How Much Basement Wall Bowing Is Acceptable?
how much basement wall bowing is acceptable
bowing walls are basement or retaining walls that curve inward.
If we look at a cross-section of a wall and drop a plumb line from the top of the wall to its base, the plumb line shouldn’t fall outside the wall’s center of gravity at the base. (The wall’s center of gravity is the middle third of the wall’s thickness.) With bowing walls, the plumb line will fall outside the wall’s center of gravity. In other words, it will fall outside the middle third of the wall.
Two inches is considered minor deflection. From 2 to 4 inches is moderate deflection; this can be corrected with tie-backs, wall anchors, or steel beams. Walls that deflect 6 inches or more are severely bowed and often must be replaced.
As a rule of thumb, basement walls that are bowing in can be reinforced to prevent further movement. Note: up to 2″ of inward bowing or less is required, anything beyond that and your wall is no longer structurally sound and should be replaced.
As a general rule of thumb, when a wall is bowing or tilting inward more than 50% of its thickness – For example, an 8” thick wall bowing in more than 4” – the wall almost always needs to be demolished and replaced. Therefore, if you see a crack, or any movement whatsoever in a wall – no matter how slight – you should contact a foundation repair professional immediately for an inspection. In other words, you should never leave a wall unrepaired to the point where it ends up bowed. Any cracks or inward movement of a wall are not OK. The sooner you catch the problem, the less expensive it will be to fix.
Reasons Why Some Basement Wall Bowing Is Within Tolerance Range
Hydrostatic overloading is a common cause of wall failure, but it is not the only cause and we have to analyze the functions of a basement wall and the distress that it can withstand as per design. For example, for a ten high wall, free groundwater will increase the loads on the wall by a range between twenty five and thirty percent.
A basement wall is a restrained wall. That means that the top of the wall should not move at all. If it does, it will shift whatever is sitting on top of it.
The typical retaining wall that you see in any real estate object is an unrestrained wall, it is designed to tilt slightly to allow the soil to carry some of the load and this can be within tolerance range and in this case, it can be considered a safe foundation.
This is where the 2-inch deformation rule of thumb comes from. A typical description of the distress suffered by a bowing basement wall is acceptable and consistent with an unrestrained wall acting as a restrained wall. The normal soil loads on the wall caused it to tilt, but the house framing prevents the top of the wall from shifting.
The deformation that translates into an inward curve is permanent. It will not ‘spring back’ once the water table lowers, or the hydrostatic pressure disappears.
The problem with unrestrained walls and clay (so I mean, soil composed mainly by clay) is that initially, the wall tilts, and the soils carry some of the load, as it is supposed to by design. Over time, many years, the clay soil creeps towards the wall, increasing the soil loads on the wall. The wall then tilts in response to the higher loads, as it is supposed to as per design, and the entire process begins again.
Over a long time, that could be thirty to fifty years, this process can cause a wall to tilt 1 to 2 feet.
Before you do repairs, you need to determine if the wall tilting was caused by hydrostatic loading or hydrostatic pressure, or an underdesigned wall.
If it is hydrostatic pressure, it would have occurred quickly in response to very heavy rainfall. This happens too in crawlspaces, they can get flooded, but only after a heavy rain, instead of standing puddles of water. And in both cases, the treatment is different.
Is the basement wet? Are there indications of water flowing through the wall, was there a very wet winter, is there a roof downspout just above the damaged section of the wall, the bowing wall? Has the bowing basement wall process been going on for several years?
Is there reinforcing steel within the wall? Something anchoring or underpinning the wall such as an anchor, a helical tieback, or a Powerbrace treatment?
If not, then exterior reinforcement will be of little help as there is nothing for them to support. The best thing to do is rebuild the wall as a restrained wall.
How much basement wall bowing is acceptable? Well, 2″ is the industry consensus considering also the functions of the basement wall as explained briefly above.
When you should straighten your bowing walls
- Straightening poured concrete walls
The strength of poured concrete walls lies in the fact that they consist of a single continuous block as opposed to many small individual pieces. They are therefore much stronger than concrete block foundation walls. However, they too succumb to the horizontal pressure from surrounding soil.
The most common signs that these walls are bowing are severe cracks especially around the corners of the walls. Cracking beams are also a sign of failure.
As a rule of thumb, you should consider having your walls straightened if the wall bulges into 30% of its thickness. E.g. if the wall is 10 inches thick, the walls should be straightened if it bulges in more than 3 inches. If it bulges in less than this, the wall can be reinforced.
- Straightening concrete block walls
Concrete block walls consist of small blocks that are joined together with mortar. Because these walls are not as strong as poured concrete walls, they need to be straightened much sooner. Signs of weakness in the wall include horizontal cracks along the mortar joints.
As a rule, concrete blocks should be straightened if the bowing is more than 2 inches inward. The wall should also be straightened if the blocks in the wall shatter or if a block in the wall shifts more than 1 inch laterally.
There are several ways to fix a wall before it collapses. The best method for your home will depend on how extensive the damage is. If a wall is only cracked, or bowing less than 2 inches, carbon fiber straps are the ideal solutions. If the wall is bowing or bulging more significantly, you’ll want to review wall anchors and helical tiebacks.
Tools For Measurement
drop a plumb line near the most-bulged area (usually the center) of the wall, perhaps fastening it to a nail in a floor joist overhead, about 4″ in from the wall. Measure from the string in to the wall at various heights up the wall.
You’ll be able to easily pinpoint the height of the most bulge or lean. This is not engineering. It’s simple a simple mason’s method to measure a wall or chimney during construction to keep it plumb.
You may need to hire the services of a licensed professional engineer, [make certain that your engineer is experienced with foundation troubleshooting and repair], especially if there is need to design a special building repair method or if there is apparent risk of possible building instability or collapse.
- Wall most-bulged in near the outdoor ground surface (commonly occurs in the upper 1/2 of the wall), perhaps at a depth equal to the frost line in climates where freezing occurs or in the top 1/3 of the wall if we suspect water or frost loading on the wall, or possibly vehicle traffic driving too close to the wall.
- Wall most-bulged in at its center height – the center of the overall height of the wall (common) – we suspect vehicle traffic or possibly water/earth loading
- Wall most-bulged in near its bottom (unusual) – we suspect earth loading or wet earth loading.
Some basement walls can be straightened, but only if there are open cracks that have never been filled in. Continuing the thought of straightening bowing basement walls, to do it properly you must excavate the soil away on the outside first to relieve the pressure allowing the wall to go back to its best position. Walls that have the cracks caulked or filled in will not straighten up. The filler binds the joint and prevents the crack that opened from closing back up which is what straightening the wall is actually doing.
Step By Step Measurement
- A plumb line, is simply a string suspended by a weight, so as to give a perfectly vertical line from which to measure back to the wall surface. We drive a nail into the side of a floor joist or into the sub floor overhead, in the ceiling over the room where we’re working, typically a basement or crawl space.
We guess by observation which portion of the foundation wall has leaned or bulged the most, and we put our nail (to suspend the string) in front of and less than a foot away from that part of the wall. (See our illustration above.)
Quiesce the plumb line, that is, make sure that it has stopped swinging or moving, before making your measurements of the foundation movement.
- A tape measure or yardstick or ruler is used to measure the distance from the face of the wall to the closest surface of the vertical plumb line.
We measure at multiple locations from the floor up to the top of the wall, writing them down in order and perhaps even making a pencil number on the wall too, so that we can identify just which part of the wall is pushed inwards or leans over the most.
- We subtract the individual measurement amounts from our “home base” starting measurement, at a point on the wall where there has been zero movement, usually at the very bottom of the wall. We don’t care about the absolute value of the various measurements, we care about the difference between these measurements.
- Our base reference point for comparing measurements is normally the bottom of the wall. Usually the very bottom of a building wall will not have moved inwards, particularly if a concrete floor has been poured against the foundation.
The entire building floor slab is acting as an “anchor” to hold the bottom of the foundation wall in place. So we take the distance between the foundation wall and the string at the bottom of the wall as our “home base” or point of assumed “zero movement”.
We compare this measured value with the other measurements between the wall and the string.
If the foundation wall or any part of it higher than the level of the floor has moved, tipped, or bulged inwards, those measurements from wall-to-string will be less than the distance, wall-to-string measured just above the floor level. That’s because the wall has moved inwards, towards the string.
Practical Example Measuring If The Bowing Of The basement Wall Is Acceptable
- We “eyeball” the “bulged” foundation wall and guess at the point at which it is bulged inwards the most – perhaps close to the center of the length of the wall (right-to-left dimension)
- We hang our string or plumb line from the nearest floor joist, keeping the string a few inches away from the foundation wall
- We measure 4.00 inches between the foundation wall surface to our vertical plumb-line string at 1″ above the concrete floor – this is our “zero point” or “home base” measurement
- We measure 2.00 inches from the same foundation wall surface to our vertical string at a height of 5′ from the floor
- We measure 3.25 inches from the same foundation wall surface to our vertical string at the very top of the wall just under the sill plate.
- We check that we’ve measured at the area of greatest inward bulge in the wall by moving our plumb line to our left, then to our right on either side of the ceiling joist we used to hang the string for our first measurement. If the distances we measure, wall to string, are greater than the distances we measured at our first trial, then that one is the point of greatest inwards foundation wall bulge.
- Finally we do the math: subtract our “higher on wall” and “closer to string” measurements from our “at the floor” and “farthest from string” measurement.
We see these results in foundation bulge measurements:
- Foundation Wall Bulge-in at floor = 0 inches
- Foundation Wall Bulge-in at 5’up from floor = 4″ – 2″ = 2″ of inwards bulge
- Foundation Wall Bulge-in at the top of the wall = 4″ – 3.25″ = .75″ of inwards lean
Back to reinforcing bowing basement walls. Steel I beams as a basement wall repair method can be used. Steel I beams are to be buried below the floor and cemented in at the bottom and properly attached to the wood structure above. In this scenario the floor below and the wood structure above are actually holding the I beam in place. Steel beams are straight so they only touch the wall where it is bowed out. The gaps at top and bottom between the wall and beam should be filled in to prevent the wall from shifting behind the beam. Downsides are steel beams are unsightly, can rust, and still can flex. Also if the soil conditions outside push on the wall it can cause the beam to put pressure on the wood structure causing it to shift or twist.
The latest basement wall repair method for bowing basement walls is carbon fiber mesh “straps” which are embedded into the face of your foundation walls. And the latest of those are made by Fortress Stabilization who has developed a system made of both carbon fiber and Kevlar meshed together giving them the ultimate in tensile and shear strength. These straps are smooth to the wall and paintable making them barely noticeable. Carbon fiber / Kevlar mesh straps are embedded into the face of the wall with a two part epoxy becoming part of the structure. How this method works is that the straps are inflexible / do not stretch so they create tension on the wall preventing movement. Soil pressure outside is diffused and the wall is permanently stabilized.
If your basement wall has a sheer, meaning a row of block has slid inward off of the row below it a half inch or more, it needs to be replaced. Some basement walls with a sheer less than a half inch can be stabilized with steel beams to prevent further movement but for the most part this is an indication of severe drainage problems resulting in water build up at the bottom of the basement wall outside. This build up results in the foundation and or mortar rotting out, becoming weak, and resulting in the shift. Often times this results in dirt and/or mud coming into the basement through the broken areas.
Tipping basement walls are a result of them not being well attached to the structure above during construction. This is a wall that is straight from bottom to top but leaning inward at the top. This is different from bowing as you will not see a horizontal crack that is opened up. Fixing tipping basement walls involves first removing the soil outside to relieve pressure. Then a system of steel beams and screws / jacks are used to slowly push the wall back into its best possible position. The steel beams can be left in place to hold the wall from further movement or other methods can be used to secure the structure above to the foundation to prevent movement.
Obviously if you have a collapsed basement wall, it would need full replacement, enough said.
Crumbling and rotting foundations occur due to the original waterproofing seal failing, allowing water into the basement wall. In many cases, if this is caught early enough the wall can be excavated, repaired, reinforced, waterproofed, and ultimately saved. The cost of going through all of that may be getting very close to replacing the basement wall and some walls with rot and decay should simply be replaced.
Settling foundations are those that are sinking into poor soil conditions. Signs of a sinking foundation include stair-step cracks, vertical cracks, and issues in the living space above such as cracked walls, ceilings, and doors that do not close properly. To stop a settling foundation it requires installing reinforcement below the foundation installed into a solid substrate. This method is often called underpinning or piering. We favor the Stabil-Loc Foundation Piering System. Its patent-pending load-bearing design installs directly under the wall being lifted. Its unique interlocking high strength steel construction can support up to 300,000 lbs. There are no bolts, no brackets, no breaking the footing, and no off-set loads. This pier carries an Engineer Certified safety factor of more than 10 to 1.