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What's the difference between old buildings and new ones?

Traditional Construction

Old building with vegetationSection through where an adjacent building has been downtaken
Solid-walled buildings are anything but.  Maybe the Greeks and Romans could afford such extravagance but we have to settle for a thin external layer of dressed stone and a thin internal layer of coarse stone with all sorts of rubble, mortar, empty spaces, soil etc in-between so it is solid insofar as it is not designed with a cavity but that's all.

Generally, stone structures range from 450mm - 800mm thick but commonly 600mm (half that at bay windows which is another issue!).  Solid brick walls can be 260mm thick and hard-plastered (directly onto brick) so they need to be in tip-top condition to have a chance of remaining dry.

Lime is hygrophobic - it absorbs moisture - but it also forces it to evaporate again once conditions are correct.  If this is substituted by using cement, the ability of the wall to allow moisture to evaporate is severely diminished or, in the case of a cement render, lost entirely.  In addition to cement preventing the passage of moisture, it also doesn't adhere very well to stone.

Some builders will recommend the use of wire mesh, rawl-bolted into the stone or brickwork to help the cement 'stick'.  It doesn't, it only prevents the failed render from falling off, which isn't the same.  Moisture will still be unable to evaporate and will be trapped behind the render making any problems worse.

Modern Construction

Brick and block cavity wallA typical brick and block cavity wall although commonly with insulation in the cavity
New construction (mainly categorised as post 1919) saw the introduction of cavity walls, where the function of external walls changed to keep water out of the building and the cavity, as a second line of defence, allowing any water that entered the wall to evaporate out of vents or drain out of weep holes - although early cavities were only vented into the roofspace and no other cavity ventilation was provided for.

Several problems have arisen with older cavity construction with failure of wall ties (which were designed to keep the internal and external structure together) or bridging of the cavity by sloppy workmanship which causes damp patches to appear in odd areas of the building.

Dampness problems still occur in cavity construction where wall-ties can corrode and fail (now either plastic or stainless steel instead of galvanised) causing bulges and cracking, or where mortar has been allowed to build up on the ties, moisture can pass through from the outer leaf and appear internally.

Hopefully you will realise just how much knowledge of different structures is needed to be able to understand faults and specify remedies for their repair.  One size does not fit all!