Details for Below-Grade Living Space
Many people live with damp or moldy basements and assume that’s just the way it is. Some basements flood every time heavy rain or melting snow saturates the soil and exceeds the capacity of the drainage system or sump pump. Over 60% of basements have moisture problems or flooding, by some industry estimates.
Keeping moisture out of your basement is not difficult in new construction, but requires multiple strategies. You only have one chance to build a dry basement economically – during construction – so it’s worth doing right the first time. Preventing basement leakage and high moisture levels requires a combined strategy of
- Drainage of rain water away from the foundation
- Granular backfill or drainage board around the foundation
- Footing drains
- Capillary breaks between foundation and soil
- Coatings or membranes to block liquid water and water vapor
- Insulation for comfort and condensation control
All six components are especially important if you are building a full basement. However, even with a slab-on-grade or crawlspace, you will want to keep water away from the foundation area. Wet soil around and under any type of foundation will result in higher indoor humidity, greater risk of insect problems, and deterioration of most building materials.
Once inside the building, water vapor condenses on cool concrete surfaces, often leading to mold and mildew on anything stored in the basement. If you later finish all or part of a basement without addressing the moisture problems, the carpeting and furnishings will grow musty and moldy as well, making the living space less than “prime”.
In addition, basement moisture can cause moisture problems in other parts of the house – even the attic or roof – as basement moisture travel up through the walls and chases due to the wintertime stack effect. In the worst cases, unmanaged water below a foundation can erode soils and undermine footings.
Water in the Soil
Water can enter your basement two ways: as liquid water or as a gas, called water vapor. Both are important and can cause a variety of problems if ignored by builders.
Water vapor. Except in arid climates, there is a lot of water vapor in the soil, whether it is wet or dry, dense or granular. In fact, if you measure the relative humidity of the soil in most areas, it is usually close to 100%. Since water vapor travels easily though concrete, block, and other masonry materials, you need to protect the foundation with either a coating or membrane that will stop, or at least reduce, the flow of water vapor.
Liquid water. Where foundation drainage is inadequate or the water table too high, liquid water can build up around the foundation and exert hydrostatic pressure, forcing itself through any small opening and into the concrete itself. This often results in true flooding of the basement.
Even without hydrostatic pressure, however, water from soil in contact with concrete will soak into the concrete by capillary action. Both the soil and concrete act like sponges, soaking up and transporting water toward the interior of the building. In England, they call this “rising damp.” Place a stalk of celery in red-dyed water to get the idea.
You can see the effects of moisture transmission in the white “efflorescence” often deposited on the basement floors or walls. Moisture passing through the concrete dissolves salts on the way through and carries them to the surface. When the water evaporates into the room, the salt deposits are left behind.
To stop the flow of water under hydrostatic pressure is extremely difficult and requires a basement built like a boat – and even boats have bilge pumps. True waterproofing membranes help, but the best approach is to provide effective drainage that eliminates hydrostatic pressure against the foundation walls or floor.
To stop the capillary transmission of water from soil to concrete requires a capillary break. This means a physical gap between the water or wet soil and the foundation wall. This can be a waterproof membrane material, a layer of granular fill (gravel or crushed stone), or a manufactured drainage membrane that goes against the basement wall.
Waterproofing vs. Damproofing
Concrete may be hard as rock and unaffected by moisture, but it readily absorbs liquid water and is porous to water vapor. This makes it a great conduit for transporting moisture into your home.
Even with good management of surface water with gutters and proper grading, there will always be high levels of moisture in the soil. If unchecked, the water vapor will migrate into your basement year ’round through the slab and basement walls.
The black coating that you see to nearly all foundation walls is called dampproofing. Dampproofing is a thin layer of asphalt applied to reduce the transmission of water vapor through concrete or block, both of which are very permeable to water vapor.
Dampproofing is not intended to stop liquid water and does not bridge the small shrinkage cracks that occur in every concrete wall. For that level of protection you need a full waterproofing system. At a glance, these often look the same.
However, a waterproof coating is much thicker and is flexible enough to bridge small gaps. If it works as planned, it will even stop water that has accumulated around the base of your foundation, exerting hydrostatic pressure against the walls. Read more on Basement Waterproofing and Dampproofing.
Drains around the foundation perimeter should be an essential part of every basement waterproofing system. The best product to use for footing drains is rigid PVC drain pipe with small holes drilled in two or three rows. I prefer Schedule 40 pipe – the same thing used for plumbing drains – as it is pretty indestructible. SDR30 is a little thinner and less expensive, but can be used successfully if workers are careful when backfilling. Remember, the holes face downward when installed! That way, rising water flows into the pipe and away from the footings.
The perforated piping around the perimeter of the foundation may be called footing drains, perimeter drains, or drain tile – referring to the clay pipes used in the old days.
Rigid vs. Flexible Pipe. Rigid PVC drain pipe is strong enough to span small irregularities in the gravel bed and withstand a moderate amount of abuse during backfilling. If it ever clogs up, it can be cleaned out with a motorized plumber’s snake. The downside of PVC is the higher cost of materials and installation since joints need to be glued.
The alternative is corrugated pipe made from high-density polyethylene HDPE with slots for drainage. Because it is flexible, corrugated pipe is easier to install and requires fewer joints. However, it is also easy to crush while backfilling if workers are not careful. In addition, corrugated pipe tends to slow down water flow and trap debris because of its wavy surface and tendency to follow the ups and downs of the gravel bed. It may also end up with low points that trap water.
There are also smooth-wall versions of HDPE pipe now sold as double-wall and triple-wall drain pipe. These have better flow characteristics than corrugated product, but are still less durable than rigid PVC and cannot be power augured. Given the cost to re-excavate and fix a crushed or blocked pipe at a later date – not to mention the potential to flood the basement – why not use the good stuff the first time?
Slope of pipe. Footings are level so it is tricky to install footing drains with much slope to the outlet. Also, it’s difficult to do the equivalent of finish carpentry in an excavation trench. While a slope of about 1/8” per foot to the drain is ideal, a level footing drain will also get the job done. Just make sure the outlet pipe is solid (no perforations) and sloped a minimum of 1/8 in. per foot to the drainage point.
Installation. While some contractors lay the perforated drain pipe on top of the footing, a convenient spot to be sure, it provides little margin for error if the system is loaded to capacity. The drain pipe should be placed low enough that it keeps water away from the top of the footing and well below the slab. In the typical residential foundation, placing the drain a couple of inches below the top of the footing (and at least 6” below the top of the slab) is the optimal location. Digging too deep for the footing drains can undermine the soil under the footing.
To keep the drain pipe from filling with sediment, it must be set in a layer of clean coarse gravel or crushed stone that is fully wrapped in a layer of heavy-duty filter fabric on all sides. After rolling out the filter fabric starting at the top of the footing, spread 3 to 6 in. of stone as a base for the pipe. After laying the pipe, fill the trench to at least 6 in. above the top of the pipe. Then wrap the filter over the top of the bed, lapping over itself to make a complete seal.
If you plan to backfill with native soil, it’s a good idea to place a minimum of 6 in. of course sand over the top of filter fabric to keep it from clogging prematurely.
Cleanout. Despite your best efforts, these systems may clog up over time. In that case, it’s very useful to have a cleanout for a power auger. As long as the pipe is not physically damaged, a drain cleaning service should be able to remove any blockage and flush the system
Discharge. Unless you have a high water table, the footing drains shouldn’t get a lot of usage most of the year. They are really meant as a back-up system for surface water that finds its way to the base of the foundation. Still, the water needs somewhere to go, away from the foundation, when it does collect. Always use solid (non-perforated) pipe from the footing drains to the outlet point.
Drain to daylight. The best option on a sloped site is to drain to daylight by gravity. This has no moving parts and is easy to clean and maintain. The slope should be minimum 1/8 in. per foot, with hardware cloth at the open end to keep out critters. Another option is to bury the end in gravel or crushed stone. This will keep out animals and allow the water to slowly disperse and percolate into the soil.
Drain to sump pump. Most people who cannot drain to daylight, drain to a sump pump in the basement. The sump is a submersible pump that sits in a basin surrounded by stone below the basement floor. It’s important that the pump has enough capacity to meet the load. Tie the footing drains to the sump pit with a solid PVC pipe cast in place through the footings.
Drain to drywells. With well-drained soil, you can drain to one or two large drywells. Keep these far enough away from the house (min. 10 ft.) that you do not return the water back to where it started. Since the performance of drywells is difficult to predict, it would be wise to connect the footing drains to a sump pit as a backup. The pipe can be capped until needed to prevent bringing unwanted water into the basement.
The general recommendation is to backfill with granular material for at least the first 16 inches next to the foundation wall. The purpose is to keep water away from the wall where it might leak through a crack, and to provide a capillary break. Any water that gets this far should drain freely to the footing drains.
With a full waterproofing system protected by a dimpled membrane or drainage board, it is generally not be necessary to use a granular fill, but you should follow the recommendations of the waterproofing contractor. Climate conditions, soil types, and the topography of the site might call for extra measures to keep water away from the foundation.
The backfill should be compacted in small lifts. This will prevent excessive setting which can cause negative grades around the foundation as well as sinking exterior stairs, driveways, and patios. Many contractors do not like to compact the backfill, claiming that it is unnecessary or could harm the foundation.
It can cause harm if done too soon or heavy equipment is brought too close to the foundation. To prevent problems, backfilling should wait until the first floor is framed and sheathed (or adequately braced) to resist lateral loads from the backfill, soil compaction, and equipment loads.
It is important to slope the backfill away from the foundation for about 10 ft. to move water quickly away from the foundation. Read more on Drainage Around the Foundation.
Subslab Moisture Control
The pore spaces in the soil around your home are filled with water vapor (or liquid water if the soil is saturated). Since air trapped in the soil typically measures 100% relative humidity, the moisture will readily pass through the slab into your home unless a vapor barrier is installed. If the concrete is placed directly on the soil, water will also wick into the concrete slab by capillary action.
The address these problems, a minimum 4 in. layer of coarse gravel or compacted crushed stone below the basement floor has become the standard practice in modern construction and required by code.
Subslab vapor barrier. The use of a polyethylene vapor barrier below the slab has also become common in recent years, although some concrete contractors still resist this. Adding the vapor barrier below the slab complicates their work because it slows down the setting of the slab and has been blamed, in some cases, for curling of slab edges.
In response, some contractors like to place a few inches of sand or gravel over the polyurethane to speed up drying, but this tends to trap water and cause other problems. In most cases, however, placing the concrete directly over the plastic is becoming the norm and concrete contractors have adjusted their workflow and concrete mixes to make this work.
A subslab vapor barrier is now a requirement of many glue-down floor coverings since some cannot tolerate high moisture levels in the slab. Whether required or not, nearly all basement floor coverings will benefit from the drier floor the drier substrate provided by a vapor barrier.
It’s best to use heavy plastic sheeting at least 10-mil thick unless you are using one of the reinforced plastics approved for this application. It is easy to damage a thin layer of standard poly while prepping for and placing the concrete.
Subslab insulation. If you ever intend to finish the basement, or wish to store materials there without concerns about mold and musty odors, you should strongly consider adding a layer of foam insulation below the slab. Energy savings may be modest, but the insulation will warm the floor enough to prevent condensation and moisture problems when warm, moist air contacts the cool concrete surface. Read more on Basement Insulation.
Many contractors build a sump pit in every basement as a precautionary measure (as well as a stub through the basement floor for a radon system if it’s ever needed). The cost to do these things during construction is minimal; not so when you have to drill or cut through concrete later.
If the site is relatively dry and well-drained, you may never need to install a pump. If it is very wet or has a high seasonal water table, a full basement may be a poor choice. If it’s somewhere in the middle, you may want to install a sump pit and pump as a precaution.
Sump basin. Make sure the pump you buy has sufficient capacity and that the basin is large enough to fit a submersible pump and provide space to store water as it is pumped. The typical basin is 18 in. in diameter by 24 in. deep, which holds over 20 gallons of water. If the sump is too deep, the bottom may fill with water during the rainy season and you will be pumping day and night for no good reason.
Pump quality. Don’t skimp on the pump. Cheap pumps are prone to fail when you need them most. The best pumps are cast-iron, rather than plastic. No-screen designs that can handle smalls solids will help prevent clogs as it is hard to keep all dirt and debris out of your system. Finally, look for pumps with a mechanical float switch designed to prevent jams.
Backup pump. If your pump runs frequently, check to make sure that your gutters, site drainage, hardscapes, and other features around the house are directing water away – not towards – your foundation. If everything checks out, you may have a high water table that gets higher seasonally or after a heavy rain. In that case, you should consider a battery-powered backup pump to kick in if the power fails during a big storm. That’s exactly when you don’t want your pump to fail. Make sure you get the right type of battery and charger to provide at least two days of continuous use.
Discharge. It is critical that the sump discharges the water far enough away from the house that it does not end up back where it started. The discharge point should be at least 10 ft. away, with a splash block, assuming that the site has a modest slope away from the foundation. On a dead flat site, you will want to go farther.
Often you can discharge near a swale or storm sewer designed to handle this type of load. In some areas you can tie directly to a storm sewer with a permit. Local codes govern how far your discharge point needs to be from property lines and roads. You don’t want to cause grief for neighbors.
The end of the pipe should be protected from critters by hardware cloth or terminate in a small pit of gravel or crushed stone – a mini-drywell. You can also get creative and put the water to good use in a raingarden or collect it in a barrel for gardening.
Testing. At least once a year, before the rainy season hits, dump a 5-gallon bucket of water in your sump pit to see what happens. If it quickly pumps the water away, you’re good to go. If not, get it working before you need it.