Steel, Stainless or Aluminium? How to Choose the Right Shim Material
04/06/26
Getting the shim itself right is the part most people focus on: the size, the thickness, the slot width. Material choice often gets treated as an afterthought. But pick the wrong material for the environment and you will have problems. Corrosion, staining, galvanic reaction, a shim that simply does not hold up under load. These are all avoidable if you make the right call at the start. The decision is simpler than it looks. For the majority of applications, it comes down to three options: mild steel, stainless steel, or aluminium. Here is how to choose.
Mild Steel Shims: The Workhorse Choice for Indoor and Structural Work
Mild steel, typically CR4 grade, is the default shim material for a reason. It is strong, consistent, and cost-effective. If you are shimming something structural in a dry or protected environment, mild steel will almost always be the right answer.
It suits levelling and aligning structural steelwork, packing below machinery and plant in factory or workshop environments, and construction applications where the shim will be enclosed or protected from moisture, such as behind cladding, within wall build-ups, or below flooring. For general alignment work indoors where corrosion is not a concern, mild steel is the practical choice.
The reason it works so well comes down to compressive strength. Mild steel machines and cuts cleanly, holds tight tolerances well, and at lower cost than stainless it makes bulk ordering practical.
Where it falls short is any environment where the shim will be exposed to moisture, condensation, or the elements. Mild steel will rust. In damp conditions or external applications, corrosion will eventually compromise the shim and potentially stain surrounding materials, particularly concrete, stonework, and light-coloured cladding. If your mild steel shim will be in an enclosed but potentially damp location, a pre-galvanised option offers a basic level of corrosion protection at a modest cost premium.
Stainless Steel Shims: For Outdoor, Damp, and Demanding Environments
Stainless steel shims cost more than mild steel. In the right applications, that cost is completely justified. In the wrong ones, you are paying for protection you do not need.
The applications where stainless earns its place include anything external that will be exposed to weather, rain, or condensation. It is the correct choice below windows and door frames in external or cavity wall situations, in marine and coastal environments, and anywhere near water: drainage, wet rooms, swimming pools, processing plant. Food production, catering, and pharmaceutical environments where hygiene standards apply also call for stainless, as does any situation where staining of adjacent materials would be unacceptable.
The reason stainless performs in these conditions is that the chromium content forms a passive oxide layer that resists corrosion without any coating or treatment. It requires no maintenance and will not degrade over time in damp or exposed conditions. Grade 304 is the standard for most general applications. For more aggressive environments, coastal, chemical, or salt-laden atmospheres, grade 316 gives improved chloride resistance.
There are two situations where stainless steel causes problems rather than solving them. If the shim will be in contact with carbon steel fixings or structures in a corrosive environment, the dissimilar metals can create a galvanic cell that accelerates corrosion of the less noble metal. And if the environment is benign, specifying stainless is simply an unnecessary cost. One practical point worth noting: stainless steel is harder than mild steel and slightly more difficult to cut or trim on site. For applications requiring any field adjustment, factor that in before you order.
Aluminium Shims: Where Weight Matters, or Where Aluminium Meets Aluminium
Aluminium is not simply a lightweight alternative to steel. It has specific properties that make it the right call in certain situations and the wrong one in others.
It is the correct choice where overall weight is a genuine constraint, since aluminium weighs roughly a third of steel for the same volume. It is also the right material when shimming aluminium extrusions, frames, curtain walling, or any aluminium structure where dissimilar metal contact needs to be avoided. Glazing applications, particularly aluminium curtain wall and window systems, are a natural fit. So are electrical enclosures, control panels, and any application where the shim may need to be trimmed or dressed on site, since aluminium cuts and files far more easily than steel.
Aluminium is naturally corrosion-resistant in most environments, forming its own protective oxide layer without any treatment. Where it is used within an aluminium structure there is no galvanic risk, and its low weight matters in applications where many shims are used or where there are load constraints on the supporting structure.
The galvanic compatibility point is worth understanding properly. Aluminium sits at a different electrochemical potential to steel. Where aluminium shims are used in contact with stainless steel fixings in a wet environment, there is potential for galvanic corrosion of the aluminium. Similarly, aluminium shims should not be used against mild steel in permanently damp conditions. In dry, enclosed environments this is rarely a practical problem, but in exposed or wet applications, matching materials wherever possible or using an appropriate barrier is the right approach.
The other limitation is compressive strength. Aluminium is not suited to very high-load structural applications such as heavy plant, bridge bearings, or heavy presses. Check load requirements against material properties before specifying.
Quick Reference: Which Material for Which Job
| Application | Recommended Material |
|---|---|
| Structural steel alignment, indoors | Mild Steel |
| Machinery levelling in factory environment | Mild Steel |
| Enclosed construction packers (dry) | Mild Steel |
| External steelwork, exposed packers | Stainless Steel (304) |
| Window and door frame shimming, external | Stainless Steel (304) |
| Coastal or marine environments | Stainless Steel (316) |
| Food production / pharmaceutical plant | Stainless Steel (304 or 316) |
| Aluminium curtain wall and glazing systems | Aluminium |
| Aluminium extrusion and framing | Aluminium |
| Weight-sensitive or site-trimmed applications | Aluminium |
A Note on Thickness and Tolerances
Whichever material you choose, all Metal Parts Direct shims are laser cut to a tolerance of ±0.25mm across all dimensions. Material test certificates are available for S275JR, S355JR, S355J2, all stainless steel grades, and aluminium, supplied on request with full traceability on every order as standard.
If your application has tighter tolerance requirements or needs a specification outside our standard range, contact us about a custom part. The options are broad and a quick conversation usually resolves it.
Shop by Material
Mild Steel Shims and Packers cover the full range of square packers, horseshoe shims and plate washers in mild steel CR4. Stainless Steel Shims are available for external, damp, and demanding environments. Aluminium Shims suit glazing, curtain walling, and aluminium structures where a lightweight, non-reactive shim is needed. If none of the standard options fit, Custom Shims and Packers are made to order in non-standard sizes, thicknesses, and profiles.
All parts are manufactured to order at our facility in Braintree, Essex. Average lead time is 3 to 4 working days*. Free delivery on orders over £100 ex VAT.
Need a little extra help?
If you are unsure about material selection, design suitability or which product is best for your application, contact us.