Why “Like for Like” Substitutions Fail on Site and How to Choose a True Equivalent

Substitutions happen on almost every job. A line goes out of stock, a delivery turns up late, the client changes their mind, or someone spots a cheaper option at the merchant. In theory, it’s simple: swap Product A for Product B and keep moving. In practice, “like for like” is one of the most expensive phrases you’ll hear on a build, because it often means “looks similar on the shelf” rather than “performs the same in the building”.

The temptation is strongest when you’re under pressure and leaning on whatever your usual trade building supplies counter has available that morning. But if you’re swapping structural timber, boards, insulation, membranes, fixings, or system components, small differences can trigger big consequences: failed inspections, callbacks, voided warranties, or a knock on effect that only shows up months later.

This article is a practical guide to choosing a true equivalent. Not the nearest match by name, but the nearest match by performance, compliance, and compatibility.

What “Like for Like” Actually Means

A true equivalent is not a product that looks similar, comes in the same thickness, or has roughly the same price. A true equivalent is one that meets the same requirements for:

Performance
Compliance
Compatibility with adjacent materials and systems
Durability in the specific environment
Installation method and workmanship tolerance

If any one of those changes, you don’t have a like for like swap. You have a redesign, even if it’s a small one.

The Five Substitution Traps That Catch People Out

1. Assuming strength grades are interchangeable

Timber is the classic example. C16 and C24 are not “close enough”. They have different characteristic strengths and stiffness. That can affect span, deflection, bounce, and overall performance. If the design, calcs, or building control expectations were based on one grade, swapping to another without checking is asking for trouble.

Even when the grade matches, treatment class and intended use matter. Internal vs external. Above ground vs in contact with the ground. If you substitute treated timber, check the use class, retention level, and whether it’s compatible with the fixings you plan to use.

2. Swapping plasterboard by thickness alone

A 12.5mm board is not always a 12.5mm board. Fire line, moisture resistant, acoustic, impact resistant, and vapour control boards have different cores, paper facings, additives, and test performance. Thickness is only one variable.

If the room is a kitchen, bathroom, garage conversion, or party wall situation, the board type can be doing a specific job. A “standard” board might physically fit, but it won’t necessarily meet the requirement.

3. Mixing insulation types without recalculating the build-up

Insulation swaps look easy until you realise the whole build-up changes. Different materials have different lambda values, compressive strength, moisture behaviour, and fire characteristics. A PIR board swapped for mineral wool might require a different thickness to hit the same U-value, and the vapour strategy might need to change too.

On top of that, you can’t assume a different brand’s board fits the same way around joists, between rafters, or behind services. Gaps and compression reduce performance quickly.

4. Treating membranes as “just plastic”

Breather membranes, vapour control layers, airtightness membranes, tanking membranes, and damp proof membranes are not interchangeable, even if they come off a roll and look similar. They have different vapour permeability, tear resistance, UV exposure limits, and sealing requirements.

If you replace a membrane without checking what role it plays in the build-up, you can accidentally create a condensation trap. The job might look perfect on day one, then become a mould and rot problem later.

5. Substituting “system parts” as if they’re standalone products

The most expensive mistakes often happen with products that are meant to work as a system: external wall insulation, roofing systems, flooring levellers, tiling backer boards, tanking kits, render systems, and many adhesives and primers.

If the manufacturer’s warranty relies on using specified components, swapping one part can void it. A primer change, a different adhesive, or a different fixing pattern can turn a compliant install into something you’re carrying the risk for.

How to Choose a True Equivalent in 10 Minutes

You don’t need to become a materials scientist, but you do need a repeatable process. Here’s one that works on site.

Step 1: Identify what the product is doing

Ask one question: what is this material responsible for?
Load bearing or structural performance
Fire resistance
Moisture control
Acoustics
Thermal performance
Airtightness
Impact durability
Chemical resistance
Finish quality

If you can’t answer this quickly, don’t substitute until you can.

Step 2: Pull the data sheet and compare the key numbers

Don’t rely on the front label or the merchant description. Use the manufacturer data sheet and look for the few figures that actually matter. Examples:

Timber: strength grade, use class, treatment specification
Boards: fire rating, moisture resistance, acoustic rating
Insulation: lambda value, compressive strength, reaction to fire class
Membranes: vapour resistance or permeability, tensile strength, UV exposure limit
Adhesives: substrate compatibility, open time, cure time, service temperature

Most of the time, if you compare these side by side, you’ll spot that “like for like” isn’t.

Step 3: Check compliance and certification

Look for recognised markings and documents: third-party certification, test standards, declarations of performance, and any approvals that are relevant to the job. If the original product was specified because it met a standard, your substitute needs to meet it too.

If you’re buying from a trade building supplies merchant, ask them for the technical sheet, not just the invoice description. It saves arguments later.

Step 4: Check compatibility with the surrounding build-up

Some products only behave properly when paired with the right adjacent materials. Think:

Vapour control and airtightness layers that must be taped with compatible tapes
Primers that match the leveller or adhesive chemistry
Fixings that match the substrate and treatment chemicals
Membranes that need compatible sealants

A true equivalent has to work in context, not in isolation.

Step 5: Document the change

Even on small jobs, get into the habit of recording substitutions. Keep it simple:

What changed
Why it changed
Which product replaced it
Which performance claim is equivalent
Any revised installation instructions

If something is ever questioned by a client, inspector, or warranty provider, this is what protects you.

A Simple Substitution Checklist

Before you approve a swap, run this list:

Does the substitute meet the same core performance function
Does it meet the same relevant standard or rating
Does it maintain the same build-up performance (U-value, fire, acoustic)
Is it compatible with surrounding products and fixings
Does it keep manufacturer warranties intact
Have you noted the change and any installation differences

If you can’t tick all of these confidently, it’s not a like for like substitution.

Where Substitutions Are Safest and Where They’re Risky

Safer substitutions are typically cosmetic or non-critical, where the performance requirements are low and the consequences of failure are minor.

Risky substitutions include anything structural, anything that affects moisture control, fire performance, or system warranties. If you’re unsure, treat it as risky by default.

When in doubt, slow down and check. Ten minutes of comparison beats a day of rework. And if you’re sourcing at speed from trade building supplies, make the counter work for you by asking for data sheets and spec confirmation, not just “the closest thing you’ve got”.

The Bottom Line

“Like for like” is not a judgement call based on appearance. It’s a technical match based on function, performance, compliance, and compatibility.

On the next job, the goal isn’t to avoid substitutions. It’s to substitute deliberately, with evidence. That’s how you keep the programme moving without quietly trading short-term convenience for long-term risk.