Case Study, Ecologic Mortar, Historic Conservation

Parging and Waterproofing the Stone Foundation of pre-civil war (1820-1840) home with NHL 5

About the House

We are the proud owners of a pre-Civil War two-story brick home in the small town of Derry, PA, about one hour east of Pittsburgh.

Although we do not yet know the exact date of construction, documents found at our local historical society date the home between 1820 and 1840. We were able to locate our home on the 1857 Westmoreland County map, so we know it was built before the Civil War began in 1861. We have also been fortunate to find several artifacts buried on our property dating back to the mid 1800’s – bricks, stoneware, glassware, buttons, horseshoes, nails, etc.

We are quite happy to own a piece of local history; however, we understand that sometimes historic homes require renovations and come with big surprises. Our surprise came in February of 2018, about 6 months after we purchased our home. I remember walking down my basement steps, turning on the light, and then stepping in about 3 inches of water. Surprise, our basement was flooded.

A Leaking Basement

The reason for our basement leaking was due to improper installation of our waterline. After excavating around the foundation, we discovered that our entire waterline was run inside of black perforated pipe (French Drainage Pipe). Due to this, a large portion of water from our front yard was flowing into that pipe and draining into our basement.

You can see from the pictures the amount of mortar loss that occurred due to this problem. After hand cleaning and pressure washing the foundation, large voids between the stones were revealed. Stones and mortar fell out as we cleaned the foundation wall. Furthermore, since the home is almost 200 years old, it does not have a typical “footer”.

The home was built on solid bedrock. We spent about a week sloping the bedrock properly so a base of mortar could be poured in effort to direct water away from the foundation and into drainage piping.

Where do we start?

When this project started, we had absolutely no idea where to begin. Several contractors recommended that we apply surface bonding/hydraulic cement to the foundation to fill the voids and stop water penetration. However, after doing some research on historic homes and mortar types, that solution did not seem correct. We consider ourselves quite lucky to have found Mr. Andy deGruchy (Limeworks.us Owner) and his team at Limeworks.us.

It is amazing what a simple Google search with the phrase “Lime Mortar + Pennsylvania” can locate. I started speaking with Mr. deGruchy in April 2019. I provided him with the details of our project, and he provided me with solutions…immediately. Saint-Astier Natural Hydraulic Lime (NHL 5) was the recommended material to restore our foundation.

Step 1

The Pointing Mixture

Mr. deGruchy personally took time to respond every email and answer all my questions. He highlighted the tools I should use for the job and provided me with 3 specific mortar mixtures to be used in restoring our foundation. They were as follows:

1. Pointing Mixture: NHL 5 (1 Part), Concrete Sand (2 Parts)


2. Level Coat/Brown Coat Parging Mixture: Binder Blend - Mix 80% NHL 5 & 20% White Portland Cement Type I (1 Part), Concrete Sand (2-1/4 Parts)


3. Finish Coat Parging Mixture: Binder Blend - Mix 80% NHL 5 & 20% White Portland Cement Type I (1 Part), Concrete Sand (1-1/2 Parts)

First, we used this mixture to make a smooth base over the bedrock so that we could lay our drainage pipe. Next, we used the Pointing Mixture to fill the voids between the last course of stone and the bedrock. Lastly, we pointed the entire stone foundation. Our foundation had a great deal of mortar loss and deep voids, so we had to replace/add several stones during the restoration. We pointed the foundation in “lifts”, gradually building up layers until the foundation was ready for parging. We used tuck pointers, furring strips, and our hands to pack the mortar between the stones. We waited 10 days between “lifts” and then another 10 days before parging with the Level Coat/Brown Coat Mixture.

Dug out foundation

repointing stone foundation prep for parging

NHL 5 Base over bedrock

repointing stone foundation prep for parging

Repointed Foundation

repointing stone foundation prep for parging

Deep Voids Filled with NHL 5

repointing stone foundation prep for parging

Step 2

Level Coat/Brown Coat Parging Mixture

We used the Level Coat/Brown Coat Parging Mixture to make our wall flat. We did this by securing furring strips (vertically) to the foundation wall every 2 feet with masonry screws. Once the furring strips were plumb and secure, we applied the Level Coat/Brown Coat Parging Mixture in 1-inch lifts between the furring strips, allowing 1 day to dry before adding another coat. We finished each layer with a masonry brush, so it was easier for the subsequent coats to stick. ***Please note that we were able to apply the Level Coat/Brown Coat Parging Mixture daily because the mixture contained White Portland Cement.***

Framing

Parging and framing of foundation

Parging Coat Lift One

Parging Foundation

Parging Coat Lift Two

Parging Foundation

Parging Coat Lift Two

Parging Foundation

Step 3

Finish Coat Parging Mixture: Binder Blend

The last layer of mortar we applied was the Finish Coat Parging Mixture. This coat was approximately 3/4” to 1” thick. We built this final coat out to the front of the vertical furring strips and finished it smooth with a trowel. We allowed the Finish Coat Parging Mixture to dry for about two weeks.

Finish Coat over framing

Parging lime finish coat

Why we used White Portland Cement in our Parging Mixture(s)

A major component of our foundation restoration project was using White Portland Cement to create a “Binder Blend” to parge the exterior foundation walls below grade. The “Binder Blend” consisted of 80% NHL 5 and 20% White Portland Cement. To this “Binder Blend”, we added 2-1/4 parts Concrete Sand (for the Level Coat/Brown Coat) and 1-1/2 parts Concrete Sand (for the Finish Coat).

Although sometimes viewed as damaging to historic masonry structures by preservationists, White Portland Cement was needed to properly restore and waterproof our historic foundation. When used in conjunction with modern waterproofing materials, (Tar, Peel & Stick Membranes, Dimple Board, etc.) parging mixtures like this that are rich in “Binder Blend” are useful for below grade applications because they discourage water infiltration. Furthermore, installing a proper drainage system can help deter water from penetrating the foundation walls, thus keeping moisture levels under control and water from entering the basement.

Finishing Touches for Parging

After the Finish Coat Parging Mixture was dry, we completed the tedious task of removing all the furring strips. When removing the furring strips, some mortar chipped away from the foundation wall. Mr. deGruchy told us that this would occur and could be easily patched when filling in the voids left by the furring strips. We filled in the voids with small stones and the Finish Coat Parging Mixture. It took about 3 or 4 “lifts” to completely fill the voids left by the furring strips. We then allowed the entire foundation to dry for nearly two weeks.

Prior to applying our waterproofing materials, Mr. deGruchy advised us to check the mortar with a moisture meter. The mortar’s moisture content had to read between 15-18% before the waterproofing materials could be applied. We tested our mortar in several places along the foundation wall and our readings registered between 9-14%. We then applied our waterproofing materials.

We used a combination of a peel and stick membrane, rubberized foundation tar, dimple board, and French Drains to waterproof our exterior foundation walls below grade. This will ultimately ensure that the historic structure above the surface is in the “the best state of conservation”.

Primer

Primer for foundation

Tar, Peel, and Stick Membranes

Tar, Peel, and Stick Membranes

Dimple Board

Dimple Board

NHL 5 - Flexibility, Breathability, and Permeability

These parging mixtures are also an excellent choice as a below grade exterior application because they combine the flexibility, breathability, and permeability of the NHL 5 with the strength of modern White Portland Cement.

The goal of our project was to decrease moisture levels, terminate water infiltration through our foundation walls, and fix voids to eliminate pest infestations. In addition to completing the exterior waterproofing, we also stripped the interior foundation walls of a previous rubber/latex waterproof coating. Coating interior historic stone basement walls with a modern rubber/latex waterproofing agent does not allow the foundation walls to breathe and release water vapor.

Should water/moisture enter the foundation wall from the outside, and the interior wall contains a latex/rubber waterproof coating, the water can become trapped and move upwards through the wall, rotting joist ends and framing members. For this reason, we removed all the waterproof coatings from our interior foundation walls prior to restoring and waterproofing our exterior foundation. NHL 3.5 (possibly mixed with Borax to inhibit mold growth) would be the appropriate parge coating to add to the interior foundation walls as opposed to a modern rubber/latex waterproofing agent or a parging coat consisting solely of Portland Cement.

Working with NHL 5

It is quite important to ensure the mortar is mixed thoroughly. We always dry mixed the materials in a cement mixer for about 15 minutes to make a consistent blend. We then added water slowly as the mixer was rotating, being careful not to add more than necessary. We allowed the mortar to mix with the water for about 20 minutes. We then let the mortar sit for about 15-30 minutes to “fatten up”. Mr. deGruchy stressed that “better mixes are drier mixes packed in” and that “water is a tool” used to help the mortar adhere to the stone.

If you attempt to point without misting the stone (especially on a hot day), the mortar can dry too quickly, crack, and fail to adhere. Furthermore, misting the mortar with water is critical to encourage a “slow cure” and deter cracking. Sometimes we misted our mortar 3 times per day when the temperature was very hot. We used burlap to cover the mortar to protect it from the elements (sometimes misting the burlap as well). One of the great properties of the NHL 5 is its unique rework ability. After pointing/parging, we were able to mist and then rework the mortar the following day to repair hairline cracks, small voids, etc.

Keeping your structure "High and Dry"

Mr. de Gruchy explained the rationale for using the parging mixture containing White Portland Cement in the following way: If moving a historic brick/stone structure, it would be ideal to place it atop a new block or poured-in-place foundation that is properly waterproofed and equipped with correct drainage. This keeps the historic structure “high and dry”. Since we are not moving our home atop a new foundation, we used the parging mixtures containing White Portland Cement combined with modern waterproofing materials to protect our below grade exterior foundation and keep our home dry above the surface. This should protect the integrity of our home for the foreseeable future.

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