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HMSG music pavilion.

The Haberdashers Monmouth School for Girls appointed Victoria Perry as their architect to expand its facilities for music and for small group rehearsal and performance in particular.  The existing music building faces west at the top of a sloping lawn with a fine distant view over the Monnow valley to the Black Mountains and the Brecon Beacons.  The new building is set into the slope so that it does not obstruct this view and also has its own terrace in front of a largely glazed west wall.  This allows it to be used for social events as well as music.
Photo: Jack Tait Gallery.

This part of the school site contains a number of older brick buildings, built in hard red bricks from the Forest of Dean set in a white lime mortar, all bonded in a version of 'garden wall bond' so that bricks of irregular length could be used.  The same bricks, mortar and bond were used in the walls of the new Pavilion.  The roof of the pavilion was made concave on section to avoid concentrated sound reflections and the internal surfaces were designed to achieve a suitable reverberation time.
Photo: Jack Tait Gallery.

Structurally the building stands on spread footings on stiff clay.  The retaining walls were designed as vertical cantilevers of reinforced brickwork laid in hydraulic lime mortar so that no movement joints were required - the low strength of the lime mortar requiring quite thick walls.  The roof is carried by complex fabricated steel beams spanning with the slope onto slender posts inside the west wall glazing.  To reduce airborne sound transmission and give some absorbtion, reinforced wood-wool slabs span between the beams carrying stainless steel standing seam roofing, and the diaphragm formed by the slabs is braced to the end walls. 

Photo: Victoria Perry.


The 'feebly' hydraulic, NHL2, lime that we used - 'hL2' then burnt from the Lias Lime in Dorset but no longer produced - proved trickier than anticipated.  Firstly, like most hydraulic limes, it needed a great deal longer in the mixer to achieve a properly workable mortar - over 1/2 an hour: the use of 2 mixers per bricklaying gang enabled that to be accommodated.  Secondly the bricks are burnt from the Carboniferous shale and have an extremely low water absorbtion so that the mortar hardly stiffened at all from initial drying.  Thirdly they were laid in an unusually wet autumn and so the mortar didn't get a chance to dry out at all until a temporary roof was built over the scaffold.  By the time the retaining wall was approaching full height but not yet backfilled, the mortar from the bed joints at its base could still be raked out with a finger.  Although no movement occurred, this was worrying so tests were commissioned.  However testing lime mortars needs a different approach from testing cement mortars and the test lab didn't know this, so the results were almost useless until it was too late.  In the end, we added 'Metastar 501' - a by-product of the china clay industry of Cornwall - as a vigorous pozzolan to the later mixes, which made them whiter than the earlier so limited repointing was carried out.  In the end, the straight hL2 mortar did set and eventually set really well, so I have no concerns about the long-term strength of the walls, but it took some time and we did have to delay backfilling the retaining walls.  I do think that avoiding movement joints was worthwhile - the uninterrupted lengths of wall are splendid.

Since then, I have tended to use NHL 3.5 limes for new walls as giving a more rapid increase in strength, but 'fat' lime with much slower-acting pozzolans, such as ground brick dust, for brickwork repairs.  I later also discovered that bricks burnt from Carboniferous shale can undergo substantial long-term expansion that is so slow that it isn't picked up by the standard tests (thanks to James Sutherland for that information) and a couple of years later worked on a most unusual problem afflicting just one corner of Gardner's Warehouse in Glasgow, to which this fact was the key.
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