Soundproof a floor with a suspended ceiling

Noise from neighbouring flats, either over or under yours, can be quite a problem. Property converted before June 1992 often had little or no sound proofing at the time of conversion so would benefit from installing some.

Noise is carried by vibrations, either through the air or through the structure of the building. Referring to 'sound proofing' is a bit of a misnomer, it would prove very difficult to full sound proof a floor/ceiling, the best that can be reasonably expect is to reduce noise intrusion to an acceptable level. To reduce noise, the vibrations must be isolated from the area. With floors/ceilings there are two options, either isolate the structure above the floor or below, each has its own advantages and drawback.

Sound proofing below the ceiling

Sound proofing below the ceiling requires fitting a false ceiling incorporating noise deadening material. The disadvantages are that:

  • up to 150mm (6 inch) of room height is lost in rooms up to about 5 metres wide;
  • This method becomes unviable for really large rooms beyond about 7 metres;
  • There's a need to drop down any ceiling fixtures (such as lights etc).

NOTE - light weight suspended ceilings (often suspended on wires) is not adequate for the job, the wires transmit vibrations and the lightweight panels have little sound deadening properties.

Sound proofing suspended ceiling A typical sound proofing suspended ceiling is shown to the right.

(An alternative method to fit a sound proofing suspended floor is given here.)

The new suspended joists are run across the smallest dimension of the room and fixed to the wall using wall hangers. It is most important that there is an air gap between the top of the joists and the original ceiling to avoid the transmission of noise, this gap can be as small as 5mm; with an old building it is best to aim for a larger gap (say 25mm) to allow for possible irregularities of the ceiling. Mineral wool is fitted between the new joists and two layers of plasterboard are fixed to the underside of the joists. If the floorboards in the room above are plain edged (rather than tongued and grooved), the floor should be covered with 6/8mm hardboard.

Fitting a sound proofing suspended ceiling.

  • The first thing is to set the level for the hangers for the new joists below the existing ceiling - this dimension is made up of:
    • The air gap, plus,
    • The height of the suspended joists (100 x 50 mm should be adequate for up to a 5 metre span).
    Once the distance has been determined, it can be easily marked by using a laser level along the two longest walls.
    If a laser level is not available, measure down on each of the longer walls the required distance.
  • Check any ceiling fixtures (lights etc) which need to be dropped, remove power from the appropriate circuit, loosen the fixture and make sure that there is sufficient length of cable to drop it the required distance to fix to the underside of the suspended ceiling. If in doubt, consult a qualified electrician, take the opportunity to consider changing the position of the fixtures - fitting a new ceiling gives you the chance to make such changes.
  • Work out the spacing of the new joists, the joists should be spaced to suit the width of the mineral wool being used. Even out the space at each end of the room as necessary as the end joists will generally not be the width of the mineral wool from the end wall. Check the length of each long wall as they may be uneven and the positioning of the joists may need to be adjusted to take this variation into account.
  • Joist hangerStart in one corner by fixing a joist hanger (right) the required distances from the end wall and under the existing ceiling. Moving to the opposite wall, mark the position of the hanger for the opposite end of the first joist. As you won't be able to drop the joist in from above, lift the joist into place and fit the second joist hanger - you may find it easier to drill and plug the holes in the wall and then screw the joist hanger in place.
    Secure the end of each joist to the hanger using nails or screws - note that there will be limited room between the first joists and the end wall, it may be easier to fix the hangers to the joist before putting them in place and then screws each hanger to the wall.
  • Moving to the other end of the room, fit the end joist in a similar manner.
  • With the two end joists in place, run a string line between the undersides of the end joist hangers on each wall.
  • Work along from one end of the room and fit each sequential joist in the same manner as the end joists, carefully measure the distance between each joist to suit the mineral wool material and line up the bottom of each hanger with the string line.
    Mark the wall under the centre of each joist hanger - this will help you find the joists when you come to secure the plasterboard to them.
  • Arrange for any ceiling fixtures which need to be positioned in the suspended ceiling.
  • The mineral wool needs to be in place before the ceiling is boarded, the wool will obviously tend to drop out from between the joists. The easiest way to hold it in place is to use tape secured to adjacent joists - plastic scrim tape is ideal for this. Only install the mineral wool over an area which you intend to cover immediately.
  • Fit and secure (using drywall screws) the first layer of plasterboard under the joists/mineral wool. Leave a gap of about 5mm between the edges of the plasterboard and the walls - the walls may be uneven, 5mm should be considered the minimum gap. As a second layer of plasterboard is going to be installed, place the first screw on each joist about 300mm from one wall and space the remainder at about 300mm centres.
  • Repeat installing mineral wool, putting up the plasterboard until the ceiling is complete. Remember where you need to drop any electrical wiring through the plasterboard and make holes through the plasterboard as necessary.
  • Install the second layer of plasterboard using longer screws and place the first screw on each joist about 150mm from the wall and space the remainder at about 300mm centres. Ensure that the edges of the second layer of plasterboard do not line up with those of the first layer. Ensure that the edge gap is still maintained and don't forget to drop through any electrical cables required.
  • When the second layer of plasterboard is in place, fill the edge gaps using a flexible sealant.
  • To finish off the corners of the ceiling, fit either plain or paper covered polystyrene coving rather than plaster coving - this will reduce any vibrations being conveyed from the walls to the ceiling.
  • Fit any ceiling fixtures to the ceiling as appropriate.
  • Finish the ceiling by skimming, papering or applying a decorative coating to suit your tastes.

The new suspended ceiling is now complete.

The floor above

If the floor in the room above has plain edged floorboard (rather than tongued and grooved), it should be covered in 6 or 8mm exterior grade hardboard (smooth side up). Secure the sheets at 150 mm (6 inch) centres all over the sheets using annular ring nails to reduce the possibility of the nails working up. The sheets should be dampened 24 hours before installation so that they can be laid flat and this should prevent subsequent unevenness.

The soundproofing is now complete, ideally the floor above should be covered with carpet using a heavy duty underlay to further improve the noise deadening characteristics of the floor.