Raised Perimeter and Post and Pier Foundations

I recently published a piece on post-tension slabs, which is more used in new construction, however it’s not the most common type across existing homes in the South Bay. While basements are not often found in the South Bay, crawlspaces are. You’re most likely to encounter raised foundation, known as perimeter or raised perimeter, also called post and pier, foundations.

 

Raised Perimeter and Post and Pier Foundation Compilation 1960s

Still images from historic reels shared by History San Jose showing one of the city’s suburban developments of the 1960s and the installation of this type of foundation. Click to go to the original video on Youtube. Edit: Unfortunately the video appears to have been removed. I’m leaving the link in case it is re-uploaded.

An Introduction to Raised Perimiter Foundations

What is a Raised Foundation, or a Perimeter Foundation?

A raised foundation, perimeter foundation, or raised perimeter foundation is one that supports a structure while lifting it a few feet above the ground level, as the name implies. It is called a perimeter foundation because the exterior walls are held up by a reinforced concrete stem wall, while the body of the house is supported by a post and pier construct. (In earthquake-free parts of the world, the stem wall may be brick or cinder block.) These are all names for the same thing.

This type of foundation is usually only raised around 1-1/2’ – 2’ high, one or two stair steps above ground level. Much taller would make a top-heaviness that becomes less stable against seismic force.

Alternatively, some floors might be set quite low. Two rooms in my single-story house are a step below the rest. They are still raised on posts and piers, but they are distinctly lower than the rest. This is called a sleeper floor. In the crawlspace, this translates to very tight quarters, and I have met professionals who will, and who will not, be able to work in that space.

What about perimeter and slab both in the same house?

If a home is described as having both a perimeter and a slab foundation, most of that time it means that the majority of the house has a raised foundation, but the family room or other room directly behind the garage is on slab. This is very common in Silicon Valley

What is a Post and Pier Foundation?

Post and Pier section of blueprint plus lowered subfloor for tile

Post and pier (or girder) foundation blueprint, including crawlspace access point and lowered subfloor and sleeper floor.

Post and pier, or beam and post foundation, supports a structure by raising it on individual posts distributed evenly beneath a structure. Each pillar of support consists of three parts: the pier, the post, and the beam.

The pier is a vertical anchor set deep in the ground, usually made of concrete (but occasionally other resistant material like steel). The pier rises a few inches above ground level and is attached to a vertical post. The post, or column, is generally foundation-grade treated wood. This post attaches to a horizontal beam, or girder, which directly supports the floor joists. Occasionally, the pier may act as a post and connects directly to the beam, but that is very uncommon in my experience.

These type of foundations can be built without the perimeter wall, but functionally it is very different. You will often see decks built this way, including decks that are attached to homes with a raised perimeter foundation. Without the continuous stem wall the cost to construct is significantly less, but the resulting structure is more vulnerable to the forces of nature, to lateral seismic force, and to pests and wildlife. It also has greater airflow beneath, which is good in places with regular flooding, but provides less insulation from below.

(more…)

Landscaping with tanbark or mulch? Use caution!

Some Silicon Valley homeowners spruce up their yards and gardens in spring and summer with tanbark or mulch. While this is a very common practice, and often encouraged as a drought-friendly gardening option, it can be a bad idea if it is too close to the structure, especially the home’s foundation.

Tanbark is simply small bits of wood, and most common mulch is often no more than shredded wood. Why is that bad? Wood is food for termites and piles of tanbark or mulch can invite and hide them as well!

 

Tanbark or Mulch?

Beware Tanbark or Mulch by the foundation!Mulch is the more widely used term and it can cover a broad scope of materials, but the most common type you will find in stores (and in Bay Area gardens) is the woodchip mulch. If you ask for mulch at a hardware store, this is most likely what they will show you. In the local vernacular, we often refer to mulch as the fine, thin, or decomposed stuff – we have a different name for the larger bark and wood chips.

I learned only recently that tanbark is something of a local term that people from other parts of the state or country may not be familiar with. Here in the Bay Area we call the stuff you commonly see underfoot at playgrounds or piled thick on the planted berms around a shopping mall parking lot by the name of tanbark. Some people may reserve the name for the large chunky bark chips while others will call just about any wood chip substrate by that name. So tanbark is, in fact, a mulch.

Homeowners and sellers wanting their home to make a good first impression are often tempted to apply mulch or tanbark in otherwise bare patches around their yard, but you can wind up with far bigger (and more costly) problems if it’s too close to the foundation!

What Was That About Termites & tanbark or mulch?

(more…)

Post-Tension Slab Foundations

Post-Tension Slab FoundationPost-tension slab foundations are found in newer homes. Here in the Bay Area, a structure’s foundation needs to withstand not only the load of the building, but expansive soils, and the ubiquitous earthquake. Certain foundations are better at handling these conditions, and are seen more frequently here. One of these which is gaining popularity in new construction is the post-tension slab foundation.

What is a Post Tension Foundation?

Post-tensioning is a technique that was developed and first put to regular use in the 1970s, and approved methods have been published by the Post-Tensioning Institute (PTI), a nonprofit organization, since 1976. Sometimes called post tensioning, or simply PT, this is a type of slab foundation with added reinforcement.

In essence, a slab foundation, aka a slab on grade foundation, is a concrete base only a few inches deep, sitting directly on earth. You might see this for a small shed or playhouse, but larger structures are almost always reinforced, usually with rebar, and a fabric water barrier is lain out before the concrete is poured.

A post-tension slab is reinforced with grids of steel cables cased in plastic sheathes instead of rebar. After the concrete has hardened around them, the cables are pulled taut with hydraulic stressing jacks. This pre-stressing of the concrete creates added compressive strength to the foundation.

(more…)

Historic Homes, Willow Glen, Foundations and Red Flags

“Red flags” are clues that something is wrong or potentially wrong. They’re the hints that we need to investigate something further, the sign that we should be on alert.

Some parts of San Jose, and Silicon Valley generally, enjoy beautiful older homes with classic styling and beautiful finishing work.  These properties and neighborhoods are prized because they are not cookie cutter, not ranch, not too new.  They may be Victorian, Craftsman, Spanish, or any number of other interesting architectural styles.

One area of Santa Clara County that is well known for both charming historic homes and unfortunately also some structural issues among those older houses is the Willow Glen district of San Jose.

Willow Glen Crumbling Foundation Red FlagsBack in 2015 I showed some clients about a half dozen homes, all in Willow Glen, and we saw a lot of “red flags” which hinted of foundation problems, among others.  I thought I’d share a few pics I snapped at one of them with my old treo camera here.  All of these were taken on the front porch of this house – all visible structural  “red flags” before we ever set foot into the house.
(more…)

Cracked Foundations, Adobe Clay Soils and Water in Silicon Valley

In Santa Clara County, as in much of California, we have adobe clay soil and it’s expansive.  That is, when the dirt gets wet, it expands, and when it dries out, it contracts. Hence it’s sometimes referred to as “shrink-swell” soils.  (Every state in the union has areas with this problem – a color-coded map on geology.com shows areas with more and less expansive soils.)

Why is expansive soil an issue for homeowners and would-be homeowners in Silicon Valley?

The trouble is that the expanding and contracting soil is far stronger than concrete and the foundations upon which a home sits. A well written and illustrated six page paper can be found online explaining the mechanics involved for those interested in more detail on the hows and whys of expansive soils. (It states that the ground can life as much as 5,500 pounds per square inch!)

What I’d like to focus on here is mitigating the risks and preventing the problems associated with expansive soils.

The trouble is not so much that the soil is wet or dry.  The problem is in the back and forth, the movement. When the soil is kept at an even amount of moisture, it does not expand and contract.

Obviously, rain is seasonal and we cannot control all moisture on or near the house. We can, though, work to move water away from the house and away from the foundation.

Keep rain away from foundations on adobe clay soil!

Winter storms can bring an enormous amount of water onto a home’s roof, and when it channels down gutters and downspouts, there can be a large amount of water exiting in just a few places.  Where does that water go?

 

downspout no extender near foundation

(more…)

How deep is the groundwater?

Valley Water depth to first groundwaterValley Water depth to first groundwater

Valley Water depth to first groundwater in Santa Clara County

In most of Santa Clara County, home owners do not own the right to drill a well and pump groundwater under their property. That will be clarified in the preliminary title report (similarly, oil and mineral rights are usually not sold with residential neighborhood parcels here). The depth of the water may be of interest, though, as a high water table may have possible risks.

High Water Table Risks

In come cases there could be problems with springs under the home. When that happens, the groundwater may percolate up under the house during times of heavy rains, causing water in the crawl space and possibly creating foundation damage later.

Another risk is underground water moving environmental hazards to your property. A plume of water could potentially move toxins released at a leaking underground storage tank (“LUST” site) to your property’s area, where you may be responsible for the cleanup! (For info on the leaking underground storage tanks, you’d want to read the JCP or similar report for natural and environmental hazards. That will tell you if there are any LUST sites within 1 mile of the property.)

Does your home, or the one you want to buy in the Santa Clara Valley, have a high water table? One way to learn is to check out the interactive map on the Valley Water website.  Much or most of Campbell seems to have first groundwater at a depth of 50 to 100 feet. Much of the low-lying areas of San Jose and Santa Clara have water at a mere 0 – 10′!

The interactive map that I saw includes most, but not all, of the valley. Enjoy checking it out!

Santa Clara County Depth to First Groundwater

 

 

Related Reading:

Historic Homes, Willow Glen, Foundations and Red Flags

Why are the hardwood floors cupping?