Foundation Details

Now we will look at typical details of a Residential or Light Commercial stem-wall and footing foundation. In particular, we will look at an exterior wall. Interior footings and details are covered separately. How to get much of the information in this discussion is also discussed separately. Refer to the attached sketch. First the items will be listed. Then they will be discussed in some more detail.

1. Stem wall height
2. Stem wall thickness
3. Footing thickness
4. Footing width
5. Stem wall horizontal reinforcement (H.S.)
6. Stem wall vertical reinforcement (V.S.)
7. Key
8. Longitudinal reinforcement
9. Transverse reinforcement
10. Supporting Soil
11. Drain
12. Backfill
13. Waterproof coating
14. Finish Grade (FG)
15. Anchor bolts
16. Sill seal
17. Sill plate
18. Crawl space
19. Interior grade (or slab)
20. Energy code requirements
21. Oiginal grade (OG) vs. finish grade (FG)
22. Stepped footings on sloped sites
23. Foundation Drain (outlet of)
24. Roof Drain
25. Other

Now we will discuss the detail items a bit further, noting that greater detail to many of the details is given in other lessons.

1. Stem wall height – is a function the required penetration of the footing to meet frost depth and the required clearance between soil and any unprotected wood (wood not resistant to decay or preservative treated). In this scenario the soil is assumed to provide insulation to the foundation system. On sloping sites it may be necessary to show a minimum height as the foundation wall height might vary in height to accommodate the terrain.

2. Stem wall thickness – the minimum is generally dictated by code (either model or local jurisdiction). Eight inches is a typical thickness. In some cases a `nominal’ thickness value may be allowed, corresponding to actual versus nominal dimension lumber (presumably used in form work).

3. Footing thickness – either calculated or prescribed. In some cases `nominal’ thickness values are allowed as noted above. Recall that for prescribed or `non-calculated’ footing dimensions the footing thickness must be at least as much as the projection of the footing beyond the face of the stem wall. Typical minimum footing thickness values are 6 in. or 8 in.

NOTE: where vertical reinforcement is required to be embedded 6 in. or more into the footing, and where the footing is cast upon earth / supporting soil, a minimum thickness of 9 inches will be required to accommodate the `concrete cover’ requirement of the `code’. As such, I often specify a 9 in. min. thickness footing (or 10 in. nominal).

4. Footing width – either prescribed or calculated and is dependent on the weight and loading of superstructure being carried bearing capacity of the soil. Minimum prescribed widths vary considerably dependent on superstructure and local soil conditions.

5. Stem wall H.S. – prescribed or calculated. Horizontal reinforcement in the stem wall will help control cracking and help the stem wall act as a horizontal `beam’.

6. Stem wall V.S. – prescribed or calculated. Vertical reinforcement helps control cracking and reinforces the stem wall as it acts as a retaining wall for differing exterior and interior backfill levels. Vertical steel is typically required to be doweled, hooked, and or lapped into the footing.

Note: the present discussion in for a relatively short stem wall. Taller walls and basement walls will be discussed separately.

7. Key (or connection of wall to footing). Some type of connection is needed to prevent the stem wall from sliding off or along the footing (due to pressure differences of soil, etc. inside and out). Older details typically show a concrete `key’ cast into the footing and filled with stem wall concrete. Such a detail is generally cumbersome to construct. The current trend is to cast the vertical reinforcement into the footing it resists sliding at the connection in addition to reinforcing the wall. As such, jurisdictions are becoming more `particular’ about the V.S. detail(s), and will dictate minimum embedment and in some cases hooked reinforcement. Typically 6 in. embedment is sufficient for a shear-type connection using Ã?½ in. diameter rebar. In my opinion the reinforcement need not be hooked, unless it is also performing some other structural function (for example, if the stem wall is not supported horizontally at the top and thus must act like a vertical cantilever).

8. Longitudinal footing steel – either prescribed or calculated. It may be detailed to be at the minimum clear distance from the bottom (3 in.) or may be detailed to be at mid-depth.

9. Lateral (transverse) reinforcement – either not needed or calculated. For modest footing widths it is generally not required. If required, it is the most effective if placed near the bottom of the footing, but still meeting the 3 in. min. clear cover distance required by code. Where lateral reinforcement is required it may be preferable for construction purposes to tie it to the longitudinal reinforcement (or visa versa) and thus have both at essentially the same level (typically 3 in. clear from the bottom).

10. Supporting soil. Probably the most important element of the foundation is the supporting soil. If you do not have good supporting soil, the foundation will break, and broken foundations are difficult and expensive to fix. I recommend being `double covered’. First, provide clear specifications in the Construction Documents with regard to the foundation soils, and, second, include a note on or referenced to the foundation details along the following lines.

“Footing shall be placed on undisturbed native soil or on controlled fill certified by a Soils Engineer to meet the specifications in these documents.”

Obviously the above presumes the native, undisturbed soils also meet the specification(s). If there is any question in that regard, I recommend that the native soils be investigated and a soils / geotechnical report with foundation design recommendations be a part of the project.

11. Drain. Proper drainage of the foundation is very important. Generally foundation designs assume that the soil backfilled against the foundation is drained of moisture. Soil that is not drained of moisture will cause problems, not the least of which are leaking and permeating of water into the crawl or living space, and excessive soil pressures on the wall.

Typically foundation drains are of hard plastic pipe with holes (perforations) to accept soil moisture. It is recommended that they be surround with permeable sand or gravel fill. If surrounding or backfill soils contain a lot of `fines’ (silts and clays), it is recommended that the pipe be protected with a permeable filter fabric. Generally I recommend that this fabric be around the surrounding gravel, or backfill zone, and not the pipe itself. I also recommend a gravel backfill `chimney’ where the adjacent 12 in. or so next to the wall is gravel. Beyond the gravel the native or other soils may be used.

The drain pipe should be sloped or should be large enough so that if flat, and partially filled with silts, there will be enough remaining volume and slope to drain the water. I recommend �½ % slope minimum for 4 in. diameter perforated pipe or 6 in. pipe flat (as long as the lengths involved are not too great).

The drain should ideally be located at or below frost depth. In no case should the drain not be lower than floor level inside.

12. Backfill materials should be free of debris. As noted above, the backfill materials next to the wall should be such as to allow free drainage of (surface and subsurface) water to the drain. If the stem wall retains a significant height of soil, the type of soil and resulting horizontal load on the wall must be taken into consideration in the design, or, conversely, the backfill materials must be specified to be wall design used.

13. Moisture barrier. Stem walls of significant height and all basement walls will generally require a moisture barrier. Check with local building code requirements and common practice.

14. Finish grade (FG) – should be shown on the detail as meeting frost depth and also the minimum clearance between unprotected wood. Also the FG should be shown and specified to slope down and away from the stem wall. A typical slope requirement would be Ã?½ ft in the first 10 ft. Grade sloping down and away from the wall is very important in directing surface water away from the foundation.

15. Anchor bolts have been discussed in other lessons. The should be shown on the Foundation plans as their placement is the responsibility of the Foundation Contractor. Specifics of the anchor bolts may be noted as being on the Bracing plan or other sheets.

16. Sill Seal – a moisture/vapor barrier between the mud sill and the concrete is becoming common practice.

17. Pressure treated sill plate. Wood in contact with concrete is typically required to be decay resistant or preservative treated. Exceptions may be possible with interior foundation elements or concrete that is not permeable and is protected from moisture accumulation. Check with local building code requirements and common practice.

18. Crawl space requirements, where applicable, should be shown and/or noted. Typical requirements might include:

Clearance between joists and earth (18 in.)

Clearance between beams or girders and earth (12 in.)

Vapor barrier over crawl space soil

Crawl space opening (18 in. x 24 in. min.)

Other (pea-gravel or other fill type might be required)

In areas of significant frost depth the min. crawl space clearance is generally easy to be met. However, in areas of shallow frost depth, clearance requirements may dictate the stem wall height and / or footing embedment. Again, check with local building code requirements and common practice.

19. Interior slab – detail where applicable. The slab should be designed to float freely with respect to the foundation wall, unless designed specifically otherwise.

20. Energy code requirements (not shown) vary from jurisdiction to jurisdiction. Check with local building code requirements.

21. Original grade vs. finish grade. The finish grade around a site is not necessarily the same as the original grade, and, in fact, is often different. On sloped sites the original grade (OG) may be (have been) significantly above FG on one portion of the foundation, and quite below FG on the other. Several important things are worth pointing out here: first, regardless of the FG location, the footings should be placed on soil not less than 12 in. (or some other number; see local requirements) below original grade. In places where the OG is relatively low on the building profile, additional excavation, backfill, or stem wall height might thus be required. Second, where the depth of penetration of a foundation into the soil profile varies signification, for example, on a very sloped site, it may be advisable to consult a soils engineer with regard to (preventing) differential settlement of the soils.

22. Stepped Footings and Foundation. On sloped sites the footing (and foundation wall) must be stepped. (Actually, there may be some allowance for mild slope of the bottom of the footing, but the top of the footing must be level.) Actual step heights and locations may be left up to the Contractor to accommodate conditions, as long as the frost depth and other requirements are satisfied.

23. Foundation drain outlet. Once the foundation drain `leaves’ the foundation area it should be non-perforated (protected from further moisture penetration) and should be sloped so as to naturally drain to an `approved’ storm or other system. In some cases the drain may be allowed to `daylight’ to grade. (`Approved’ means `approved by the Building Official‘.)

24. Roof drains – should NOT be allowed to drain into the foundation drain system or area. Let me say it here: water and foundations are a bad mix. If I were to rank the leader in cause of foundation problems, it would be water, and then poor foundation soil (though it may be the other way around, and the two are not necessarily unrelated).

25. Other – `Chamfer’, color, or other features.