Waterproofing Basement During rear Addition

Installing a rear addition means taking some precautions to prevent a wet basement.

Construction worker pours gravel in a French drain beside rear addition.

A typical rear addition requires expanding a building’s footprint which usually involves excavating the ground behind the house to create a new foundation for what’s being built. This is an ideal time to install waterproofing solutions to ensure the concrete remains dry while the work is being done, and for decades afterwards.

Construction manager shows basement cinder block wall width.
Original basement wall had no waterproofing.

A recent home renovation in Burlington saw our team extend the building foundation beyond the cinder block barrier deposited by the original builders in the 1960s. The project required Eastview break and then expand the basement wall in two places.

Prior to the nineteen seventies, cinder block foundations were standard fare across Ontario, and today nearly every house that’s over fifty years old sits on these ubiquitous cement blocks. There are some builders who believe this is still the best approach, but Eastview values poured concrete walls because of their lateral strength. Reinforced concrete doesn’t easily buckle, while block wall foundations are prone to warping and even separating and repairs are costly. That being said, the cinder block wall, when built correctly, is stronger than the poured wall for compression strength, which means it can hold more weight, and that’s why many commercial buildings still employ blocks. While both foundation types can be completely waterproofed, block walls tend to have more leaks because of the grout lines. Ground water will wear down the mortar over time and cause leaks.

In our recent project, Eastview had to break the basement wall in two spots and then tunnel under part of the house which is a major exercise. We needed to be able to get under the entire property to add reinforcements to support the major overhaul happening above, but there was no room for any heavy equipment down there. Much of the early excavation was done by hand, with picks, shovels and wheelbarrows. Outside we were able to dig down more efficiently and two teams worked simultaneously to meet in the middle.

Notice in the photo below how the exterior concrete has a waterproofing membrane attached. This two-ply plastic laminate prevents water intrusion. The sheet is made of a high-density polyethylene resin that’s specially formulated to last for decades despite freezing and thawing, and it’s made to resist acids and other deteriorating ground agents. There’s an air-gap inside the materials which channels rainwater away from the cement. We’ve chosen this manufacturer because their product comes delivered in five foot rolls which can be applied in one continuous strip that leaves no seams.

Concrete foundation wall with plastic resin membrane for waterproofing.
Concrete foundation has plastic resin waterproofing membrane applied positively along circumference.

Depending on the structure and need, the waterproofing membrane can either be applied positively to the outside, as seen above, or negatively applied to the interior. A negative application is less preferable as having wet concrete is never optimal. A negative application is usually only done to remedy fractured foundations in aging houses. Positive waterproofing membranes are preferable and are applied all exterior surfaces. Positive waterproofing is a critical step in the construction of rear additions because it prevents moisture infiltration and protects both metal and cement. It also keeps the walls from experiencing the harshest freeze-thaw cycles and protects concrete from automotive fuel spills and other corrosive agents such as dog urine.

Waterproofing Basement With Dricore Subfloor

A basement subfloor is a layer of wood or plastic that’s placed below the floor covering (be it carpeting, laminate or tiling) and above the original concrete surface poured by the home-builders. Subfloors are recommended if the basement gets damp in the springtime, or is located anywhere close to water that could rise and cause dampness. We used special prefabricated tiles with plastic bottoms and channels which collect and drain water. When installed properly, the Dricore brand sub flooring can also be considered part of the home’s waterproofing solution.

Eastview employee installs Dricore sub floor particle board panels in residential basement
Eastview employee installs Dricore subfloor to battle moisture and make softer, warmer basement floors.

Waterproofing Solutions Are Built Into Basement Renovations

Dricore basement subfloor showing tread pattern
Dricore basement subfloor

Untreated basement floors are made of porous concrete which will always absorb and emit moisture. Concrete is an innately water resilient material and can withstand massive amounts of water before it breaks. When floods happen, they tend to erode the ground behind or under the concrete which causes cracks as the weight shifts. Groundwater moving at ten miles per hour can be just as strong as hurricane force winds up to 200 mph. Although the neighbourhood in question is not known for floods, Dricore panels will protect the homeowner’s expensive flooring from suffering any moisture whatsoever.

Damp basements can eventually become moldy basements and this is where the Dricore subfloor really shines. The product helps guard against moisture which can lead to mold and mildew growth in hard-to-reach places. This subfloor protects the homeowner’s surface flooring investment, regardless of whether they select engineered hardwood, luxury vinyl tile (LVT), laminate or plush carpeting. This subfloor also makes unyieldingly hard concrete feel softer on the foot. The wood adds some cushion as well as insulation, and so these subpanels make the ideal bottom when creating a cozy subterranean spaces.

One hundred and eighteen Dricore subfloor panels were installed in a day. You can see below how they’re made tongue-in-groove and fit together to make a seal. Interestingly, the manufacturer recommends they be installed as a floating floor, and it’s not advisable to nail, glue or screw Dricore panels to the concrete as this punctures the integrity and the screw holes break the seal.

cross section of DriCore basement sub floor product - particle board
Cross section of the Dricore basement subfloor panel showing the particle board’s groove receptacle.

While pouring the new concrete floor for the two sections where we expanded the basement, we added grates and drains and took some other precautions. In the north east corner we hollowed out a cylindrical pit the standard size receptacle for a sump pump. More than just a hole in the ground, you can see there’s plastic drain pipe buried here which can bring more water from other areas. A sump pump will be deployed in here, ready to drain the bilge. That hole is the sump, a specially constructed excavation which is a round depression in the basement floor. The strange word originates from the German word sumpf which was a mining term that meant ‘a deliberate swamp’. This handy basin will hold the house’s sump pump which will, in a perfect world, never be powered-on (except for testing). That’s because the pump is equipped with sensor valves which are triggered by escalating water levels and this is the last line of defense. Should rising flood water find a crack in the foundation, the water will collect here first. Ideally it’ll never be needed. If the house has adequate waterproofing, the sump should remain dry. But as the house ages, this safeguard becomes increasingly important.

Empty hole in basement needs sump pump before spring flood
The sump pump hole awaits the mechanism which will live down here safeguarding the basement.

Every homeowner has a seasonal checklist for regular maintenance, and testing the sump pump is an important element. Regular testing is important for confirming that everything is functioning properly before the snow melts and the basement has an inch of water. 

Outside the House, We Added Capacious Gutters and Downspouts

aluminum downspout subcontractor measuring the length for eves gutter trough
Subcontractor estimates the lengths required after determining the path and placement of the downspouts.

At one point, the new roof we’d just constructed still needed gutters and downspouts and we still had to repair the existing piping that was removed when the old exterior siding was demolished. This became an issue with our wet spring and our project not having proper eves troughs and drainage due to the exterior renovation happening simultaneously (as is the case with most rear additions). Gutter pipes should be replaced every twenty five years anyway and those present were long past their prime.

Labourer wheelbarrow gravel for French drains waterproofing house
Gravel trenches under downspouts are ‘French drains’.

We added slightly longer downspouts which run more diagonally to drain the water further away from the house.  The top of this conduit connects to a hole in the gutter channel and the bottom bends away to carry the rainwater away from the roof and clear of the building’s foundation.  In some properties the groundwater can return to the basement especially in situations where improper drainage makes puddles and ponds in the backyard.

How far should downspouts extend from the house? Six feet is normal but ten feet may be possible.  There are different types of curved downspouts, and special pipes can be ordered such as extensions, splash blocks, downspout drains, and buried pipes. Let’s remember, municipal bylaws generally forbid residents from venting their roof water at their property line or too close to their neighbour’s property.

In addition to everything already mentioned, we installed proper gutter guards at the roof’s edge to prevent clogged and overflowing drains up top. Although we’re not landscapers, and we don’t own heavy-duty earth moving equipment, we did make some effort to improve the grade around the house down below, and we installed French drains which are shallow trenches filled with gravel which give water an easier path away from the house.

These are just some of the precautions Eastview takes on site, and they’re helpful tips our blog readers can use to better protect their own properties and keep their own basements warm and dry all year round.

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