Strengthen a Wood Floor

Bouncy floors can transmit a fair amount of noise, both due to poor acoustic Insulation and due to noise created by the floor structure flexing in use.

Sometimes one finds a floor structure that is simply not upto the task in strength terms. These may be badly converted lofts where lightweight woodwork has not been upgraded for habitable use. Or they may simply be old properties with low spec original floor structures.

Fortunately strengthening a wooden floor structure is not too hard.

Structure

A typical suspended timber floor structure consists of 3 layers:

_________________
_________________  <-- wood floor, typ 19-25mm
 | |         | |
 | |         | |   <-- wooden joists, 3-12" deep
_|_|_________|_|_
-----------------  <-- plaster ceiling, typ 10-30mm

1. The top layer is the floor, which is either sheet wood (usually chipboard), or plank wood laid at 90 degrees to the joists. These are laid on the joists and nailed or screwed in place.
2. The central load bearing layer is a series of rectangular wood beams or joists, usually wood, which are fixed to walls at each end. Most timber joists are 2" wide, though 1.5", 3" and 4" are also used at times. Common joist depths are anything from 3" to 11", though again other sizes are sometimes found.
3. A plaster ceiling is fixed to the underside of the joists. This is usually plasterboard sheets screwed in place, with either the joints filled or the whole layer skimmed with plaster. In old houses, L&P ceilings are often found instead.

Variations

   _ _________________________ _
  | |                         | |
 _| |_                       _| |_
|     |                     |     |
|_____|_____________________|_____|

concrete beam & block construction

Some variations are also found:

• Lath & plaster ceiling instead of plasterboard. Common in old houses.
• No ceiling added, with the beams all visible.
• Joists can sometimes be made of steel, or be engineered composite material joists
• Flitch joists are occasionally found, these are steel reinforced wood joists.
• Reinforced concrete is sometimes used for suspended floors, especially in purpose built blocks of flats.
• Other less usual constructions are also found now and then.

Bounce, Strength & Risk

Rigidity and strength are 2 different characteristics. Joist stiffness is proportional to width x depth cubed.

Sometimes people have become alarmed at the amount of movement in a wood floor. However wood is to some extent a springy material, and a great deal of bending is possible before breakage occurs. For instance a 20' joist can normally bend by a more than foot without breakage.

Rotten wood is different. If rot is the cause of flex, the wood will break rather than flex significantly.

Heavy notching much weakens joists. If half the depth of the wood has been cut away its rigidity has been reduced by a factor of 8.

Noise

Noise generated and transmitted by unimproved floor structures depends first on movement, and thus rigidity. The less rigid the structure, the more noise is generated as creaking, and the more noise is transmitted from room to room.

Improved floor structures are also affected by damping, for example with materials such as mineral wool and sand.

Finally floors can be quietened by decoupling the floor from the ceiling.

Methods to reduce noise:

• Rigidify the joists by:
  • gluing & screwing more timber along the top of them to increase their total depth
  • adding new larger joists between the old ones
  • adding noggings, either timber or crossed metal straps
  • Use stressed box construction, see further down
  • Ensure there's a ring shank nail in each floorboard at each joist crossing
• Dampen the structure by filling the floor cavity with:
  • rockwool
  • a thin layer of sand
  • a shallow layer of scattered small scrap timber
• Stiffen the plaster layer by:
  • adding another layer of plasterboard to the ceiling, ideally sticking it to the existing layer to maximise rigidity
  • pouring wet plaster onto the ceiling from above
• Reinforce creaky floorboards and block sound path gaps by:
  • laying sheet chipboard under the floor
  • laying hardboard on top of the floor before carpeting
  • Or replace floorboards with thicker ones
• Isolate ceiling from floor by:
  • Add a false ceiling below the existing one, supported on its own independent joist structure
  • On top of the existing floor, add a layer of underlay plus a floating timber floor
• Adding carpet to a bare wood floor gives it a little damping and isolation from footfall, reducing noise to a useful degree.

Joist Height, Width, Spacing & Span

All these factors affect a floor's strength and rigidity.

• Doubling a joist's width doubles its strength and rigidity.
• Doubling a joist's height doubles its strength and quadruples its rigidity.

In Beam theory, doubling a rectangular beam's depth, decreases its centre deflection by 8 (2^3). A beams ultimate strength is normally only of theoretical interest. Rigidity is the reciprocal of deflection.

The maximum deflection recommended for new structures is 1 in 360 or 3mm per meter span for wood. Existing structures often have significantly higher deflection.

Rot first

If a floor structure has been weakened by rot, the rot damage and cause of rot (damp) need to be rectified first.

Liability

It is impossible to write an article that will cover every possible situation. When you undertake work, you decide what to do and you accept liability for the result. You may wish to seek qualified advice on your specific floor if necessary. This article is intended to explain the problem in general terms and its various possible solutions.

Strengthening Strategies

Stressed box construction

Sheet ply or OSB flooring is used, and is screwed to the joists every 6".

This couples neighbouring joists, sharing loads, and makes the floor layer structural. The box structure adds rigidity.

Increase Joist Height

A joist can have its depth increased by gluing and screwing more wood along the top. The new wood runs along the old joists, not at 90 degrees to it.

This is often a good solution, especially for loft floors. It does however raise the floor level to a small extent.

The new wood is glued in place & screwed every 6".

Its not essential to use full length timber. Using cut timber finger jointed gives almost the same rigidity. Square cut wood butted firmly end to end also gives a lot of additional rigidity, if not as much as continuous or finger jointed timber. Use a mallet to prevent gaps between ends.

Using reduced length timber avoids the need for a crane and part removal of roof covering.

Noggings

Noggings rigidly couple neighbouring beams, spreading loads across several supports, thus greatly reducing point load deflection. They also stop joists from bending sideways, improving stiffness and again reducing vertical deflection.

Straight

Straight noggings are pieces of wood at 90 degrees to the joists, joining neighouring joists. They are apx the same size as the joist, and are inserted between the joists, lying level & flush with them.

Straight noggings should be cut accurately so they fit without a gap.

| |       | |
| |       | |
| |       | |  <-- 2 joists
| |_______| |
| |_______| |  <-- nogging
| |       | |
| |       | |
| |       | |
| |       | |

Herringbone

Herringbone noggings are a method using less material. Instead of one solid nogging they are two much smaller pieces of Wood fitted in an X shape, thus linking the top of each joist to its neighbour's bottom, and vice versa.

 _          _
| |'.    .'| |
| |  '..'  | |
| |  .''.  | |
|_|.'    '.|_|

Herringbone noggings between 2 joists.

Noggings can be in timber or metal. Pictures:

• metal
• in use
• timber noggings

Flitch beams

Steel reinforced timber beams.

• External article

Cross beam

A wood or steel supporting beam run across under the joists greatly increases their strength by reducing their span. However it is visually intrusive and not usually the best option.

Thicker or stronger floor

Floor rigidity usually makes a small contribution to total structure rigidity. Extra thick self-supporting flooring is occasionally used without joists. A floor structure may be as little as 3" deep this way.

It is also possible to use this approach to reinforce an existing floor structure, by fitting jointed flooring 2" or more deep.

This is not a common means to strengthen and stiffen floors, since it uses a lot of wood for a given result. It can be a useful option where headoom is critical, and/or where the ceiling can not be disturbed, perhaps due to special decorative features.

Support Column

These are rarely added. They greatly improve strength & rigidity by reducing joist span.

Levelling

Sometimes a floor is sloping, often due to past subsidence in old houses, or timber frame rot. If its desired to flatten it, this can be done by putting wood strips on the top of the exposed joists to bring them all up to the same level. Each strip is cut to the necessary height, with at least one joist needing no wood adding.

The new wood can be glued in place, in which case it contributes something to increasing strength, or it can just be held in place temporarily, relying on the flooring nails to hold it when the floor's relaid.

Where joists are sagging badly, the new timber will want to be cut curved to compensate.

There's no need to use new timber here, all sorts of junk can be used. Sometimes other materials get used, such as bits of scrap plastic, card, scrap wood, and whatever's to hand and firm enough.

Levelling an uneven floor raises the level at one end. Usually some work is needed to deal with the new level, such as a step fitting, door trimming, or fitted furniture refitting. In some cases the change in level creates more work than is practical, so the floor is left sloping. There are also cases where the new level would be unworkable.

Wood grades

• C16
• C24
• TR26
• Ungraded

Screw vs Nail

Hammering Nails in can crack or break a plaster ceiling. This is more likely with small size joists and L&P ceilings.

Screwing causes much less damage. However driving 4" screws can require a relatively high torque screwdriver.

See Also

• Sheet Materials
• Timber basics
• Wiki Contents
• Wiki Subject Categories