Concrete Screwbolts Explained: Types, Sizes and When to Use Each

Choosing the right concrete screws for your installation makes a bigger difference than most people expect. Whether you are suspending threaded rod from a prestressed concrete ceiling, fixing strut channel to a block wall, or anchoring a base plate into a concrete column, the wrong fixing costs time and money. This guide covers every type of concrete screw we stock, so you can pick the right one first time.

What Are Concrete Screws?

Concrete screws, also known as concrete screwbolts or thunderbolts, cut their own thread directly into the base material as you install them. Because they work without plugs, sleeves or resin, installation is straightforward. You drill the hole to the correct diameter and depth, clear the dust, and drive the screw home.

Since they do not expand in the substrate, concrete screws are well suited to edge fixing and reduced spacing, where expansion anchors would risk splitting the material. Most types in this range are also removable and reusable, which makes them practical for temporary installations and adjustable support systems.

Strut Fix Concrete Screws

Our Strut Fix range is built specifically for strut channel, cable containment and mechanical support installations. Trade packs of 50 make them cost-effective for site use, and the dual thread versions work with both M8 and M10 threaded rod, so you carry fewer SKUs on the job.

Strut Fix M8/M10 x 35mm Dual Thread Female Concrete Screw (Pack of 50)

The female version accepts both M8 and M10 internal thread, so it works with a wider range of threaded rod sizes without any adapters. The drill hole and embedment are identical to the male version at 6mm and 35mm respectively.

Strut Fix M8/M10 x 55mm Dual Thread Female Concrete Screw (Pack of 50)

Where greater embedment depth is needed, such as when a render coating is present or where the substrate requires deeper engagement, the 55mm version provides the additional penetration. Drill to 6mm diameter and a minimum depth of 65mm.

All Strut Fix screws come in packs of 50 and are available on our concrete screwbolt category page.

Fischer FBS Concrete Screws

The Fischer FBS is one of the most widely specified concrete screws in UK construction and mechanical services. ETA and CE approved, it carries fire resistance classification R120, making it suitable for HVAC and MEP ceiling installations in prestressed concrete. It is also the recognised equivalent to the Hilti HUS3 range.

Fischer FBS 6x35mm Male Threaded (M8)

This version suspends M8 threaded rod directly from the concrete ceiling. The external M8 stud measures 19mm in length, the drill hole is 6mm, and the minimum hole depth is 45mm with 35mm embedment. It is the go-to fixing for standard HVAC and electrical containment ceiling drops.

Fischer FBS 6x35mm Female (M8/M10 Dual Thread)

The female version accepts both M8 and M10 internal thread, giving more flexibility across different threaded rod sizes on the same installation. Projection length is 26.5mm, and the drill specification is identical to the male version.

Fischer FBS 6x55mm Male and Female Versions

Both 55mm versions suit applications where greater pull-out resistance is required or where the available drill depth allows deeper engagement. The drill hole is still 6mm but the minimum hole depth increases to 65mm. Fischer sells these individually, which suits smaller jobs where exact quantities matter.

For full technical data on the Fischer FBS range, you can also refer to Fischer UK.

Fischer FBS II Ultracut

The FBS II Ultracut serves a different purpose from the FBS anchor. Rather than suspending threaded rod from above, it fixes brackets, back plates and structural frames directly to concrete, brick or block surfaces. Its saw tooth cutting geometry drives quickly into the substrate, so installation is fast without a separate clearance hole stage. Like the FBS, it carries ETA and CE approval and uses zinc plated steel.

8mm Diameter Versions

Five lengths are available in the 8mm range: 70mm, 80mm, 90mm, 110mm and 130mm. All require an 8mm drill hole and come in boxes of 50. These are well suited to fixing strut channel back plates, cable tray wall brackets and support framework bases to masonry.

10mm Diameter Versions

Four lengths are available in the 10mm range: 70mm, 90mm, 100mm and 120mm. All require a 10mm drill hole and also come in boxes of 50. The larger diameter suits heavier loads and thicker fixtures where the 8mm version would be at the limit of its rated capacity.

Rawlplug Flanged Concrete Screw Bolts

For lighter duty applications, the Rawlplug M6 flanged concrete screw bolt is a practical option. The hex head has an integrated flange that spreads the bearing load and copes well with slightly oversized holes. Because it uses a non-expansion design, it is safe near edges and in grouped fixing patterns where expansion anchors would cause stress concentration.

Five lengths are available: 50mm, 75mm, 100mm, 130mm and 150mm. All require a 9mm drill hole. These work well for conduit saddles, cable clips, light brackets and other applications where M8 anchors would be too large for the load.

Which Concrete Screw Should You Use?

 

Application	Recommended Product
Suspending M8 threaded rod from concrete ceiling	Strut Fix or Fischer FBS 6x35mm Male
Need M8 or M10 flexibility on the same fixing	Strut Fix Dual Thread Female or Fischer FBS Female
Deeper embedment or render coating present	Fischer FBS 6x55mm Male or Female
Fixing bracket or back plate to concrete wall	Fischer FBS II Ultracut 8mm or 10mm
Light duty fixing, M6 thread, non-expansion	Rawlplug Flanged Concrete Screw Bolt
Trade pack for site use	Strut Fix range, packs of 50

Installation Tips

Getting the drill specification right is the single most important factor in concrete screw performance.
For the FBS 6x35mm range, drill to 6mm diameter and a minimum depth of 45mm.
For the FBS 6x55mm, increase that to a minimum depth of 65mm. For the FBS II Ultracut, match the drill diameter exactly to the product size, either 8mm or 10mm.

Always clear the hole of dust before driving the screw. Compressed air gives the cleanest result, though a brush works well on smaller jobs. Furthermore, avoid drilling into reinforcement bars, and check that the embedment depth requirements are achievable in the actual substrate thickness before specifying the fixing length.

For fire-rated installations in prestressed concrete, the Fischer FBS carries R120 classification, which meets the common specification requirement for HVAC and MEP ceiling support systems.

Browse the Full Range

You can find all products mentioned in this guide on our concrete screwbolt and thunderbolt category page. Compatible products including threaded rod, pipe clamps and strut channel are also available across our wider fasteners and fixings range.

For advice on your specific application, call us on 0121 270 8507 or email sales@directchannel.uk.com and our team will point you in the right direction.