Sunday, May 6, 2012

Block and Base Foundations: A Brief over view of design and why they move

I've been getting a lot of questions over block and base foundations/pier and beam foundations recently. These homes are experiencing a good deal of movement over the past months, and customers constantly ask me "why do I need to re-adjust my home every 3-5 years?" This post is an attempt to explain how these foundations are designed, why they constantly move, appropriate maintenance techniques, and proper repair methods.  

Block and Base
A Block and base foundation is a wood framed house that sits on blocks.  Blocks are to be solid masonite blocks that sit directly on the soils surface.Interior and exterior beams should be 4x6 treated lumber or larger.  Typically these houses have exterior beams and one interior beam running front to back down the center of the home.  Spans between interior and exterior beams should not span greater than 15 feet. Beams should be a minimum of 18" off of the ground in order to keep water and wet soil directly off the beam.

Common Problems

Drainage:  When these homes were built 50+ years ago, the lots originally had a slope away from the house.   Over time, pounding water from roof ejection during rains has eroded that slope.  There are usually "drip lines" right under the roof lines where you can see the effect.  Combine that with organic matter build-up (i.e. dead tree leaves and you now have a slope back towards the home. As there is no exterior barricade, water will flow underneath the home and sit.  This will in turn cause each individual block under the home to act as a piston of sorts, moving up as the soils swell, and moving down when they dry out.  Blocks will also sink into muddy soil when certain areas become too wet.  So in some cases you have the weight from the house pressing downward, and the exterior landscaping building up, so the drainage is getting consistently worse.  Drainage is the leading cause for interior doors to become non functional.

Take this home for example, can you see how the lot drains to right underneath the front porch?  This is where the water will collect during heavy rains, causing differential movement.

Bad Supports: Termite Damage, Dry Rot, Improper foundation supports.  Remember that these homes sit directly on the surface of the soil.  As the soil moves the supports will also move.  Blocks over time can break, tilt, rotate and even drop from under the support.  I've seen everything from car jacks, to car tires, to 2x4's used as supports.  Rotted or termite eaten wood can no longer support the weight of the home. Wood will compress and in turn floors will drop. This will need to be replaced so the house can be lifted back into place.

Most common Signs of problems: Sticking/Non working doors, drywall cracks, separation in the floor boards, separation in the window casings, zig zag patterns in the exterior siding, and in severe cases the walls separating from either the ceiling or floor.

Most Common Repair Methods
  • Readjustment/Reset-Reshim:  This is mechanically lifting the home until all of it's structural components are realigned.  I hesitate to list this under repair.  It's not a permanent solution.  A readjustment lasts only as long as the soil conditions allow.  It's also only as feasible as the wood underneath the home allows.  Wood is often warped or bowed due to age and it's impossible to get these homes level.  I always make the home owners aware that we can't get it level, and we can't guarantee that doors will be perfectly aligned when we are done.  While we can't make it perfect, we can usually make a noticeable difference in  condition.  Occassionally, new wood beams are needed in order to lift areas in between the origional foundation layout.  If your bathroom door doesn't sit on the interior beam, it will be impossible to correct without new wood.
  • Wood Replacement
  • Drainage Correction:  This needs to be done two ways, one is by the addition of gutters and the other is by the instillation of surface drains.  Gutters need to be in place to catch water ejected by the roof during rains, and the surface drains to catch surface run off that goes back to the house.  French drains are normally not effective with block and base homes, and can be a very expensive, non functional direction.
  • Drilled Piers: This is actually uncommon due to cost, but it's the only repair method in which we can guarantee stability.  Drilled piers would basically take the place of the block supports, and would be to a depth to negate any future drawbacks.  The benefits of piers is that they provide long term stability, the draw back is that in order to install interior piers flooring would have to be removed.   
Pier and Beam Foundations
Pier and Beam Foundations are homes that have interior piers, and an exterior concrete beam.  The exterior beams are commonly 24-30 inches deep and 12 inches wide.  The piers are the interior are usually 24-30 inches in the ground and 12x12 in width and depth and sit 18 inches above ground.  In Houston, these are rare, usually around Rice University or Zoo district.  These houses normally have brick siding and have 4x6 interior and exterior beams.  As with block and base homes the beams should not extend beyond 15 feet in width.
The piers should also line up with beam intersections as well.  Mason vents on the exterior should also be in place.  These vents should be no more than 10 feet apart (however it has more to do with cubic crawl space area than linear feet.  The vents allow for humid air to vent out from the foundation.

Most Common Problems:
Drainage:  See what I wrote about block and base foundations.  Same rules apply here.  The only difference is that wood rot can be an enormous problem with these foundations.  Most of these homes are undercut, meaning that the soil level inside the crawl space is actually lower than the soil level outside the home.  Water will always seek its own level, so often the crawlspace will fill up with water when it rains.  Since the exterior contrete beam acts as a barricade to hold water, wood will absorb moisture.  Without proper airflow fungus, mold and bacteria will eat away at beams. 

Settlement:  Just as with slab foundations settlement can occur with pier and beam foundations.  Some of these foundations were built with builders piers, which are bell bottomed piers that were doweled into the exterior concrete beam when the original construction took place.  Over time large trees or erosion has led to differential movement in the exterior beam.

Beam Erosion: Years of sitting in highly acidic, often moist soil can erode even the strongest concrete.  Beams will become soft, and you can often see "spaulding" where the concrete is crumbling and falling apart.  Rebar will often protrude from the beam as well.  Severe beam erosion can be very difficult to replace.

Ventilation:  Often the vents are not creating airflow.  This can be for several reasons.  Most commonly is that there is something blocking airflow.  Repaired beams often are not cut to work around the vent, they simply cover them up.  When I drop in a crawlspace the first thing I do is turn off all the lights to see how many vents are actually uncovered.  The second thing that keeps them from working is another structure too close to the house to allow wind to flow through the crawl space.  Large wooden fences or the neighboring houses are the most common culprits.

Termite Damage:  See above.

Pier Erosion:  Same as beam erosion, only with the interior piers.

Most Common Repair Methods
Exterior Piers
Drainage Correction
Excavation of interior soil
Addition of Vents
Addition of Vent Fans
Wood Replacement
Adding of new support beams.

Proper Repair Techniques
Reset-Reshim:  It is important that all supports are aligned at beam intersections.  1/4" galvanized steel shims are most typical.  Steel shims are honestly inexpensive, and if a company can't afford to put those on you should probably think twice about going with that company.  Wooden shims compress over time, and are often cut at an angle.  Angled wedges are hammered under the beam to save time.  A little basic geometry should help you understand that an angled wedge and a flat beam don't work well together.  The beam will rotate, and gravity can actually pull it off the support leaving you with a large problem. Shims made out of crushed coke cans, roof shingles, cut siding and any thing else are simply not acceptable. 

Blocks that are angled or rotated should be reset to fit flatly on the ground to give perpendicular support to the beam.  This is not possible with piers. Because piers sit in the ground they can not be dug up and reset.  Blocks should also be solid, not halite blocks.  Halite blocks do not have the compressive strength to support the weight of a house. 

Wood replacement: It is vital that the same wood that is taken out is also put back.  Wood should be no smaller than 4x6.  If a 4x6 is pulled out, then a 4x6 should be put back.  Also, the wood should stand 6" tall, and 4" wide, not vice versa.  The compressive strength of the wood is in the height, not the width.  Beams installed flat should be rotated.  If a sill plate on a pier and beam has rotted, the anchor bolts will need to be cut, the wall will need to be lifted to relieve pressure from the sill plate, it will need to be removed and replaced.

Piers: On a block and base, it should only be drilled piers, due to the weight of the house.  On a pier and beam, either piering system is sufficient on the exterior beam, unless the concrete grade beam has degraded.

Drainage: As stated earlier, gutters and surface drains are the most effective types of drain systems.  Surface catch basins should be no farther than ten feet on center.   Ideally you would like your gutter downspouts tied into the drainage system.

Vent Fans:  Vent fans should be placed in the actual vents themselves.  They should be directed in a way to generate airflow in the crawl space.  There should be one vent fan for ever 500 cubic feet of crawlspace.  I like to angle them to create flow from one side of the house to the other.  They should never be placed to suck air into the crawl space, however if all of them are set to push airflow out of the crawlspace that is acceptable as well.

To finalize, what I would like to say is that when it is impossible to inspect these foundations without actually getting underneath them.  If your repair man hasn't gotten under your house, he has no idea whats going on with it.  I would also encourage all of you to set your head under there and see what the company did.  It's pretty easy to see whether they actually used steel shims, or if they put whatever was closest by and easiest to install.  When I get under a house, I actually take a camera with me so that the homeowner can see what I see.  I think proper communication and expectations are key when it comes to working on these homes.  If you have this type of foundation and it needs to be repaired, or if you just would like some helpful tips on how to keep it more stable give me us a call.  Perma-Pier Foundation Repair has a great deal of experience with repairing these homes, and I've crawled under and evaluated several hundreds of these in my time here.

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Thanks for Reading, and talk to you soon!


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  2. Thank You, Thank You, Thank You! If we were anywhere near Texas we would use your services. We have been searching everywhere for this information. Great Service!

  3. Hi Brian. We are house shopping in the Magnolia/Conroe area. I've just spotted a house that is block and beam. (MLS # 58909195) My husband is leery and wants to stay with concrete slab foundations. We are about 10 years away from retirement and want our next house to be our last house. Would you agree with him?

    1. Thanks for asking! In Magnolia/Conroe there is not a perfect system. Hills, Large trees, volatile clays, and old homes can be tough on the houses. Both slabs and block and base houses can form issues here. Both slabs and block and base homes will require general maintenance(I.E. watering, gutters, and drainage corrections) in order to keep the foundation stable. Block and base homes (under proper maintenance) can be just as sturdy as a slab. If you have any questions, or would like for me to help you with this, please feel free to contact me @