Don't use 3/8 rib lath for stucco

Sir - Are you familiar with instances of hairline cracking along ribbed lath rib lines? We have a project with exterior cement plaster over sheathing, using 3/8" deep ribbed lath. Unfortunately, the plaster system was indicated to be 3/4". There are some hairline cracks which seem to follow the rib lines. Also, ther appear to be some stress cracks which follow the control joint legs and at the edge of the control joint, itself. We used zinc-coated steel lath and aluminum CJ extrusions (steel was specified). I suspect these may be due to differential expansion. Thanks for your help.

3/8 rib lath put on vertically will crack along every rib

3/8 rib lath put on vertically will crack along every rib, every time.

I saw an interior plaster job many years ago near Baltimore where the lath was put on vertically and there was a crack along every rib in the brown mortar.

Outside stucco would probably crack worse.

If 3/8 rib or high rib lath, is to be used it should always be put on horizontally, and backwards, that is with the point of the V touching the wall to be plastered. This allows mortar to key in behind the lath.

Still this can be a path to failure unless the mortar is squihed in real good along the bottom of the V.

We used 3/8" rib lath many years ago for suspended plaster ceilings on channel iron framing on schools, hospitals, etc. This lath may work well for this, but not for anything else.