Strengthen a Wood Floor

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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.


Contents

Structure

Upstairs floors

A typical upstairs suspended timber floor structure consists of 3 layers:

_________________
_________________  <-- wood floor, typ 19-25mm
 | |         | |
 | |         | |   <-- wooden joists, 3-12" deep
_|_|_________|_|_
-----------------  <-- plaster ceiling, typ 9-30mm
  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.
Floor construction 3.gif

Ground floor floors

A typical ground floor suspended timber floor structure consists of 2 layers:

_________________
_________________  <-- wood floor, typ 19-25mm
 | |         | |
 | |         | |   <-- wooden joists, 3-12" deep
_|_|_________|_|_
|    |             <-- sleeper wall
|    |
  1. The top layer is the floor boards or sheets, usually 3/4" - 1"
  2. The central load bearing layer is timber joists, usually 2" wide, various possible heights.
  3. Brick sleeper walls are often used under the timber floor to support the joists and firm up the floor. Its impossible to clearly show both joists and sleepers from one point of view in ascii art, maybe a clear photo one day. Sleeper walls run at 90 degrees to the joists, and have holes to allow airflow.


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 occasionally, such as vaulted floors, plaster floors, joistless timber floors, etc


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, the rigidity of the cut away area is 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
  • Damp the structure by filling the floor cavity with:
    • rockwool batts
    • a thin layer of sand
    • a shallow layer of scattered small scraps of timber, plasterboard, etc
  • 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 isolation from footfall, reducing noise to a degree.
  • Add more support by repairing existing sleeper walls or adding a new one.


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.


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.


Causes of weakness and bounce

Rot

If a floor structure has been weakened by rot, the rot damage and cause of rot (damp) need to be rectified first. Failure of supports, whether end supports or sleeper walls, also causes bounce on ground floor timber floors.

When a joist rots away at one end, and is thus no longer fixed to the wall there, it can be repaired by bolting a new length of wood onto the side of it, and fixing that to the wall, using either a metal joist hanger or a new hole in the wall. when inserting the new end into a hole in the wall, the wood should be rot preservative treated and wrapped with a membrane first to minimise risk of rot.

Sleeper wall failure

Ground floor timber floors are often supported part way along their span by sleeper walls. These are dwarf brick walls just high enough to support the floor joists. If these collapse or sink, the floor is no longer well supported, and becomes bouncy. If collapsing they can be repaired. If sinking they can be given wider footings, or a new sleeper wall can be constructed with a wider footing. Always leave a number of holes in sleeper walls to enable sufficient ventilation, otherwise rot can occur.

If a sleeper wall has only sunk slightly, it can just be packed with bits of wood or ply glued in place.

Adding a new sleeper wall is of course one way to rigidify ground floors.


Woodworm

Minor woodworm makes no noticeable difference, but severe woodworm can eat much of the wood away, leaving a bouncy weakened floor. If its got this bad, a complete new timber floor structure is generally needed. If the worm is active the new timber should be treated to avoid reinfestation.


Structural movement

When a joist comes out of its socket at one end due to wall movement, the first necessity is to address the structural movement. That done, the existing joists can then be rehung on joist hangers.


Undersize Timber

A lot of upstairs floors in old houses are bouncy simply because the builder economised on timber. Various ways to improve them are given below.


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 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

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

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.


Herringbone

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

Herringbone noggings between 2 joists.

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.

Noggings can be in timber or metal. Pictures:

Flitch beams

Steel reinforced timber beams.

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't 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 level it, there are a few ways to do this:

  • wood strips are put 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.
  • bits of scrap packing are used to raise the joist tops to the right level
  • whole new timber strips are bolted to the side of the existing joists
  • If the holes the joists ends are in have the room, cement and slate can be inserted under the joist end to raise it

New wood added on top of a joist can be glued on in place, in which case it sometimes contributes a bit to increasing rigidity, or it can just be held in place temporarily, relying on nails to hold it when the floor's relaid.

When packing the top of joists, there's no need to use new timber, 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, or just reduced in slope. There are also cases where the new level would be unworkable.

To level a floor:

  1. remove skirting where its height will be changed
  2. remove the floorboards or chipboard sheets
  3. find the highest point of the joists around the edge
  4. build up all joists to this level, using any of the methods above
  5. re-lay floor
  6. refit skirting, trim door bottoms


Wood grades

Wood grades differ in strength, and to some degree stiffness

  • C16
  • C24
  • TR26
  • Ungraded


Fixings

Plaster ceilings can be a bit fragile. Hammering Nails in can crack or break a ceiling. This is more likely with small size joists and L&P ceilings. Screwing is less likely to damage plaster ceilings.

When nailing floorboards back, ring shanked nails are much less prone to pulling out later than plain round or oval nails. Cut clasp nails are too, but these make later lifting of boards without damaging them difficult.

When attaching new ends to joists, a few threaded rods (studs) can be used with penny washers and timber connectors. Epoxy glue and plenty of screws are also an option. For ground floor use, in principle its also an option to use less thorough fixings and construct a sleeper wall under the join to support both pieces of wood.

See Also

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