What Does EPDM Do in a Roofing Screw Washer?
What Does EPDM Do in a Roofing Screw Washer?
The small black ring under a roofing screw's head plays a major role in weather sealing. Here's how it works, why installation matters, and how to select the right fastener.
EPDM stands for ethylene propylene diene monomer, a synthetic rubber used in many outdoor seals. In a bonded roofing screw washer, it compresses against the roof sheet around the screw penetration to help resist rainwater ingress. It does not make every installation automatically waterproof: the correct screw, washer, roof profile and tightening method all matter.
What does an EPDM washer actually do?
Roofing sheets need fasteners to connect them to a supporting structure. Each fixing creates a potential path for water to enter. A typical hex-head roofing screw can include a metal-backed washer with a bonded EPDM rubber layer beneath it. When the screw is installed as specified, the elastomer makes contact with the roof surface and helps seal the penetration.
EPDM is often chosen for exposed construction because suitably formulated compounds have good resistance to weathering, ozone, sunlight and water. These are material-family properties, not proof of a particular washer's service life or certification. Compound quality, geometry, compression, exposure and installation still determine performance.
| Part of the assembly | Primary job | Important limitation |
|---|---|---|
| Self-drilling screw | Creates and holds the mechanical connection in an approved substrate | Drilling capacity and structural strength must be verified for the exact screw |
| Metal backing washer | Supports the rubber and distributes contact under the head | Does not compensate for an unsuitable roof profile or fixing location |
| EPDM sealing ring | Conforms to the roof surface to help resist water ingress | Does not guarantee sealing if under-tightened, over-tightened or misaligned |
| Protective coating | Helps protect the metal fastener from corrosion | Its suitability depends on the specified environment and coating system |
Anatomy of a bonded EPDM roofing screw washer
The sealing ring works with the screw head and metal backing. Its function is different from an ordinary flat metal washer, which provides a bearing surface but does not itself create an elastomeric weather seal.
OnlyScrews' listed CORROSHIELD hex-head self-drilling roofing screws specify a 2.5 mm EPDM washer thickness. That listing value is useful for product identification, but it is not a universal compression target or a tested watertightness rating.
Under-tightened vs correctly seated vs over-tightened
Correct washer compression is critical. Under-tightening may leave insufficient contact; over-tightening can distort the elastomer or roofing sheet; and driving the screw at an angle can prevent even sealing. Use the screw manufacturer's recommended driver, depth stop and installation instructions rather than an arbitrary torque number.
A sealing washer is not a substitute for the correct fixing location, roof slope, sheet support or specified fastening pattern. For wind-uplift or other engineered roof connections, follow the roof-system designer's approved details and tested fastener data.
EPDM vs plain metal and other rubber washers
A roofing screw's washer should be selected for its intended sealing function. A plain metal washer cannot form the same compressible seal as a bonded elastomer. Other rubbers, including neoprene, may be used in certain systems, but material choice should be based on the manufacturer's tested assembly and exposure conditions rather than colour alone.
| Washer type | Typical role | Selection note |
|---|---|---|
| Plain metal washer | Provides bearing area | Not an elastomeric roofing seal |
| Metal-backed EPDM washer | Provides contact support and a compressible weather seal | Check washer size, material, fit and installation guidance |
| Other bonded elastomer washer | Can provide a sealing interface where specified | Confirm weather, temperature and chemical compatibility for the actual compound |
EPDM typically has strong weather and ozone resistance but is not generally the preferred elastomer for prolonged exposure to petroleum oils or hydrocarbon fuels. If the fastening point will contact chemicals or oils, verify the washer compound's compatibility rather than assuming all rubber is interchangeable.
How to choose a roofing screw with an EPDM washer
Start with the roof system, not just the screw length. Check the sheet profile and thickness, the supporting substrate, the fastener's thread form and drilling capacity, the specified fixing location, and the expected corrosion exposure. Finally, confirm that the sealing washer is compatible with the screw and roof sheet.
| Check before ordering | Why it matters | What to verify |
|---|---|---|
| Thread diameter and pitch | Influence engagement with the substrate | Specified fastener and substrate compatibility; do not choose solely by TPI |
| Length and clamping thickness | Determine whether the assembly can be secured correctly | Sheet stack, support thickness, point length and required engagement |
| Washer size and EPDM condition | Help create the intended seal | Correct backing, intact elastomer and manufacturer-approved seating |
| Coating and environment | Influence corrosion durability | Roof material, local exposure, compatible coating and approved service conditions |
Five common mistakes that can cause roofing fastener leaks
- Overdriving the screw. Excess compression can distort the seal or roof sheet.
- Leaving the screw loose. The EPDM may not seat against the roofing surface.
- Driving at an angle. Uneven contact can compromise the sealing interface.
- Using the wrong fastener. A mismatch in substrate, drilling capacity, length or washer geometry can undermine the connection.
- Ignoring damaged washers or roof surfaces. A torn gasket, burr, debris or distorted sheet can create a leak path.
For a related overview of tip styles, pilot holes and roofing applications, read Self-Tapping vs Self-Drilling Screws: Which One Do You Need?
Shop CORROSHIELD roofing screws at OnlyScrews
The linked CORROSHIELD products were verified against the OnlyScrews Shopify catalogue before publication. Each lists mild steel, a CORROSHIELD coating and a 2.5 mm EPDM washer. Verify the exact application and drilling range before installation.
CORROSHIELD 10g · 4.8 × 20 mm
Hex-head self-drilling roofing screw · 16 TPI · listed 2.5 mm EPDM washer
View live product →CORROSHIELD 12g · 5.4 × 25 mm
Hex-head self-drilling roofing screw · 14 TPI · listed 2.5 mm EPDM washer
View live product →CORROSHIELD 12g · 5.4 × 35 mm
Hex-head self-drilling roofing screw · 14 TPI · listed 2.5 mm EPDM washer
View live product →CORROSHIELD 12g · 5.4 × 45 mm
Hex-head self-drilling roofing screw · 14 TPI · listed 2.5 mm EPDM washer
View live product →
CORROSHIELD 12g · 5.4 × 32 mm
Hex-head self-drilling roofing screw · 24 TPI · listed 2.5 mm EPDM washer
View live product →
CORROSHIELD 12g · 5.4 × 65 mm
Hex-head self-drilling roofing screw · 24 TPI · listed 2.5 mm EPDM washer
View live product →The 14 TPI and 24 TPI variants are not presented as interchangeable. Thread pitch and size alone are insufficient to establish suitability for a particular steel or timber substrate.
Frequently asked questions
What does EPDM stand for in a roofing washer?
EPDM means ethylene propylene diene monomer. It is a synthetic elastomer commonly used in outdoor seals because suitably formulated compounds resist weathering and ozone.
Does an EPDM washer make a roofing screw waterproof?
It helps form a weather seal at the fastener penetration when used in a compatible system and correctly installed. No washer alone guarantees a leak-free roof.
How tight should I make an EPDM roofing washer?
Use the fastener manufacturer's depth-control and installation instructions. The aim is proper seating without damaging the gasket or sheet. There is no safe universal torque value for every roof and screw.
Why does a roofing screw leak even with a rubber washer?
Possible causes include under- or over-tightening, angled installation, a damaged seal, an unsuitable fastener, poor roof support or a wrong fixing location. Inspect the complete connection.
Can I use a normal flat washer instead of an EPDM washer?
Not where the roofing specification calls for a bonded elastomeric sealing washer. A plain metal washer does not provide the same conformable weather-sealing interface.
Is EPDM better than neoprene for roofing?
EPDM generally offers good outdoor weather and ozone resistance, but the correct choice depends on the exact compound, temperature, chemical exposure and tested roofing assembly. Follow the manufacturer's specification.
Final takeaway
An EPDM washer is a sealing component, not a guarantee of waterproofing or structural capacity. Choose the complete roofing screw for the sheet, substrate and environment; install it straight with the specified depth control; and inspect the washer for correct seating. For material-selection context, see SS304 vs Mild Steel Fasteners.
Engineering note: This guide is educational. For roof systems, wind loads and safety-critical connections, use approved fastener specifications, applicable codes and manufacturer installation data.