A double storey extension by Marble Construction in Surbiton, Surrey.

Double story extensions are most useful for those that need to massively increase the size of their property, do not have a loft suitable for conversion and for those that want to loose as little garden space as possible.

Most upper floors are constructed using wooden beams which slot into joist hangers which are in turn built into the block work of load bearing walls. The longest a wooden beam can be is 4.5 metres, if the required distance is longer than this RSJs (rolled steel joists) will be required which are slightly more expensive. Because Timber is hydroscopic steel cross strutting is used to prevent warping and shrinking as the water content decreases.

In the past floors have been constructed using softwood joists. However these are very prone to shrinking, this is the main reason for squeaky floors and creepy creaking in the middle of the night. To prevent this and the wide array of other longterm problems that can arise, Marble offers the use of I-joists as an alternative, these have a similar “I” shape to RSJs and can bridge distances of 6 metres as opposed to 4.5 metres, are quicker to install and lighter than their softwood adversaries. The only downside is that they must be custom ordered as cutting them reduces their structural integrity.

Timber Joists Quality Checking

Marble will only use construction material of a high enough quality for the job to prevent problems arising later and to ensure the property meets building regulations. If the customer wishes to check the timber joist’s quality and the quality of their fitting the following are the basic tests:

Holes, burrowing and wiggly lines like this are signs of woodworm
  • Joists are securely and tightly fixed to the joist hangers.
  • For inner walls, joists should fit tightly around the brick/block work.
  • The distance between the joints should be either 400mm, 450mm o 600mm depending on other factors.
  • The depth and width of each joint should be as specified.
  • Where there is an upstairs wall, joints below should be doubled up.
  • All joists should be level at the top.
  • The stress grading should be adequate for the distance the beam spans.
  • The beams should be free from rot, woodworm, bowing or twisting.
  • Wood should be treated to prevent insect infestation.



Building regulations only require insulation in the floor where the area below is not heated, ie a garage or passageway underneath. However it does not cost much to install insulation between the wooden joists during construction and it will save the homeowner in the long-run through reduced heating costs.


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