Can You Waterproof a Fieldstone Foundation?

Yes, you can waterproof a fieldstone foundation, but the approach is different from sealing a modern concrete wall. The National Park Service's guidance on historic masonry favors moving water away from the stone and collecting what gets through, not coating the wall. NPS Preservation Brief 39 states that moisture "should not be sealed into the wall with a waterproof coating, such as cement parging or vinyl wall coverings, applied to the inside of damp walls."

Repointing follows a rule that runs against instinct. NPS Preservation Brief 2, the federal guide to repointing, says new mortar "must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units," and as soft or softer than the historic mortar. The brief explains that a mortar harder than the stone "will not 'give,'" so stress gets relieved through the masonry itself, "resulting in permanent damage." This post covers why stone walls leak, why they need to dry out, how to repoint them, what drainage works, and what to avoid.

What is a fieldstone foundation, and why does it leak?

A fieldstone foundation is a basement wall built from natural, mostly uncut stones set in mortar, sometimes called a rubble stone wall. HUD's Rehab Guide on foundations (1997) noted that foundations of older houses "built more than 60 years ago, particularly those in rural areas, are often made of stone."

Most of these walls were laid with lime mortar. NPS Preservation Brief 2 says mortar of mostly lime and sand was used for thousands of years, that Portland cement was not in common use across the U.S. until the early 20th century, and that by the 1930s most masons used equal parts Portland cement and lime putty.

Water gets in the same way it reaches any basement, just with more paths. Preservation Brief 39 explains that when soil is saturated at the base of a building, moisture "will wet footings and crawl spaces or find its way through cracks in foundation walls and enter into basements." In a stone wall, those paths are the mortar joints. Brief 2 notes that in traditional masonry, moisture "was able to migrate through the mortar joints rather than the masonry units." For a wider look at age-related leaks, see what makes older Pittsburgh homes leak more.

Why does a stone and lime mortar wall need to breathe?

The mortar in an old stone wall is meant to be the weak link. Preservation Brief 2 describes historic mortar as a bedding material, "not unlike an expansion joint," rather than a glue, and calls mortar "sacrificial," protecting the masonry around it.

That design only works if water can leave. According to Brief 2, "if the mortar does not permit moisture or moisture vapor to migrate out of the wall and evaporate, the result will be damage to the masonry units." It adds that high lime mortars are more permeable than denser cement mortars.

Sealing the inside face causes its own trouble. Preservation Brief 39 says capillary action lets moisture in saturated masonry "wick up or travel vertically," and that a waterproof coating on the inside of a damp wall "will only increase the pressure differential" and force that moisture, and its damage, higher up the wall.

How do you waterproof a stone foundation without trapping moisture?

Stone foundation waterproofing works best as a sequence: reduce the water outside, repair the joints, then collect what still comes through.

Start with grading and downspouts

Preservation Brief 39 says the ground and building stay much drier by redirecting rain away through sloping grades and carrying downspout water well away from the building. Its checklist suggests aiming for 6 inches of fall in the first 10 feet around the foundation. The U.S. Department of Energy's Building America Solution Center gives a similar target of at least 0.5 inch per foot for the first 10 feet, with downspouts discharging at least 5 feet away, or underground at least 10 feet away.

Brief 39 also sets the order. It says excavating foundations and adding dampproof coatings and footing drains "should only be used after" those ground moisture measures are in place.

Collect water on the inside

Where water still enters, Brief 39's checklist lists adding "interior perimeter drains and sump pump." HUD's Rehab Guide describes a perimeter drain "cut into the juncture of the wall and the floor" and notes that a sump pump lowers the groundwater level to a point below the basement slab. The drain sits at the base of the wall, so the stone face stays uncoated. Our guide to how you waterproof a basement from the inside walks through the process, and interior vs exterior basement waterproofing compares the two.

Manage humidity with care

Brief 39 suggests dehumidifiers for seasonal humidity control in closed, unventilated spaces, with a caution not to "create a problem with pulling dampness out of walls."

How should a fieldstone foundation be repointed?

Fix the water source first. Preservation Brief 2 calls it "erroneous to assume that repointing alone will solve deficiencies that result from other problems," and warns that without repairs to the source, "any repointing will have been a waste of time and money."

When the joints are ready for work, Brief 2 gives specific guidance:

• Remove old mortar to at least 2 to 2.5 times the joint width. For stone masonry with wide joints, it may need to come out several inches deep.

• Match the new mortar to unweathered samples of the historic mortar. A lab analysis can help, and the brief says its most useful result is identifying the sand by gradation and color.

• Use a mix that is softer and more permeable than the stone and no harder than the original mortar.

Brief 2 also says surface grouting, smearing thin mortar over joints, "cannot substitute for the more extensive work of repointing."

What should you avoid on a fieldstone basement wall?

Several common fixes work against a stone wall.

Paint-on sealers are the big one. NPS Preservation Brief 1 explains that waterproof coatings "seal the surface from liquid water and from water vapor," and that water-repellent coatings, though vapor permeable, can still hold liquid water in when vapor condenses inside the wall. The brief is blunt: "A water-repellent coating should never be applied to a damp wall." It also says a water-repellent coating will not solve rising damp and "may be likely to exacerbate it."

Freezing adds to the risk. Brief 1 says water that enters a wall from the ground can freeze in cold weather and cause spalling. At Pittsburgh International Airport, NOAA's 1991 to 2020 normals list a January mean temperature of 28.8 F and 44.1 inches of snow a year.

Also avoid:

• Hard Portland cement repointing mortar, which Brief 2 links to permanent damage in softer masonry

• Cement parging or vinyl wall coverings over the inside of damp walls, per Brief 39

• Treating the wall surface when the cause is outside. Brief 1 says water entering masonry buildings is "seldom due to porous masonry" and usually traces to leaking roofs, clogged gutters and downspouts, missing mortar, or open joints

Stone walls can also bulge or shift over time. If yours leans, read about what causes bowed basement walls in old homes before choosing a water fix.

What does it cost to waterproof a stone foundation?

The cost to waterproof a stone foundation depends on how much work the wall needs. The main factors are:

• Whether grading and downspout changes solve most of the problem

• How many joints need raking and repointing, and how deep

• Whether an interior perimeter drain and sump pump are needed, and how many feet of wall they must cover

• Whether exterior excavation is required, and what is in the way, such as porches, driveways or landscaping

• Whether the wall has structural movement that must be addressed first

An on-site inspection is the only reliable way to price a specific basement. See our basement waterproofing page for the wet basement warning signs we look for, and basement waterproofing for older homes for more on the challenges older foundations bring.

FAQ

Can you finish a fieldstone basement?

Only after the water is under control, and with care. NPS Preservation Brief 39 warns against sealing damp walls with coverings like vinyl, which trap moisture in the masonry.

What kind of mortar should be used on a stone foundation?

NPS Preservation Brief 2 says repointing mortar should be softer and more vapor-permeable than the stone and no harder than the historic mortar. High lime mortars are more permeable than dense cement mortars. A mason experienced with historic masonry can match the mix.

Can I paint a stone basement wall with waterproofing paint?

NPS guidance advises against it on damp walls. Preservation Brief 1 says waterproof coatings block water vapor and that water-repellent coatings should never go on a damp wall.

Is interior or exterior waterproofing better for a fieldstone foundation?

Preservation Brief 39 puts grading and downspout fixes first, and says excavation, damp-proof coatings, and footing drains should come only after those steps. Interior perimeter drains with a sump pump are also on its list of options.

Should I run a dehumidifier in a stone basement?

Brief 39 supports seasonal dehumidifiers in closed, unventilated spaces. It also cautions against a setup that pulls dampness out of the walls.

Is a fieldstone foundation a bad foundation?

Not by itself. HUD notes stone was a common foundation for older houses. How well one holds up depends on the condition of its mortar, the drainage around it, and whether past repairs used compatible materials.

Stone Foundation Help From A Better Choice, Inc.

A Better Choice, Inc. has waterproofed basements across the Greater Pittsburgh area since 1996, serving over 26,000 satisfied customers in Westmoreland, Allegheny, Butler, and neighboring counties. If your older home has a fieldstone or rubble stone basement, our family- and veteran-owned team offers a free inspection to find where water is getting in and explain options that suit the wall.

Visit A Better Choice, Inc. or call 1-800-343-2357 today. Contact us for more information.