External Plastering (Rendering on Metal Lath Using St. Astier Products)

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The main difficulty in working on lath is ensuring the lath is suitable for the job. Metal laths come in a wide variety of types and designs, some of which have specific functions and are not suitable for a broad range of rendering applications. It is imperative to check with the manufacturer of the lath what is it designed to cope with.

The lath Manufacturers recommendations for fixing should be sought, however as most fixings are specified to accommodate cement renders or plasters, it may be necessary to decrease the centers of the fixings to 4″ (100mm). to reduce to the minimum any movement during application. Laps should be firmly fixed.

Lath can be plastic, metal, galvanized or stainless steel. All can be used with NHL mortars.
Simple expanded mesh lath usually requires greater numbers of fixings to reduce play.
The ribbed laths span more readily between supports and are generally more robust.

Backgrounds:
Brick, block, concrete or even straw bales. It is also commonly used on framing, either timber or metal studs or to bridge between two completely different backgrounds and supports. If the work spans over two different types of backgrounds, the choice of application and mix will be determined by the weaker support.

Application:
Ensure that the first coat covers the laths by a minimum of 5/16″ – 3/8″ from the outside face of the metal (including any ribs or overlaps), particularly where the lath is spanning between framing.

First coat: use NHL 5 and a well graded sharp sand at a ratio of 1:2. The mortar must be allowed to cure and harden sufficiently, at least 1 week in good weather conditions, longer if possible, before a second coat is applied. Internal work should not be force
dried under any circumstances.

Further coats can be applied as per the general guidelines for NHL Renders and plasters (2 or 3 coat work).
Where lath is well fixed to a solid background, use of a spray gun with a NHL 5 1:1.5 can be used for the first and all subsequent coats.

Protection and curing:
Always follow best working practice, pay particular attention to protection against knocks and bumps on fresh mortars.

Note:
If lath has been fixed by another trade, the plasterer should only apply NHL mortars when he is fully satisfied that the lath work is rigid and sound.

 

All above information is given as general indication only. Dosages may vary in accordance
to materials and aggregates used and site conditions. Please liaise with your St. Astier distributor to finalize mortars mixes.

• KINDS OF APPLICATIONS

 

General

Careful and proper installation is important for ensuring the long-lasting beauty of your Proline Stone Veneer. For nearly all exterior applications, required installation includes a metal lath and scratch coat (also known as cement plaster coating) over a water-resistive barrier. The water-resistive barrier generally consists of two layers of properly applied Grade D paper, which is then covered with metal lath and a mortar scratch coating (cement plaster).

What can I put stone veneer on for an exterior application?

The metal lath/scratch coating may be applied over structurally sound wall surfaces of plywood, OSB, concrete board or gypsum sheathing, supported by wood or steel studs, and over concrete or masonry walls.

What can I put stone veneer on for an interior application?

For interior applications, stone veneer can be installed over cement board, drywall, and all of the above.

How do I apply veneer to cement board or drywall?

We recommend installing metal lath and scratch coat. The metal lath will prevent the weight of the stone from pulling the nails through the cement board. See www.masonryveneer.org

•  VAPOR BARRIER

Do I need to have a vapor barrier?

Yes, two layers of Grade D (15lb felt) paper is required for exterior application.

• METAL LATH

What size metal lath should I use?

2.5 pound galvanized, self-furred, metal lath is recommended.

What surface do I need metal lath on?

For all exterior applications, install metal lath and scratch coat on the structurally sound rigid sheathing.  Note: However, for clean and unpainted concrete and masonry surfaces, stone veneer may be directly applied without use of metal lath/scratch coat.   As a further note, when in doubt, install the metal lath and scratch coat.

How should the lath be applied?

The lath should be applied horizontally; Surface should be rough as you rub your hand upward. This will make the lath angle down against the wall forming little cups that will help in catching the mortar for the scratch coat.
There should be NO bubbles or bounce in the lath. 

What size nails do I use?

Coated roofing nails or screws will suffice.  Be sure to penetrate stud 1 inch, and nail every 6” vertically. Overlap the lath 4” at all seams - wrap all corners 16” or attach to stud. When corners are wrapped it is important to nail into the corner stud from both sides.  This is where the most stress will develop. If you don’t wrap the corners with lath, you will have cracks next spring!

•  MORTAR

How much mortar should I mix?

See bag instructions.

Can I Drystack the stone?

We do not recommend drystacking for exterior applications.  The grout line in traditional installations helps to secure stone and keep water out.  However, if you do choose a dry stack application, use an upgrade polymer mortar.  Be sure to order an additional 15-20% on your stone order.

What type of mortar do I need?

Type N or S.

Can I add a bonding agent to the mortar mix?

Yes, adhesion can be improved by adding a liquid bonding agent to the mortar mix. Follow manufacturer's recommendations.

•  SCRATCH COAT APPLICATION

How thick should the scratch coat be?

All lath should be covered with a 1/2" to 3/4" coat of mortar.

How soon can I apply stone after I finished scratch coat?

Wait until scratch coat is dry or tacky to touch.

What should I use to apply the "scratch" to the scratch coat?

Scratch coat can be roughed up with a stiff brush for better adhesion.

•  STONE LAYOUT

What guidelines should be followed when laying out the stone?

Lay out a quantity of stone near work area, blending multiple boxes to ensure proper mix of size shape & color.

•  APPLICATION OF THE STONE

What is "Buttering" the stone?

To butter the stone, completely cover the back of the stone with ½” mortar.  This will ensure a tight seal.

How hard should I push to apply the stone to the wall?

Apply enough pressure so mortar begins to ooze out to the sides of the stone.  Use a wiggle action to further set stone.  If mortar mix is made correctly, the stone should “hold” fairly quickly – less than 10 seconds.

If I need to cut a stone how do I do that?

Cutting or trimming the stones are done best with a mason blade in a skill saw, a diamond saw, a mason hammer or shingle hammer.

Additional reading:
What is the best fencing for schools?
SS PROFILES
What is high rib mesh used for?

Wusheng Hardware contains other products and information you need, so please check it out.

Where do I begin laying the stone?

Begin by snapping a line to ensure level installation (or use a level occasionally to make sure you are on track).  Use the bottom of the stone for determining whether level  - this is where the eye focuses.   If installing corners, apply corners first, starting at the bottom.

How far do I space the stones apart?

You can have larger or smaller grout joints, but typical installations are ¼ to ½ inch.  Whatever your grout joint, be sure  the stone is completely sealed around the edges with at least ¼” grout exterior

What if I get mortar on the stones?

Rinse off film or residue with 1 part vinegar / 4 parts water, using a soft bristle brush, rinse repeatedly with plain water.
Remove mortar from stone face, as necessary during installation.

DO NOT ALLOW MORTAR ON STONE FACE TO SET OVERNIGHT

•  GROUTING

Where can I get a grout bag?

Grout bags are available at any hardware store. If the bag has a metal tip remove it and cut a 3/8” hole in bag otherwise it will clog

How do I grout the joints?

After the stone has been applied to the surface, fill grout bag 2/3 full with mortar.   Fold 2 corners in and roll up end. Fill around stone in a continuous motion.

Be sure to cover any noticeable broken or cut edges with mortar

•  STRIKING THE JOINTS

How do I "strike" the joints?

Use a striking tool to push mortar into any cavities or voids, creating a seal around each stone.

What is the purpose of striking the joints?

To fill all holes or gaps, to prevent the occupation of pests or water that may cause future problems.

•  BRUSHING

What can I use to brush out the joints?

A mason or horsehair brush can be used to brush the joints

A smaller paintbrush is an advantage to getting those smaller areas

What is the purpose to brushing the joints?

Brushing the joints helps to create that clean natural stone look. It will remove any tooling marks made in the mortar.

How do I clean mortar off of stones?

A mild vinegar and water solution will work; do not use just water as it will leave a milky residue on the stone.

DO NOT USE ACID WASH

•  SEALERS

Does the stone need to be sealed?

It is not necessary to seal the stone.  If sealing is preferred the sealer MUST be breathable.

What should I use to seal the stone?

Use clear non-film forming – Breathable Sealer

DO NOT APPLY MORE THAN THE CONTAINER RECOMMENDS

DO NOT USE

              - Stone Mason Products

              - Any type of non-breathable clear sealer

•  CLEANING THE STONE

What can I use to upkeep the look of my stone?

We recommend a mild solution of granular detergent and water. Soft bristle brushes and rinse with water

What shouldn't I use when cleaning the stone?

Do not use; acid, salt, or other de-icing materials or a wire brush, these will damage the stone.

•  SILL APPLICATION

How far should I overlap the flashing? 

Flashing should be 4” under the siding and ½” – ¾” over the top of the sill.

How should I set the sills?

Snap a chalk line and fasten 2 x 2 zinc coated angles. Butter sill and set on top of the angles to support. Apply stone underneath sill as normal, covering the exposed leg of the angle. Grout and finish joints.   After sills have set, caulk the joint along the back of the sill and the building.

•  HEARTHSTONE APPLICATION

How much overhang is recommended for the hearthstones? 

1/2 inch is what we recommend for both visual and strength qualities.

What do I need underneath the hearthstone?

Surface should be level. Use plywood, tar paper, metal lath and about 1/2" of mortar.

•  STONE & DIMENSIONS (approximate)

What size are the stones?

River Rock Flats                3” – 12” Diameter           1” – 3 1/4 ” Thick    
River Rock Corners           3” - 6" returns
Split Rock/ Granite             4” – 12” Diameter           1” – 11/2" Thick
Split Corners                      4" - 7" returns
Ledgestone                        4” – 10” Long                  1” Thick
                                          1 ½” – 3” High
Ledgestone Corners          4" - 8"  returns
Limestone                          4”– 14” Long                  1 ½ “- 2” Thick 
Limestone Corners            3” – 9” returns
Fieldstone                          5” – 15” Diameter          1" – 2 5/8" Thick
Fieldstone Corners            3" - 6" returns

Weatheredge                    4” – 20” Long                    1” – 2 5/8” Thick
                                         1” – 6” High
Weatheredge Corners      3"-9"

 

What size boxes do your stones come in?

100 sq. ft. boxes measure 40 x 48 x 30

10 sq. ft. boxes & 8 ln. ft. boxes measure 23 x 12 x 19

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