Concrete Spalling in Singapore: Causes, Risks, Repair Methods and Who Must Fix It

A brown stain on a bathroom ceiling, a hairline crack that slowly widens, a patch of paint that bulges and finally a piece of concrete on the floor: that is how spalling usually announces itself in a Singapore home. HDB describes spalling as a common issue in older buildings, especially in damp areas such as kitchens and toilets.¹ MND told Parliament in 2020 that HDB received an average of about 10 reported cases a year per 1,000 sold flats, and 23 per 1,000 rental flats.³

Spalling is not just a cosmetic problem. Behind the bulge is corroding steel reinforcement, the part of reinforced concrete that carries tension. Left alone, the corrosion spreads, more concrete falls, and the steel loses cross-section. HDB warns that unattended spalling spreads to a bigger area and weakens the building structure.¹ On façades, ledges and car park soffits, falling concrete can injure people below.

This guide explains why concrete spalls (carbonation and chlorides), the signs to look for, how serious each kind is, how spalling is treated under BCA’s Periodic Structural Inspection, how proper repairs are done, who must pay in HDB, condominium and commercial buildings, and when you need a Professional Engineer.

Why concrete spalls: the chemistry in plain English

Steel bars embedded in good concrete do not normally rust, even though concrete is porous and damp. The reason is alkalinity. Writing for the Institute of Corrosion, corrosion engineer John Broomfield explains that the cement chemistry raises the alkalinity at the steel surface to around pH 12.5 to 13, which passivates the steel with a thin protective oxide layer.⁶ Two things destroy that protection.

Carbonation

Carbon dioxide from the air slowly diffuses into concrete and reacts with the alkaline compounds, neutralising them. Broomfield describes the carbonation front as a transition in pH from about 12.5 to 8.5, a few millimetres wide, that moves inward over time; its depth is approximated by a diffusion relationship, growing roughly with the square root of time.⁶ When the front reaches the reinforcement, the steel loses its passive layer and can corrode wherever there is enough moisture and oxygen.

HDB identifies carbonation as the main cause of spalling in flats, calling it a natural deterioration process that causes the steel bars in the ceiling slab to corrode, cracking and bulging the concrete cover.¹ HDB adds that a humid environment speeds carbonation, which is why bathrooms and kitchens are the usual locations, and that paint helps protect ceilings against it.¹

Practical consequences:

• Thin cover spalls first. Bars placed close to the surface, common in older slabs, are reached by the carbonation front sooner.
• Cracks and holes are shortcuts. HDB recommends sealing ceiling cracks and holes, including old fixing holes, to stop moisture and carbon dioxide entering.¹
• Moisture decides the rate. Dry carbonated concrete corrodes slowly; damp carbonated concrete corrodes quickly. Leaks from above, poor ventilation and condensation all accelerate damage.

Chlorides

Chloride ions break down the passive layer locally even where the concrete is still alkaline. Broomfield explains that chlorides build up in the cover until they reach a threshold concentration at the steel surface, generally taken as 0.2 to 0.4% chloride by mass of cement for ordinary carbon steel in Portland cement concrete.⁶ Chloride attack tends to cause localised pitting that can cut a bar’s cross-section faster than the general corrosion typical of carbonation.

In Singapore, chloride exposure is most relevant for coastal and marine structures, which is one reason BCA extended PSI to jetties, wharves, bridges and similar civil engineering structures, with specific checks on corrosion in marine environments.⁹⁷ Chlorides can also be present in some older concrete from the materials used to make it.

Why corrosion cracks the concrete

The rust that forms takes up much more room than the steel it replaces. Broomfield notes that the common corrosion products are porous iron oxides with “a volume several times that of the steel consumed”, generating tensile forces that crack and delaminate the cover.⁶ Concrete is weak in tension, so the cover splits along the line of the bar, then separates and falls. By the time you see spalling, corrosion has usually been active for some time.

Sources: Institute of Corrosion;⁶ HDB.¹

Signs of spalling and corroding reinforcement

Spalling develops in stages. Catching it at the first or second stage keeps the repair small.

A simple tapping test with a light hammer helps map the extent: sound concrete gives a sharp ring, delaminated concrete a dull thud. BCA’s PSI guidelines require the inspecting engineer, wherever spalling concrete is observed, to carry out simple tests such as tapping in other areas to identify the risk of further spalling.⁷

How serious is it? Risk to people and to the structure

There are two different risks.

Falling concrete. A delaminated piece of cover can drop without warning, onto a bathroom floor, a car park bay, or a pavement below a façade. Concrete elements are within the scope of BCA’s Periodic Façade Inspection, which covers any part of a concrete wall, slab, beam or column on the exterior, and lists spalling, delamination and cracking of concrete as common façade defects.¹⁰ For façade spalling, the Competent Person must recommend immediate measures where needed, such as cordoning off, hoarding and removing loose elements.¹⁰

Loss of structural capacity. Corrosion reduces the steel area, weakens the bond between steel and concrete, and removes the concrete that confines and protects the bars. In a ceiling slab with minor spalling, the structural effect is usually small. In a cantilever, a heavily loaded beam, a slender column or a transfer structure, the same corrosion can be significant. BCA’s PSI guidelines single out structures without redundancies, such as cantilever structures and cantilever balconies exposed to the elements, for specific checks for signs of distress, cracks, deformation or corrosion.⁷

How BCA classifies spalling in the PSI

BCA’s standard PSI report template, issued with Circular APPBCA-2024-19 in December 2024, gives structural engineers examples of how to classify defects.⁸

Source: BCA Circular APPBCA-2024-19, Annex D (standard PSI report template), section 3.10.⁸

These are examples in a report template, not legal thresholds, and the engineer’s judgement governs. They are nonetheless a useful, Singapore-specific way for owners to understand why one patch is “minor” and another triggers a full investigation.

Spalling and the Periodic Structural Inspection

PSI applies to all existing buildings except detached, semi-detached, terrace and linked houses used solely as residences, and temporary buildings: every 10 years where at least 90% of the floor area is residential, and every 5 years for all other buildings.⁷ Spalling is one of the conditions the engineer must look for. The guidelines list it among the signs of structural defect, deformation or deterioration to be recorded, with comments on extent, possible causes and seriousness, and they allow judicious removal of plaster or finishes to reveal the structure beneath.⁷

Several PSI rules matter specifically for spalling:

• Severity assessment. Each problem is assessed as of no structural significance, requiring remedial action or monitoring, or a suspected defect of structural significance needing a full structural investigation and immediate action.⁷
• Extended coverage. Where defects are caused by degradation of material due to age, such as spalled concrete, BCA expects inspection coverage to be increased throughout the building, for example by inspecting more units or accessing ceiling panels at closer intervals.⁷
• Corrosion in similar exposures. Where corrosion is found because of exposure conditions, inspection should extend to other areas with high humidity or similar exposure.⁷
• Past repairs. The engineer is expected to review the effectiveness of past repairs.⁷⁸
• Follow-up. After BCA accepts the report, it writes to the owner to follow up on the engineer’s recommendations; owners should carry out remedial works promptly and submit the engineer’s certification of completion where applicable.⁹ A planned sale is not a reason to avoid the repairs.⁹

Read our full PSI owner’s guide for cycles, timing after TOP and penalties.

Repair methods

Good spalling repair has one aim: stop the corrosion and restore the protective cover so it does not come back. That means removing all damaged and contaminated concrete around the bars, treating the steel, rebuilding the section with a compatible repair material, and protecting the surface.

Small domestic patches: HDB’s method

HDB says owners can repair spalling in their flats themselves if the area is small, and sets out the steps:¹

1. Remove the spalled concrete.
2. Scrape and clean the exposed steel bars, using a wire brush to remove rust.
3. Apply two coats of anti-rust paint to the steel bars.
4. Apply a bonding agent to the affected surface.
5. Patch the hacked area with polymer-modified cement mortar.
6. Paint over the patch to match the rest of the ceiling.

If the area is large, HDB advises engaging a contractor promptly, before more concrete dislodges and potentially causes injury.¹ Under HDB’s Home Improvement Programme, the essential repair is described as removal of loose concrete, patching and painting of the affected areas.⁵

Larger or structural repairs: the professional sequence

For larger areas, beams, columns, cantilevers, car parks and façades, repairs should follow a specification from a Professional Engineer. The European EN 1504 series, Products and systems for the protection and repair of concrete structures, is widely referenced for this work; Part 9 sets out general principles for the use of repair and protection products and systems.¹⁶

Choosing repair mortar

Repair mortars are not ordinary plaster. EN 1504-3 classifies repair mortars into classes R1 to R4. Manufacturers’ technical guidance describes R3 and R4 as structural repair mortars, with R4 having a compressive strength of over 45 MPa.¹⁵ The engineer should specify the class and product according to whether the element is structural, the depth of repair and the exposure. Using a lightweight cosmetic filler on a beam or column is a common reason repairs fail.

Sources: HDB;¹ BCA PSI Guidelines and template;⁷⁸ BCA PFI Guidelines.¹⁰

Protective coatings and prevention

After repair, protection slows the return of corrosion. HDB’s prevention advice for flats is to paint ceilings regularly, seal cracks and holes immediately, and ensure ventilation in moisture-prone areas.¹ MND has likewise described spalling as generally a surface maintenance issue that regular ceiling painting helps to mitigate.² On larger buildings, an engineer may specify a dedicated anti-carbonation coating, waterproofing to roofs and decks, and in chloride environments additional corrosion protection.

Who must repair spalling concrete

HDB flats

MND’s position, stated in Parliament in 2016 and repeated since, is that flat owners are responsible for maintaining the interior of their flats, including spalling concrete, and bear the cost of repairs.⁴ For spalling due to wear and tear after the defects liability period, owners can ask for help under the Goodwill Repair Assistance scheme, under which HDB arranges the repair and co-pays 50% of the cost.²⁴ Under the Home Improvement Programme, repair of spalling concrete and structural cracks is an essential improvement, fully funded by the Government for Singapore Citizens; MND has noted that HIP does not prevent spalling recurring later, as wear and tear continues.⁵² MND said in 2018 that after the 1-year DLP, an average of about 13 spalling cases per 1,000 dwelling units are reported to HDB each year.² Check current scheme terms with HDB, as these statements date from 2016 to 2020.

Condominiums and strata buildings

BCA’s summary of the Building Maintenance and Strata Management Act (renamed the Building (Strata Management) Act from 1 October 2025, when its maintenance duties moved into the Building Control Act) amendments that took effect following gazettal in November 2018 states that the definition of common property makes clear that key structural elements, namely foundations, beams and columns, are common property, and that slabs, excluding underlayment, are common property.¹¹ The MCST must properly maintain and keep common property in good and serviceable repair, including renewing or replacing it where reasonably necessary, under section 29(1)(b) of the BMSMA.¹² The High Court has held that duty is not strict; an MCST is liable if it fails to act reasonably, not merely because common property is damaged.¹²

In practice, spalling of a structural slab, beam or column in a strata development, including where it shows on the ceiling of your unit, is generally for the MCST to investigate and repair. Report it in writing with photographs. Plaster, ceiling boards and fittings inside your unit remain your responsibility, and BCA reminds owners that structural elements such as slabs, beams, columns and walls cannot be altered without a Professional Engineer’s assessment and the Commissioner of Building Control’s approval.¹⁷ Where cause is disputed, for example spalling triggered by a leak from the unit above, see our guide on inter-floor water seepage.

When BCA steps in

Where a building is in such a condition as to be dangerous, section 24 of the Building Control Act empowers the Commissioner of Building Control to order the owner to inspect, carry out works, shore up or hoard the building, or suspend its use; non-compliance can attract fines of up to S$100,000, imprisonment of up to 12 months or both.¹³ Under section 25E, the Commissioner may serve a notice to maintain where a building, common property or exterior feature is not kept in good repair; non-compliance carries a fine of up to S$20,000, up to 12 months’ imprisonment or both, plus up to S$250 a day for a continuing offence.¹⁴

When you need a Professional Engineer

BCA’s PSI guidelines state that major repairs and strengthening works are treated as building works, so the procedures for approval of plans or a permit, and supervision, apply.⁷ Where façade repairs involve structural elements, BCA’s PFI Guidelines require structural plan approval and a permit, where applicable, before work starts, and supervision by a professional engineer.¹⁰ For how PE endorsement works and why independence matters, see our guide on PE endorsement. If the work involves hacking or altering structure, see hacking walls, slabs and structural elements.

An illustrative cost-of-delay comparison

Illustrative. The figures below are assumptions for explanation only, not market rates. Suppose a bathroom ceiling shows one bulging patch about 200 mm across. Repaired promptly with HDB’s method, the job involves a small break-out, steel treatment, a patch and repainting, typically done in a day. If the leak above is not fixed and the patch is ignored for several years, delamination can spread across much of the ceiling, the bars may lose section, and the repair becomes a larger break-out with possible bar replacement, scaffold or platform, waterproofing works in the unit above and an engineer’s assessment. The cost multiplies, and the risk of a piece falling on someone in the meantime is real. The general lesson, supported by BCA’s observation that routine inspections and timely repairs avoid major, costly and disruptive repairs later,⁸ is that early repair is almost always the cheapest repair.

Key takeaways

• Spalling is concrete cover breaking away because the reinforcement has corroded, most often after carbonation in damp areas, or from chlorides in coastal or contaminated concrete.¹⁶
• Rust expands to several times the volume of steel, cracking and pushing off the cover; by the time you see spalling, corrosion is established.⁶
• Small ceiling patches can follow HDB’s six-step method; larger areas, beams, columns, cantilevers and façades need an engineer’s specification.¹⁷
• BCA’s PSI template treats localised spalling under 300 mm as a minor structural defect and spalling over 300 mm as a structural defect.⁸
• HDB flat owners maintain their flat interiors, with GRA co-payment and HIP funding available in defined circumstances; in strata developments, structural slabs, beams and columns are common property for the MCST to maintain.²⁵¹¹¹²

For related problems see cracks in walls and ceilings, popped and hollow tiles and roof and wet-area waterproofing. Terms are defined in our glossary.