Adding Second Story On Existing House
by John F Mann, PE
Adding a new second story on an existing house can often be a cost effective way to increase living space. However, the following basic structural requirements must be addressed;
(1) Capacity of existing foundations to support new weight (load)
(2) Design for wind resistance
Various other issues may also influence structural design, such as protection of living space during construction.
Capacity of Existing Foundations
Unless existing foundation elements bear on rock or are underpinned, the addition of new weight (load) to an existing building must cause settlement due to compression of soil that currently supports existing foundation elements.
The key issue of course is how much settlement can be expected.
Expected settlement can be estimated by calculation only if detailed information about underlying soil is available. In general, at least one soil boring would have to be taken to obtain such information about soil under existing foundations. Two borings are preferred.
Cost of one soil boring, taken to a depth of 20 feet below base of existing foundations, is likely in the range of $1,000 to $2,500, depending on factors such as general location, ease of access for the truck-mounted drilling rig and drilling effort (time).
For the vast majority of residential projects, soil borings are not taken unless there is evidence of existing settlement problems with the house or in the local area. Another reason would be if walls of the new second story were to be masonry (on top of existing masonry walls).
Key problem with evaluating settlement potential of soil is that the relation between applied load and soil compressibility is not linear. In practical terms, this means that, at some level of load, the rate of soil compression may increase much faster than before such level of load.
Yet, despite the various risks, experience with adding a new, wood-framed second story shows that, unless settlement is already a problem (for existing house), existing foundations can support load from the new second story with minor settlement.
However, the following conditions must be evaluated;
(1) General condition of foundation walls to determine if remedial work is required.
If walls around an existing basement are cracked and pushed inward (from lateral soil pressure), careful structural evaluation is essential. Extensive reinforcement or even replacement of damaged foundation walls may be warranted.
(2) Whether there is a separate footing under foundation walls.
For many (perhaps the vast majority of) houses built before 1940, foundation walls were built directly on soil, without a concrete footing. Soil pressure at base of foundation wall will therefore be much greater (for existing and proposed conditions) compared to foundation wall on a concrete footing that is wider than the wall. However, as long as base pressure remains relatively low, overall settlement potential will be essentially the same.
(3) Depth (below existing grade) of existing foundation elements.
For entirely new construction, the building code includes a standard requirement for minimum depth of foundation elements below finished grade. In New Jersey, this depth is typically 36 inches (north half of NJ) or 30 inches (south half of NJ). Primary reason for this requirement is to protect against "frost heave". However, another reason (often not stated) is to ensure that footings bear on firm soil, below compressible soil often found near the surface (topsoil, fill).
For many existing houses built before 1945, depth of footings (or base of foundation wall) is less than current requirements. As long as there is no evidence of prior frost heave or settlement, and as long as the footing is at least 18 inches below grade, such conditions should be considered acceptable. However, a report by a qualified professional engineer is often required by code officials when such condition is encountered.
(4) Whether additional load will be applied to existing first floor framing, including main girder or beams that currently support first floor. Such load (from new second story) would be applied through walls or columns on the first floor. New columns and footings may be required.
In general, size and capacity of existing footings that support existing interior columns or piers (in basement or crawlspace) is not known without detailed inspection. Even if design plans might be available, showing footing sizes and details, some inspection to verify as-built construction may be warranted. Such inspection typically requires drilling holes through existing concrete slab around columns to determine size of footings.
An alternative to detailed inspection of existing column footings is to install new columns and footings based on conservative assumptions about existing columns and footings.
New footings can be placed on top of an existing concrete floor slab as long as the slab has firm support from underlying soil. Such firm support is generally indicated by lack of severe cracking. The slab is certainly as good as a layer of compacted soil or crushed stone.
(5) Potential for damage to finish materials, especially tile flooring
Minor settlement can cause cracks in finish materials, such a drywall and flooring. Tile flooring is especially prone to cracking in response to movement.
Design For Wind Resistance
Any building must have capacity to resist wind pressure, including wind uplift pressure on roof surfaces.
For new buildings, the building code includes extensive provisions related to design for wind resistance. However, even without prescriptive code provisions, the building must be designed to resist wind pressures specified by the code, which includes hurricane force winds where applicable.
For a new second story added to an existing one-story house, design for wind resistance can be complicated. For many such projects designed by architects, this issue is all-too-often almost entirely ignored.
At the very least, new roof framing should be properly tied down, just as for new construction, to resist wind uplift forces.
The first key issue is connection of new second story to existing building below. Connections must resist tension force caused by; (1) Wind uplift applied to roof, and (2) Overturning due to horizontal wind pressure acting on walls (both inward and outward).
In general, steel straps can be used to securely tie new walls to existing wall studs. Careful attention to detail is required near corners, especially if new walls include large openings.
Much more difficult is evaluation of tiedown requirements between existing walls and existing foundations. Wind force applied to the new second story greatly increases overturning forces at base of walls on the foundation.
A simple moment calculation can be developed for a rectangular house with wall height H, length of windward wall L and wind pressure W.
For one-story house, total wind force is WHL and overturning moment is WHL times (H/2) or W(H^2)L/2.
For a two-story house, with second floor wall height of H, total wind force on second story is WHL. Additional overturning moment (at foundation) is WHL times (H/2 + H) or 3W(H^2)L/2.
Total overturning moment is then 2W(H^2)L, which is 4 times greater than for the one-story house.
Wind force applied to walls is distributed vertically to foundation and a horizontal "diaphragm", which is ceiling or roof for a one-story house. This diaphragm distributes forces to the side walls, which act as shearwalls (braced walls). Overturning moment must be resisted by shearwalls, along with shear force.
Shearwalls are always parallel to direction of the wind force.
For a one story rectangular house, with 8 feet high walls, overturning moment (for each shearwall) is calculated as;
M-ov,1 = w psf x (8 feet / 2) x (L feet / 2) x 8 feet = 16wL ft-lbs
w = wind pressure (psf), assumed uniform for simplicity
L = Length of windward wall (feet)
For a two-story rectangular house with the same plan dimensions and 8 feet high walls, additional overturning moment at base of shearwall (on foundation) is calculated as;
M-ov, 2 = w x (8 ft/2) x (L/2) x (16 ft + 8 ft) = 48wL ft-lbs
Total overturning moment = 16wL + 48wL = 64wL
This is the same basic result derived for the overall overturning moment previously calculated.
Even with this large increase in overturning forces, extensive remedial work (to install new tiedowns into existing foundations) is often not required considering resistance from weight of house and typical plan dimensions. However, for narrow houses, and especially for houses near the ocean, careful evaluation is warranted.
If detailed analysis shows that new tiedowns are required, complete reconstruction of the foundation at each tiedown location may be necessary to provide adequate uplift resistance.