After Macquarie: Conserving Heritage Places: Clarendon, Tasmania, 2010

Disasters in building conservation — ten things not to do!

David Young

Ten common mistakes in repairing and caring for old buildings can cause more problems than they solve. Though unintentional, the resulting damage can be substantial, difficult and expensive to rectify, and in some cases may lead to permanent disfigurement.

1. Don’t sandblast masonry or timber
Sandblasting is often used to remove dirt, paint, plaster and render from masonry surfaces and also paint from timber. It is quick, relatively cheap and very damaging! It will remove the outer surfaces of bricks and stones, surfaces that may be critical to their durability. Mortar joints can be deeply eroded, and the combined damage to bricks and mortar can lead to excessive water penetration through walls. Sandblasting gives timber rounded corners and a weathered, fibrous look. Though commonly called sandblasting, a range of abrasive grits can been used and the broader term gritblasting is more correct. Much of the damage done is due to the very high water pressures used. The use of apparently-benign sodium bicarbonate as an abrasive (sodablasting) can be double trouble, due to the damage caused by the residue of salt in the walls, in addition to the abrasive action.

There are forms of gritblasting (or air-abrasive blasting) that are acceptable for use on old walls. These are based on specialised abrasives and generally use very low pressures. Even so, they require well-trained operators to ensure that no damage is done. Unless it is falling off in sheets, paint should generally be removed by chemical stripping.

2. Don’t replace a timber floor with poured-on-fill concrete
One of the worst mistakes of renovators is to remove a ventilated timber floor and replace it with a concrete slab poured on sand or other fill. The concrete and its associated damp-proof membrane prevent evaporation, and the soil moisture rising beneath the building becomes focused on the walls. Rising damp problems are almost guaranteed, whereas before there may have been no significant damp, even though the walls may have lacked effective damp-proof courses. External concrete paving against old walls will compound the problem.

3. Don’t install a damp-proof course without also taking the salt out
Rising damp carries soluble salts up into walls. Most decay of masonry materials is due to salts which form crystals in the pores of the material and cause fretting and loss of surfaces as the crystals grow with periodic wetting and drying. Once there is enough salt in a wall, damage due to crystal growth can happen simply from changes in humidity — and a damp-course will not prevent that. Successful treatment of salt attack and rising damp (salt damp) requires dealing with the salt, as well as the damp. Sacrificial mortars and plasters (which need to be replaced as they fail) are used to manage salt attack and to prevent it damaging adjacent masonry. A range of techniques including captive-head washing and poulticing using highly-absorbent materials can be used to extract salts from walls.

4. Don’t repoint lime mortar joints with cement
Replacing lime mortar with lime mortar is not only correct from a philosophical point of view, it is also technically correct, for three reasons. Mortars should always be weaker than the surrounding bricks or stones, so that any structural cracking occurs through the joints, rather than the masonry units. Mortars should also behave sacrificially in relation to the masonry, they should decay first and so protect the adjacent bricks or stones from salt attack.

Additionally, mortars have an important role in the way a wall ‘breathes’, in the way moisture moves in and out after rain. For each of these reasons, lime mortars should always be repaired with lime, rather than with cement which is too strong, and too impermeable.

Unfortunately, much repointing work is poorly executed, with mortar smeared over the faces of bricks and stones, producing very unsightly results. Though not related to choice of materials, it compounds the problem by adding aesthetic issues to the technical ones.

5. Don’t seal walls with water repellents or modern impervious paints
Masonry materials such as stone, brick and mortar are porous – they contain air in the tiny pores that naturally occur in these materials. Walls ‘breathe’ as the air expands and contracts with changes in temperature. Moisture also moves in and out of walls with natural changes in temperature and humidity. Attempts to ‘seal’ or ‘protect’ walls with water repellent coatings or modern impervious paints risk trapping moisture behind the coatings, which may lead to damage to the masonry. The problem is made much worse when the masonry contains appreciable salt, and the older the wall, the more salt it is likely to contain. Salt blocks pores in the coatings and prevents breathing, leading to failure of the coatings and increased damage to the masonry that you’re trying to protect. Acrylic paints have some breathing capacity (vapour permeability), yet they perform poorly when salt is present, they should not be used on valuable old walls. Use traditional limewashes instead.

6. Don’t mulch walls and block underfloor ventilation
Garden beds look nice against walls and the plants hide the decaying brickwork — that they are helping create! And heavy mulching is good for the garden but bad for walls, whether masonry or timber. Watering of garden beds increases the rising damp ‘stress’ on masonry walls, while salts in the soils and in the added fertiliser lead to inevitable salt attack on the masonry. Floor timbers and wall timbers are also at risk, but from fungal rot, borers and termites, due to the elevated moisture levels in the wood. Higher moisture levels may be caused by direct contact of garden beds with walls, and also by overspray from watering. Additionally, garden beds and plantings may block air vents which are designed to allow ventilation so that the humidity of the underfloor spaces is kept reasonably low.

Garden beds should be pulled back a minimum of 300 mm away from walls, and preferably 500 mm. The space between should be a sterile zone of gravel or other porous paving and should be well drained to prevent water lying against the base of walls.

7. Don’t mix dissimilar metals on the roof drainage system
When pairs of dissimilar metals like copper and zinc, or copper and steel are in electrical contact in water, the less noble metals (the zinc and the steel) corrode rapidly and in doing so protect the more noble (the copper in both cases). This principle of galvanic corrosion is used to protect steel by coating it with zinc in a process known as galvanising. Should the zinc coating be scratched or otherwise damaged, the less noble zinc will corrode and protect the more noble steel beneath it.

Mixing metals on a roof risks causing galvanic corrosion, even though the metals may not be in direct contact. For example, if a copper roof has galvanised (i.e. zinc-coated) steel gutters, water flowing over the more noble copper will lead to rapid corrosion of the zinc coating and then the steel of the gutters. Roofs and their drainage systems should preferably be made of a single metal type throughout — a copper roof should have copper gutters and downpipes, and a galvanised steel roof should have galvanised gutters and downpipes.

8. Don’t coat valuable timber floors with polyurethane
Old timber floors were left bare, oiled, waxed, stained, or covered with rugs or carpet. Today we don’t have maids for waxing, nor money for the regular maintenance required. Current taste often favours a hard gloss finish produced by tough durable coatings like polyurethanes. And so we sand into the old floor removing millimetres each time before coating with modern finishes. These soak into the tongue and groove joints and glue adjacent boards together. The result is a tightly sealed floor (at least on the top surface) which stays tightly sealed until the next big dry spell, when the boards shrink. Under normal circumstances the shrinkage would be taken up by tongues moving slightly in the grooves. But now that the tongues are firmly glued into the grooves, the weakest points are the upper parts of the groove, the ones that have been sanded down — and so the boards split a few millimetres in from their edges, through the groove. Then, after fifteen or twenty years when the polyurethane finish is worn through and needs redoing, the floor is once again sanded down and the floor boards get progressively thinner and thinner until the tops of the grooves are lost — leaving tongues showing and the floor has to be replaced.

Valuable old timber floors should never be coated with hard modern finishes like polyurethanes. Instead, where bare boards are not acceptable, alternative finishes should be considered. These might include oils such as tung oil which soak into the timber and can be recoated without re-sanding.

9. Don’t remove complete elements just because they’re a bit damaged
The damages may be an important part of the story of the place — in which case they should be kept. Where they should be fixed, repair only the section that is damaged. For example, if the bottom of a verandah post has rotted, then replace only a section of it, scarfing the new piece in with traditional joinery details. The same approach applies to an area of failed plaster on a wall — don’t replace the plaster from the whole wall just because it sounds a bit drummy in places. Patch repair only the area that really needs it. If done well the new patch will help stabilise the rest of the plaster.

These cases are examples of a key maxim in building conservation — do as much as necessary but as little as possible, so that valuable existing fabric can be retained where practicable.

10. Don’t forget routine maintenance of roofs, gutters and drains
This really should be the first one on the list, as failure to do basic maintenance is the main cause of much damage to old buildings. Keeping roof drainage systems working effectively is critical to maintaining a building in good condition. Regular inspections should be made of roofs, flashings, gutters, downpipes and drains. Get inside the roof space to check for leaks after heavy rains, and go outside during the heavy rain to look for blocked downpipes and overflowing gutters. Clean gutters regularly and adjust the cleaning regime to suit the season and the location of trees close to the building. Ensure that roofwater is taken well away from the base of walls. Clear sumps and stormwater drains regularly.

Prepare and make use of a maintenance plan, which should include a schedule of tasks and the frequency with which they need to be done. Include the changing of smoke alarm batteries, and checking of termite bait traps for any evidence of activity.

More information
Go to the technical information pages of the web sites of State heritage agencies, such as:

     http://www.heritage.nsw.gov.au/13_subnav_01.htm
     http://www.heritage.vic.gov.au/Publications/Technical-information.aspx
     http://www.environment.sa.gov.au/heritage/resources/pub_built.html
     http://www.heritage.tas.gov.au/practice_notes.html

Evans, I. 1988. Caring for old houses. Flannel Flower Press, Mullumbimby, NSW. ISBN 1-875253-09-2.

Evans, I. 1989. Restoring old houses: a guide to authentic restoration. 3rd edn, Macmillan, Sydney. ISBN 0-33-29881-0.

Historic Scotland publishes excellent Technical Advice Notes and Guides for Practitioners.
     http://www.historic-scotland.gov.uk

Murray, W. & Croker, A. 2005. Traditional joinery: Sydney houses 1810–1915. Watermark Press, Boorowa, NSW. ISBN 0-949284-71-8.

NSW Heritage Office. Various dates. Maintenance Series of information sheets. e.g.
Long, Elisha. 2005. Repair of tongue and groove floorboards. Information Sheet 5.4
     http://www.heritage.nsw.gov.au/docs/maintenance5-4_tongue_groove.pdf

Technical Advisory Group, Heritage Council of NSW. 2010. How to care for heritage buildings and sites: a first guide for owners/managers of heritage listed places. Department of Planning.
     http://www.heritage.nsw.gov.au/13_index.htm (available soon)

Stapleton, I. 2008. How to restore the old Aussie house. 3rd edn, Flannel Flower Press, Mullumbimby, NSW. ISBN 1-875253-16-6.

United States National Parks Service. Preservation Briefs. 40+ technical information sheets available on line at:
     http://www.cr.nps.gov/hps/tps/briefs/presbhom.htm

Walker, M. & Marquis-Kyle, P. 2004. The illustrated Burra Charter: good practice for heritage places. Australia ICOMOS Inc., Burwood. ISBN 0-9578528-2-7.

Young, David. 2008. Salt attack and rising damp: a guide to salt damp in historic and older buildings. Heritage Council of NSW, South Australian Department for Environment and Heritage, Adelaide City Council, Heritage Victoria, Melbourne. 80p. ISBN 978-0-9805126-4-9 (print) ISBN 978-0-9805126-5-6 (online)

Young, David. Forthcoming. Mortars — materials, mixes and methods: a guide to repointing mortar joints in older buildings. (working title) Heritage Council of NSW, South Australian Department for Environment and Heritage, Heritage Victoria.
Meanwhile see:

Young, David & Long Elisha. 2009. Repointing lime mortars joints — some important points.
     http://www.heritage.nsw.gov.au/docs/repointing_joints_dy_el_update.pdf

Acknowledgement
Thanks to Elisha Long for her helpful suggestions on this paper.