Soffit Fixings, aka Wedge Nuts

The soffit of any composite slab is, of course, formed by the deck profile itself. This makes it ideally suited to receive special fixings for supporting suspended ceilings and cable trays; no drilling of slab and deck is necessary, no specialist tools are required and the deck surface remains undamaged.


What and Why

The R51+ profile is known as ‘re-entrant’, and the service fixings for this type of deck, consisting of wedge shaped nuts with drop rods, are inserted into the narrow dovetail ribs and clamped into place against the underside of the deck.

The trapezoidal TR profiles, TR60+ and TR80+, also have a small dovetail re-entrant channel running along the top of each rib which can take a similar nut and rod, again clamped against the deck.

The nuts and rods are easily adjustable and hence accommodate future alterations to service layouts.

In buildings where sound insulation is a key design requirement, the use of suspended ceilings enhances the acoustic performance of the slab.



Both the ‘V’ Nuts (used with R51+) and the ‘TR60’ Nuts (used with TR60+ and TR80+) are manufactured by Lindapter International Ltd specifically for these deck profiles.

V-Nuts VN10 are designed for M10 drop rods with a safe working load of 2.1kN. The TR60 Nuts are supplied in three sizes – TR6006 (M6 rods), TR6008 (M8 rods) and TR6010 (M10 rods). These have a safe working load of 1.0kN.



The wedge nuts and their rods must only be installed and loaded after the concrete slab has cured.

The slab in its final composite condition should have been designed to take the expected loads of both the suspended ceiling construction and the services that will be required to run beneath it.



To avoid potential localised overloading of the slab, soffit fixing locations should be based on a nominal 600mm grid. This is the minimum recommended spacing and the SMD Technical Team should be consulted if closer groupings are required. Factors such as slab depth, deck profile and specific design criteria for the particular slab will need to be taken into account.

Installation of each wedge nut and rod requires only a few simple steps.

Installation of V-Nuts (R51+ decking)

  1. Screw the V-Nut (VN) onto the threaded rod.
  2. Insert the VN and rod into the dovetail rib by keeping the nut in line with the rib.
  3. Rotate the rod and VN through 90° so that the tapered edges engage the sides of the re-entrant shape. The rod can then be tightened by hand and a washer or locking plate set against the deck soffit.
  4. Mechanical tightening can be used to finish but the final torque should not exceed the maximum recommended by the manufacturer.

Installation of TR60 Nuts (TR trapezoidal profiles)

Ensure you have the correct size nut and threaded rod for your particular application.

  1. Pre-assemble both the locking plate and wedge, with its flat surface facing up, onto the threaded rod.
  2. Insert the wedge into the re-entrant channel at the top of the deck rib and rotate it until the chamfered cams engage on the sides of the channel.
  3. Slide the plate up the threaded rod and over the wedge to lock it into position in the channel.
  4. Tighten the locknut beneath the plate to hold the assembly in position without exceeding the manufacturer’s recommended torque.

What ifs

There may be cases in which the required suspended loads exceed the wedge nut/drop rod manufacturer’s recommendations. The designer must check that these loads are within the slab’s design capacity before considering any alternative fixing options.

If bolting fixings into the slab is proposed, non-percussive methods should be used in order to minimise the disturbance of the bond between concrete and deck. Bolts should always be positioned in the trough sections of the slab and be used in conjunction with an appropriate spreader plate that is sized to suit the load applied.

The applied loading, taking into account its location if localised, should be checked using SMD Elements® design software.

Wedge nut fixings and their rods should only ever be subjected to static downward loads. Any transverse motion of the load is likely to cause the wedges to twist and become loose.


Roof Deck

At the top

Lightweight ceilings and services can also be suspended from our SR roof deck profiles. There are two methods for achieving this.

The first is use of proprietary fixings that are drilled through or fixed to the profile. nVent produces both trapezoidal and dovetail deck hangers in their Caddy range.

Alternatively, a Unistrut channel can be fixed to the underside of the profile using stitching screws in every trough. Ceilings and services can then be hung from the Unistrut channel.

Whichever method is selected, the suspended load must not exceed the lesser of:

  • the maximum UDL allowable for the profile when considered with all other applied loads and
  • the pull-out resistance of the proprietary fixing or stitching screws (when using a Unistrut system).

To avoid local overloading, hangers should be spaced at centres no closer than alternate ribs of the profile.


More Technical Help

The SMD Technical Team will happily assist with further recommendations and designs to suit your bespoke circumstances or refer to TGN Online regarding further information and details.

Download our Product Data Sheet for more information on these items

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