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Question DetailsAsked on 3/11/2014

The back room on our home which was an addition home has suddenly started seperating from the origonal wall. Help!?

This was an addition prior to the purchase 7+ years ago. It is Built on a slab.

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4 Answers


Sounds like the addition was built on uncompacted fill which as started to settle. I am not sure if you are looking for what type of contractor you need or what exactly you need. I would call a concrete contractor or better yet one that deals with slab jacking. If the house crack is not that big it is possible that slab jacking might save the room. Slab jacking is a proccess where holes are drilled in the slab and concrete is pumped in under high pressure and lifts the slab back into place. I and other experts that answer questions here would probably need more info to advise you much more than that.

Is it a true slab construction with no frost walls going into the ground , is the house on a slope, what type of soil (sandy, clay, etc.), is the slab the finished floor or is there a crawlspace or any other particlulars you can add.


Answered 6 years ago by ContractorDon


The "suddenly started separating" could be a sudden movement, or could be because the settlement over the last 7 years has reached the point where it is finally exceeded the amount of slack and "give" in the walls so is finally causing visible tearing, even though it may have been going on for 7 years. Obviously, if the crack is opening up rapidly and visibly, you need an immediate inspection by a civil engineer as you may have something dramatic going on. If just slight cracking appearing and not visibly growing, there is likely time to be more deliberate about it. Obviously, if you live in an unstable soil area subject to landslides or sinkholes, or located on a steep hill or near a bluff, etc then assume it is serious and get an engineer to look at it ASAP.

Don gave you a good start, and a photo or two of both the cracking and outdoor photo or two of the overall addition and surrounding area would certainly help us address this more definitively:

A list of possible causes:

1) poor compaction under foundation footings (including slab) - this is VERY common, due to laziness, thinking tapping it with a backhoe bucket is "compaction", or workers raking loose fill in to fill low spots just before pouring concrete, without compacting it

2) failure to remove unsuitable materials under footings before placing base fill and slab, either poor native materials or washed in mud

3) poor footing design, so unable to carry design loads without excess settlement - this is rare except in very soft soils and expansive soil areas

4) water infiltrating in under slab, or coming from broken pipe, which is washing the fines in the soil further down into the ground or out under the slab, causing settlement of the remaining fill material or natural soil; or water causing more rapid consolidation under the slab, which sounds like a possible cause if you have had unusually wet weather recently or roof runoff has suddenly started accumulating by the slab for any reason

5) broken sewer pipe causing erosion under footing, leaving gaps the fill is settling into

6) slope instability - i.e. soil the addition is built on is sliding or creeping downhill due to excess moisture, landsliding, nearby erosion, etc

7) soils that are subject to expansion and contraction with water changes

8) soils that dissolve upon contact with water like gypsum and limestone and salt among others, causing sinkholes or gaps enough to remove support from under the foundation/slab

9) differential settlement - let me explain in a bit more depth on this one, as it is a very common cause of this type of problem. All fill (and natural soils) progressively settle or compact with time - due to crushing under load, due to natural consolidation (tighter packing of soil grains) and due to squeezing out of water from non-free draining soils. Some materials like crushed competent rock see almost all the settlement very quickly under normal construction loads (hence used for road and driveway bases, for instance), whereas others continue to compact or settle for hundreds of years - "fat" clays and the silt and clay in river deltas being a prime example, causing for instance New Orleans to go from an above-ground city when started to one that is 10-20 or more feet below original ground surface in many areas. So, using better quality fill and properly compacting it during construction reduces post-construction settlement, whereas loose or poorly compacted fill and poor natural materials under the construction site can result in long-term settlement. Your main house slab has likely already seen most of its settlement occur, but when you add an addition it will settle for some years, causing it to move downward relative to the original house. Commonly, the slabs between the two houses are tied together to at least csome extent, either intentionally or just by pouring one slab against the other, "hung up" by friction - so that end tends to stay more or less in place many times, whereas the other end undergoes full free settlement, resulting in a crack or gap at the intersection of the addition wall to the main house which is wider at the top than the bottom. The same thing happens if fill was placed for the addition because of a natural slope away from the house - instead of excavating to a uniform depth to get down to suitable soil and provide a uniform fill thickness under the addition (hence uniform settlement), most contractors only excavate to suitable soil conditions over the entire surface, leaving a sloping soil surface under the addition. Therefore, the fill depth placed to pour the slab on at the end furthest from the house ends up thicker than by the house, so it settles more, making for a slab tilted down away from the house, and the same tension crack at the main house - wider at the top then at the bottom. If the slabs do not connect or hang up, or are properly separated and isolated during construction, then the crack at the addition/house interface will be more uniform width, commonly with some crushing and diagonal cracking as the drywall and siding shears with the addition settling more or less vertically.


10) You need to find the cause of the settlement, because while mudjacking may relevel the slab, if there is a water leak settlement can continue, and if a sewer leak you can end up pumping your sewer full of grout.

11) If not serious enough that the slope on the floor slab is objectionable, or if your budget is limited (as mudjacking is pretty expensive), you can have the two buildings cut apart - physically separated - proper support given to the disconnected walls, then a loose, flexible airtight and bug-proof liner (commonly ice and water shield) with folds and slack installed in the crack, the gap filled with compressed insulation so it can adjust to movement, and the crack covered with decorative trim inside and out (fastened with slotted fastener holes or fastened on only one side of crack) that can be removed to redo the foaming if the crack widens too much in the future. By the way - by physical separation I mean structurally not joined at all - generally, certain structural restraints have to be kept in place, but bolted or nailed connections are replaced with hinged plates or cables or keywaysornew portal framing atthe interface, for instance, to allow some movement but not so much that there is "out-of-plane" displacement of walls and roof at the junction.

12) Things that also need to be considered are slack in plumbing and wiring at the interface so you do not get leakage or fire hazard, and providing protection against roof damage that could open up the shell to water infiltration - typically an overlying flashing fastened at only one side of the junction between addition and house.

13) My recommendation - I would use a water meter check (if metered) during a time when zero water is being consumed to check for water leak, or if no meter then use a stethoscope to listen at water lines for leaks. You can locate a leak under a slab that way, but you do not have to actually trace every line - just turn off water heater and furnace so they are not gurgling, heating or pumping or consuming fuel (which you WILL hear through pipes, even a pilot burning), and any other fans or motors or TV/music so the house is totally quiet, then listen at pipes around the house. To hear what a leak sounds like, make a faucet drip just a bit to see what a leak sounds like at different distances. Also listen at inside and outside walls so you know what outside traffic noise sounds like. Then listen around the house at different pipes to see if you have a leak - if so, track it down.

14) Sewer leak is tougher to rule out - requires a camera run to be sure that is not the problem. If the tilt on the slab is uniform across whole addition, highly unlikely to be a sewer pipe leak - usually they will only open a void a few feel in diameter unless split wide open under the length of the slab, which with a 7 year old addition is highly unlikely with modern piping unless the addition was laid on top of original iron or clay sewer.

15) If you want to track down the cause and get remedy recommendations, a civil engineering firm with both structural and geotechnical engineers on staff is what you would want - to assess the damage, determine cause, and design remedial measures - probably from $400-1000 depending on how serious your issue is, assuming it is NOT gross land movement or landsliding. If not serious enough for that and no significant cracking is evident in addition slab, then a General Contractor specializing in remodels could probably take care of separating the sections of the house and installing flashing and seals and piping/wiring slack to allow for movement without further damage.

16) Cost obviously dependent on your specific situation, but mud jacking commonly runs several thousand and commonly up to about $3-5000 for a good sized addition - separating the house junction and putting in sealing and flashing and such can run from maybe $400-800 with additions who have separated roofs and siding already, to $1000-1500 range if they were built to act as one unit. If in doubt, et the engineer on board.

Answered 6 years ago by LCD


First, I am not an expert. I don't know about your situation, and the people who have just posted know a lot more than I do. I.But your question was interesting to me because I have a similar situation, i.e., ongoing difficulties with an add-on. In my case, already had six piers put in under an addition back in 2007. In my case, the addition was not separating, but back door was sticking, and some wall boards were protruding. So sometimes I think the thing is on its way to separating but hasn't yet. Back in 2007, I did not get an engineer's report before hiring a foundation company. I have since. But back then, the piers did not correct the problem. I called the company back, in 2008 or 2009, and was told that piers prevent downward motion (we have clay soil) but do not prevent upward movement. So we learned to accept the problem. Fortunately ours has never separated from the original wall (yet), so your situation is a lot worse. I have since asked a structural engineer if anything can be done about it and was told that add-ons are just lighter than the rest of the house and that they all have problems. He said my add-on was about average. He indicated that if I were concerned that I could contact the foundation company to dig to the piers to see if water was flowing well (or something). I also asked a handyman once, and he said, just nail the boards back in. So in my case--badly sticking door, varies with rainfall--boards protruding--I just live with. The door frame is mangled from the seasonal/rainfall sticking. (For that matter, my backyard gate sticks as well, dependent on rainfall.) When I googled once, I learned that rising is a cosmetic issue but is not generally considered serious in the way that sinking would be. Of course, when walls start separating, as yours is, that is another matter, and your foundation might well be falling. But I did find out that there is a difference in seriousness generally (excluding yours, where the whole thing is separating) in sinking and in rising, and in the behavior of add-ons and the rest of the house. Might not apply to your situation, and definitely you should ask a structural engineer and/or a soils engineer if you can find one.

Answered 6 years ago by Guest_988593721


The guest commenter made a good point - one of the first signs of settlement or soil expansion is commonly either drywall cracks or sticking doors and windows. The cracking is a symptom, not the problem, so spackle and paint takes care of that as far as cosmetic issues go. It is not in the least uncommon to have to go back after 3-10 years and remove and reinstall, or at least cut the nails and reshim and renail, at least a few windows or doors on additions to remove the binding.

The engineer's comments regarding it moving more because it is lighter must have been for a single-story addition on a two story house. However, additions do tend to have difficulties at the connection with the main house due to differences in soil consolidation, different building weight per square foot on the foundation (especially with slab versus strip footers), and also because they are commonly T'eed off the main house, so their movements due to natures forced tend to be at right angles to each other, causing cracking at the interface. The flat slab issue is one I have run into a lot, because unlike stem wall or strip footer designs, there is nothing holding them down, so they tend to move around a lot more. I had one sunroom I worked on which would have been funny were it no sad - frost jacking (they left it cold in the dead of winter) was moving it away from the main house, and the slab and addition were almost 2 feet away from where it was poured - nothing a bit of cleaning out the interface area and a trio of 200 ton jacks could not fix - slipped right back into place without a visible crack in the slab !

Answered 6 years ago by LCD

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