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 reinforced for floor use. Or they may simply be old properties with low spec original floor structures.

Fortunately strengthening wooden floor structures is not too hard.

Structure

Describe normal suspended timber floor structure

Bounce, Strength & Risk

Rigidity and strength are 2 different characteristics.

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 insance 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 can also cause breakage before significant deflection. If half the depth of the wood has been cut away its strength has been reduced to ---.

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 ---s its strength and ---s its rigidity.

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 accept liability for the results. You may wish to seek qualified advice on your specific floor if necessary. This article is intended to explain the problem generally and its various possible solutions.

Strengthening Strategies

Glue & screw the floor

Stressed box construction

Increase Joist Height

A joist can have its depth increased by gluing and screwing more wood along the top.

Glue & screw new wood on

Noggins

Herringbone

Straight

Flitch beams

Cross beam

Thicker or stronger floor

Floor rigidity makes some contribution to total structure rigidity. For a given thicknes of wood, plywood gives more rigidity than floorboards, which give more rigidity than chipboard.

This is not a prime means to strengthen and stiffen floors, but if you are already planning to replace the floor, you can choose materials that will maximise any benefit.

Wood grades

C16
C24
Ungraded