Expert: Bruce E. Johnson - 4/10/2009

Question
We are in the final stages of designing a new home.  We are very fond of natural stone, and interested in considering using slate instead of more conventional exterior siding materials.  We live on an island in the Pacific Northwest (mild temps, moderate rain).  How should slate be installed for this purpose? Any particular mortar or underlayments? Any reasons not to consider this material?
Thanks,
Julia

Answer
Hi julia, slate can be installed pretty much the same as any other stone.  If you are installing thin slate (3/4" thick) over a wood frame sheathed with plywood you would need to cover the building with a good vapor barrier of heavy felt paper and top that with metal lath as if going to stucco the exterior.  Then you begin with a stucco or mortar scratch coat pressed into the lath. The stone is then attached to the scratch coat using traditional mortar.   Thicker stone needs to be tied into the wall using brick ties keyed into the mortar joints.  A little different system is used if your walls are concrete masonry units/ cinder block.  The slate is attached directly to the CMU using mortar and then sealed with a good sealer to prevent moisture penetration.  I can't think of any reason not to use slate.  I have seen slate roofs that are over 200 years old.  Which is another option.  You can use slate shingles for siding also.  Slate is a beautiful stone and holds up to extreme weather conditions if cared for properly.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com