Wooden laths: how should we use them, and should we even be using them at all? Laths were widely used until the mid 20th century, but they have now been largely replaced by modern solutions such as plasterboard. However, many owners of period properties will encounter laths when renovating or restoring.
In their most basic form, laths (or lathes) are traditionally timber strips attached to a stud frame. This studwork frame is usually comprised of panels. These are fixed onto the floor and ceiling. The wooden strips, or laths, have gaps between them that allow plaster to settle behind and bond more strongly to the lath itself. Once the plaster has oozed through the laths and adhered to the backside of the lath, this forms a 'key'.
Lath and plaster ultimately derives from an earlier technique, wattle and daub, which has been in use for at least 6,000 years. From around the 18th century, lime plaster was used on brick, stone, or laths. Metal laths can also be used. These date from around the late 19th century, and they have increased strength and rigidity in comparison to wood. Conversely, the angular edges of metal laths mean that working with non-hydraulic lime plasters is more difficult. Both metal and wooden laths serve a similar function.
Before you begin work on your laths, ensure that they are clean. This includes being free from any greasy substances, dirt, or plant matter. Similarly, if the laths are damaged in any way, they should be replaced prior to plastering. At Best of Lime, our Douglas Fir laths are sawn 'green', so have a raised grain texture that also acts as a key for the plaster. Before applying a scratch coat to the laths, spray them with water.
At Best of Lime, we find the easiest way to illustrate the correct spacing of wooden laths is through 'Goldilocks'. Use your little finger as good guide - the gap between the laths should be around 1/4 of an inch, or the width of the average little finger. The first set of photos show laths that are too close, and the plaster won't push through to form the key. However, too widely spaced, and the key is likely to fall off the back. The final image shows laths that have been spaced just right!
The following photos show how lime plaster on laths should look from behind.
There are two key benefits to wooden laths: their sound-proofing properties, and their role in delaying the spread of fires. In comparison to modern plasterboard, lime plaster and laths play a role in absorbing sounds, particularly low frequencies. Due to the non-uniform keys, echoes can also be deflected. It is also proven that lath and plaster can limit the spread of fires, as a result of the density of lath and plaster construction limiting the oxygen supply needed to fuel the fire.
The question is not how we use timber (or metal) laths, but should we be using them at all? At Best of Lime, we develop and manufacture low carbon lime plasters for use internally and externally. We like to think that the longevity of our plaster is measured in generations. The use of timber lath ticks the conservation box, but the thermal efficiency of lath as a background is poor, so from an energy conservation perspective we should be looking at alternatives.
In 2020 we feel the use of laths externally has had its day. By 2050 (or surely sooner) they will be obsolete old technology that may have to be stripped and replaced. At Best of Lime, we now promote the use of the PAVATEX range of wood fibre boards. Their use is not detrimental to the fabric of an historic building and used correctly can make our built heritage future proof and fit for purpose for the next generation.
Do you have questions about using our products with wooden laths? Get in touch with Best of Lime today.
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