints where they are damaged, brush everything out, and vacuum if you can. Nothing bonds to dust.
- Match the brick.The patch will be visible for ever if the brick is wrong. Take a sample and match size first, then texture, then colour. A reclaimed brick from the right period usually beats a new one on an older wall.
- Wet the opening thoroughly.Soak the surrounding brickwork and let the surface water go. Dry brick pulls the water out of the mortar instantly and the mortar never hydrates, which is why so many patches are powdery.
- Bed the brick in a matched mortar.Mortar that matches the existing in strength and colour — weaker than the brick, and lime-based if the wall is lime. Butter the bed and both ends, push the brick home, and pack the top joint from above with a slim pointing tool, because filling the top joint of a patch is the fiddly part.
- Point to match the surrounding joints.Wait until thumbprint firm, then tool the same profile as the rest of the wall. This step is what makes a patch disappear or stand out.
- Fill a small hole in layers.For a mortar-only repair, press mortar firmly into the back of the hole, build up in layers of no more than about 10mm each, letting each stiffen. A single deep fill shrinks and cracks.
- Make good the cavity where relevant.Where the hole went through the outer leaf of a cavity wall, clear any debris out of the cavity below before closing it up, so it cannot bridge across and cause damp.
- Protect it and let it cure.Cover from sun, rain and frost for a few days, held clear of the face. A patch that dries too fast is a patch that falls out.
Tips
- Expanding foam has no place in a masonry repair on an external wall. It is not weather resistant, it takes up water, it degrades in sunlight and it cannot be pointed.
- For a hole in a rendered wall, fill in the masonry first, let it cure, then repair the render as a separate operation.
- Photograph the joint pattern before you cut anything back, so you can rebuild the bond correctly.
- If the hole is directly over a window or a door, do not enlarge anything until you are sure nothing structural is involved.