How To Repair Cracks In Internal Walls and Ceilings

  • Cracks in your walls and ceilings are both unsightly and potentially hazardous
  • Learn how to deal with both hairline and deeper cracks in your home
cracks in corner of a ceiling

Cracks in your walls and ceilings can be very unsightly and on rare occasions, they may signify some more serious issues. Often they are rooted in one or more of the following causes:

  • Substandard cement and materials
  • Incorrect ratio of sand, aggregate, and cement
  • Dampness and water damage (groundwater or rainwater)
  • Impact damage
  • Seismic activity/shock
  • Excessive load or stresses
  • Fluctuations in temperature, particularly sharp ones and repeated ones.
  • Vibrations from heavy traffic, nearby highways, underground rail, and nearby construction activity.
  • Shifting or settling of the foundation or the house itself
  • Normal ageing

These different causes damage structures with varying frequencies depending on geographical locations. For instance, dampness and water damage cause cracks to a fair number of walls and ceilings of structures in Scotland but not so much in Turkey. On the other hand, seismic activity and shock damages walls and ceilings of houses throughout Turkey but not in Scotland.

Also, one and the same cause, incorrect ratios, may result in cracks, but through entirely different intermediary causes. Using too much cement in the mix will result in high heat of hydration and stresses due to shrinkage, with consequent cracking. On the other hand, using too little cement will result in a weak and non-durable concrete that cracks under normal, or even moderate, stresses and loads.

Besides temperature fluctuations, even the normal passage of seasons and their more gradual temperature variations can cause certain types of walls or ceilings to crack. The walls of houses that have wooden frames under drywall without any intermediary layer, contraction joints, and/or control joints crack quite often during and immediately after winter because of the very different ways in which wood and drywall contract and expand as a result of temperature changes.

Are Wall Cracks Normal?

This really depends on the cause and also on how one interprets (what is) ‘normal.’ Cracks that appear in the walls of very old structures are normal because – like everything else – walls are not forever. Cracks caused by foundation or the house itself settling or shifting are, quite honestly, also normal.

As for when, say, substandard materials are used or the components are mixed in an incorrect ratio, cracks are to be expected and are a ‘normal’ outcome of the underlying mistakes; however, it would be unwise to view and treat these cracks as ‘normal’ because the underlying mistakes are not ‘normal.’

In any event, normal or not, all serious cracks should be repaired at first opportunity.

What are the Types of Internal Plaster Wall Cracks?

Internal plaster walls are basically of two kinds: load-bearing walls and partitioning walls. Cracks in load-bearing walls should be treated seriously. While cracks may result from any and all of the causes listed in section What Causes Cracks in Walls and Ceilings?, their types are several. First, a crack may be hairline, deep, or gaping (that is, you can see right through it). Next, it may be vertical, horizontal, or angled/slanting. Finally, it may originate at the juncture of the wall and ceiling, originate at a corner, or occur in the middle of the wall.

Cracks that begin as hairlines are often caused by an incorrect ratio of sand, aggregate, and cement; specifically too much cement and too little sand and aggregate.

Horizontal cracks are usually caused by a settling or shifting foundation. Cracks originating near the juncture with the floor, especially if dampness is visible, are caused by groundwater.

Cracks originating at jambs or doorframes point to poorly installed frames or a wall that is not strong enough to bear the load and stresses of the frame and doors.

When is a Crack Serious?

The least serious type of crack is a pattern hairline crack in the middle of a partition wall.

A deep slanting crack in any type of wall, but especially in a load-bearing one, is a serious crack.

A long gaping horizontal crack in a load-bearing wall is certainly the most serious type of crack.

A cracks in a ceiling is much more serious when it occurs in or near the middle than when it is along or at an edge or in a corner.

Apart from the location, length, and width of the crack, the condition of the immediately surrounding surface will also indicate whether or not the crack is a manifestation of serious structural issues. Is the surrounding plaster flat, firm, and dry, or is it uneven, bloated, and spongy? Does gentle tapping or even knocking have no effect on the damaged area, or does it slump, deform, or break away? The answers to these questions will indicate the seriousness of the crack.

Are there any Safety Issues?

Yes, there are a couple of safety issues to consider.

Preliminarily, find out whether or not the paint surrounding the crack is lead-based or not. The reason for doing so is that you may well need to do some scouring and widening, and cleaning up with a brush, and lead-based paints are carcinogenic and extremely toxic. You can determine whether or not the paint is lead-based by using an affordable lead testing kit. If the paint turns out to be lead-based, leave the job to experienced professionals.

Before proceeding with any crack repairs, wear a dust mask. Even if you will not be using a scraper or craft knife, you will still need to brush out or blow out dust and debris from the crack. And if you will be using a scraper or craft knife, consider wearing work gloves. And, as a few caulks and fillers contain allergy-inducing (or occasionally even toxic) ingredients, you have all the more reason to wear a mask and gloves.

Underneath we explain how to repair different kinds of cracks and gaps. Crack repair is not an overly-technical process marked by a complicated series of steps – you have to do but three or four simple steps. What is important to understand is that the art of filling, smoothing, and shaping is a learned technique. And as with all such techniques, practice makes perfect.

How to Repair and Fill Plaster Walls

Hairline Cracks

Counter-intuitively, before you repair and fill in a hairline crack you have to make it ‘worse’!

As the first step, use a scraper or craft knife to scour into and through the crack to make it sufficiently deeper and wider. If and when necessary, go through to the brick. This step is necessary because any sealant has to get through and between the material to be bonded but this is not possible with a hairline crack.

If there is any brittle surrounding plaster, break it up and remove it. Scoop out loose dust and debris. Use a thick, stiff paintbrush to clean out the crack.

You have three options as to how to repair a hairline crack.

Method 1: Caulk.  Apply or simply squeeze caulk directly into and on the crack. A good choice is Caulk Once. Push the caulk into the crack by pushing it in with your fingers or palm as you make small circular motions. Though many professionals smooth caulk with their bare hands and fingers, the substance technically is toxic. On the other hand, some work gloves will cause caulk to stick to them and pull it away. Gloves made of nitrile are quite effective when working with caulk.

Method 2: Polyfilla.  First, using a small brush apply a coat of PVA and allow it to dry. Next, mix Polyfilla professional patching plaster on a board. (If you have heard ‘Poly Filler’ being mentioned in the context of crack repairs, this is actually Polycell’s Polyfilla, an established brand name.) Using a flexible filler trowel or plasterer’s float, scoop up the Polyfilla and push it into the crack, and slather it in and around the crack. As soon as it is semi-dry, scrape off the excess, shape it, and smooth it. Apply a second, finishing layer as and when necessary. This too should have the excess removed, and shaped and smoothed when it is semi-dry.

Method 3: Polyurethane foam.  Polyurethane (PU) foam cans are sold in quite a variety of formulations so you will need to choose the type that is meant for filling and sealing wall cracks.

This type of foam can also be injected using specialised equipment, this being a foam injection machine which is widely used by professionals. DIYers doing home repairs could certainly make do with PU foam cans, however.

Using a small brush apply a coat of PVA and allow it to dry. Shake the can of foam vigourously and liberally squeeze the foam into the crack. Carefully read the instructions on the can as you may be required to hold it upside-down when squeezing out foam. Promptly push the foam into the crack using a flat trowel or plasterer’s float.

After the foam has dried, trim the excess off the wall with a scraper, and level off the foam with the wall’s surface.

Larger and Deeper Crack Repair

Oddly enough, though repairing larger and deeper cracks is a more serious proposition than taking care of hairline ones, in a way it is somewhat of a less technical procedure, and this is because you do not need to make the crack any ‘worse’ than it already is!

First, brush out all loose dust and debris from in and around the crack. Then, gently use a scraper to dislodge loose sand and debris. Now brush out the dust and debris once again, or use a handheld vacuum cleaner with a narrow crevice attachment to clean out the crack. If the crack is (what is called) a ‘gaping crack,’ you could even use a blower to blow out loose debris and particulate matter.

As a preliminary step, apply plaster primer into and around the crack. It will act as a sealant. Leave it for 24 hours.

Method 1: One Strike. Everbuild’s One Strike Multi Purpose Filler is a product that is ready to use ‘out of the box’ and is ideal for beginners and DIYers. Available in tubs, all you need to do is to spread it on a board, scoop it up with a flexible filler trowel or plasterer’s float, and slather it into and around the crack. It dries quickly and does not require any follow-on applications.

Method 2: Polyfilla. Use Polyfilla professional patching plaster, mentioned above. It is as good for repairing long, deep cracks as it is for hairline cracks. Of course, you have to prepare it with water as stated earlier.

Method 3: Gyproc Easifill 60 or Easifill [with no trailing number]. Gyproc’s Easifill is a gypsum-based filler that is available in bags. It is suitable for experienced DIYers and is an economoical option when there are many cracks to fill. Prepare it with water according to the instructions and mix it thoroughly. Scoop it up with a flexible filler trowel or plasterer’s float and push it into the crack and slather it in, quickly scraping off the excess. Wait for about 60 minutes and then apply a second, thinner coat, again scraping off the excess. Allow to fully dry and then sand it down.

Instead of doing repairs the time-honoured way by using trowels or floats, you could make an investment in a caulking gun or sealant gun and learn how to use it to fill long, deep cracks. These guns take cartridges or canisters. Everbuild One Strike Multi Purpose Filler is available in such cartridges in the same formulation as it is in tubs. Polyfilla is also available in cartridges but in a somewhat different formulation. Gyproc Easifill is ideally suited to use with caulking guns but you would need to prepare a batch and then charge (or load) the caulking gun with the paste.

How to Repair Ceiling Cracks

If you’re a beginner, first lay some newspapers or plastic sheeting on the floor beneath the crack. Though you can vacuum up dust and debris, glops of fast-setting compounds are another matter entirely!

Hairline Ceiling Cracks

You will need to widen and clean out a hairline crack in the ceiling before you can repair it. Tightly hold a firm scraper and insert a corner into one end of the crack and drag along the scraper, slightly twisting and jiggling it, to expand the crack just a bit. Then, with the scraper, remove any loose plaster to the extent necessary and undercut the plaster around the crack to remove thin strips. Do not otherwise damage good, sound plaster. 

Next, take a self-adhesive mesh tape, like EuroScrim or ADFORS Fibatape, and stick it along the crack from end to end.

For the repair step, use a fast-setting joint compound like USG Sheetrock All-Purpose or Knauf Fill & Finish. These are also known as ‘drywall mud,’ ‘hot mud,’ and by similar trade lingo.

Repairing a ceiling crack is different from repairing a wall crack in one fundamental way: you are working against gravity. For this reason, fast-setting compounds are necessary, as a consequence of which you will also have to work fast otherwise the compound will begin to set and harden in your tray or on your board!

‘Drywall Mud’ compounds like the ones recommended above are ready to use but you may dilute them with water just a touch. The mud should be thick enough so you can apply it overhead on the ceiling without it separating or dribbling down.

Make one smooth, tight application along the length of the crack. Scoop up good-sized dollops with a flexible filler trowel and gently and quickly press the paste into the crack. After about 15 minutes, repeat by liberally applying it once more along the length of the crack, pushing in the paste. Go over it with the trowel (without any more compound) along the length of the crack a couple of times, applying moderate pressure, to even it and smooth it out.

Large Ceiling Cracks

Before you get started on a large ceiling crack you need to determine if it is ‘just’ a large crack or something more (and worse). Sometimes large ceiling cracks are caused by water or damp; if so, the ceiling itself around the crack will be damaged. If you push against the surface close to the crack and it is flat, firm, and dry, all is well. However, if it is moist, unstable, and spongy, it is not only a large crack but a larger repair job that your ceiling requires as the old plaster, drywall, or other material or substrate will have to be removed, structural damage or defects uncovered, and other repairs, including water-resistant sealing, will have to be undertaken.

If you are dealing with ‘only’ a large ceiling crack in drywall, get a professional-style platform to stand on. If you use a stool, you will constantly be climbing down the stool, moving it, and getting back up. Not only will this result in a poor filling job because of breaks in continuity, the time wasted will also contribute to the fast-setting compound hardening in the container or on the board.

First, use a stud-finder to locate the wooden joists above and inside the ceiling that run across or close to the crack, and mark the locations of the joists. Drill a pair of 3-centimetre screws on each side of the crack, about 4 centimetres to the side of the crack, at regular intervals as best as possible to secure the ceiling to the joists.

To repair a large ceiling crack, you have to make a judgement call on whether or not the crack needs to be scoured out or widened as explained in section Hairline Ceiling Cracks. As a rule of thumb, if the crack’s width is 3 millimetres or more and the surrounding ceiling is tight and firm, do not widen the crack. At most, insert a scraper or craft knife into the crack and run it along at an angle, leaning it on one side on the first run-through and then the other side on the second run, to give the crack a ‘V’ or wedge-shaped taper. Simply clean out the crack and remove dust and debris with a brush or use a hand-held vacuum with a narrow crevice attachment. However, if the surrounding plaster is loose or crumbly, then, of course, you will have to scour around the crack with a scraper, remove loose plaster, and clean it out before you can begin your repairs.

Apart from these differences, to repair a large ceiling crack follow the instructions in section Hairline Ceiling Cracks from the step outlining the application of self-adhesive mesh tape onwards, with one key determinant influenced by the length of the large ceiling crack, as follows:–

The fast-setting compounds used to fill ceiling cracks do not give you much time to work with. Therefore, even if you use a platform (which you really should), if the crack is very long, say 300 centimetres or more, and you are a beginner, you may find that the compound starts to set before you are quite done! In such a case, work with a helper who will keep scooping up the required amount in a small container or board and keep handing it to you.

A Final Word

Regardless of whether you are repairing wall cracks or ceiling cracks, and no matter what type of compound you use, run your trowel or float along each edge of the applied coat, that is where the newly-applied compound meets the original wall or ceiling, so as to smooth or ‘feather’ it clean and flush with the surface.

After the repair-job has cured, you can sand as necessary by hand using a high-grit sandpaper like P120, or, if the repair covers a large area, with an orbital sander. Finally, you can apply a coat of PVA over it.

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