Friday, November 12, 2010
Changes to the MVMA guidelines raises questions about the use of Spider Lath. Here are some answers when using Spider Lath in place of metal lath:
For starters, I typically refer people to the Masonry Veneer Manufacturers Association(MVMA) Guidelines as a starting point. MVMA is made up of the leading manufacturers and other professionals in the industry.
You can find their guidelines at http://www.masonryveneer.org/pdf/MVMAManualDesign_web.pdf It is important to recognize that installing Adhered Concrete Masonry Veneer on ICF’s is outside of the scope of the MVMA guidelines. Before you start, you want to make sure that the lath is pulled tight.
Some installers use a 3 or 4 foot 2x with nails that is used as a stretcher (see attached photo). MVMA calls for ½” nominal scratch coat, where the lath is fully embedded (1/4” behind and ¼” in front of the lath) – this is required whether you are using self-furring metal lath, flat metal lath with furring strips or fiberglass lath with furring strips.
Most installers aren’t aware of this requirement and also don’t typically apply this thick of a scratch coat. We have found it easier when you apply the mortar up the wall first and then across the wall (to push the mortar behind this fiberglass). It is certainly a bit different than metal, but it works pretty well and most importantly it ensures you are getting a full 1/4” behind the lath. After spreading about a 10sft area, come back and apply another coat – ensuring the full ½” that the MVMA calls for. Once thumbprint hard, scratch with scarifier.
Letting the scratch coat set up will provide a very rigid surface to apply thin veneer to. In regards to installing thin veneer over ICF. There are a few manufacturers of thin veneer who give recommendations on how to install over ICF.
I would suggest obtaining this information from the ICF manufacturer as typically you would need to tie in to the metal or plastic flange that typically runs from the top of the form to the bottom.
I’ve spoken to a couple of ICF manufacturers and they recommend that you attach lath with screws and some sort of washer (head) 6” on center. I think the important point is that we should all take some time to read the MVMA guidelines. There are a lot of enhancements that help us build better walls.