Current Work:
We are excavating a very rough 5′6″-5′11″ basement with sloping and uneven patched floors to a new 7′6″ finished height. The new basement perimeter walls have been engineered to carry the load of the old structure, effectively replacing the old foundation with a new code compliant system. There will be a new basement bathroom and laundry area.
As our client is a contractor who specializes in finish work, we are only performing the excavation, concrete and structural framing work, and the client is acting as his own general contractor managing the other trades. We are coordinating with the plumber and electrician for the ground work that has to take place before the foundation footings and walls are poured.
In the process of the excavation, we discovered 1) a total foundation failure in a previously remodeled area which would explain a heavily sloping area of the first floor, and 2) that the kitchen drain connection to the main sewer line was in tatters, possibly accounting for a significant amount of the water intrusion into the basement.
Blue print courtesy of Nava Contracting & Engineering, Inc.
General:
Most of our basement clients are growing families in 1910-30 houses in relatively close-in Portland. They like their neighborhoods and schools. They have limited lot sizes. There may be space for a traditional addition, but they are reluctant to lose yard space. A full scale second story remodel may be in the future, but often there is an existing converted attic space. They don’t want to move to newer larger construction further out from downtown, but they still need more space.
Our typical estimates for a basement dig out with finish come out between $30,000 and $75,000, but there are a great many variables involved. Existing conditions and designed goals can swing square foot costs from $50/sq ft to $150/sq ft or more.
Note that we do structural basement work and excavation with shoring, but not house lifting. Going from no basement to a full basement is usually best done by lifting the house, so in most cases that would not be something for us .
Four common situations:
Excavation
- It is possible to convert a half basement into a full basement.
- It is also possible to shallow dig out a basement floor in order to obtain some extra ceiling height for code compliance, to improve the floor, to install radiant heat, or to bring two sections of a basement floor into plane with each other.
- Excavation is often less expensive than building up, and in some cases it may be necessary before building up.
The basement dig shown below converted an existing 6′+ tall half-basement to a 7′ 7″ full-basement. The existing basement was full height on one side of the main bearing beam and a crawl space on the other. When finished, the project will have, radiant heat in the new concrete slab, a new egress window, a new full bath room and add approximately 1,000 square feet of finished living space. The engineering called for 1/2″ thick steel C channel scabbed onto the original bearing beam.
The photos below are from a basement that was over-excavated by a prior owner. The crawlspace earth had been removed to near flush with the foundation walls, no new concrete had been added, the shoring was inadequate and the house had started to sink in that corner. We installed extensive temporary shoring (not shown) while building the new walls and slab. The current owner also asked us to install foundation anchor plates as a seismic retrofit which allowed him to obtain an earthquake rider on his home owner policy.
“Blind Side” Pours
Typical residential concrete wall work employs double-walled forms. The opposing walls are connected to each other by ties that are embedded in the concrete. The concrete is poured between the walls, and the weight of the fluid concrete presses outward and downward evenly against the forms. The forms need to be very strong and rigid, but the forms are relatively self-supporting. Only modest external bracing is needed to keep the forms from tipping over.
In our basement work, the wall forms are single sided or “blind side” forms, where the earth is used as the outside of the form. It’s not practical to anchor the forms to the exterior earth, so all of the concrete weight and form bracing is on the interior side of the basement. The loads involved when pouring concrete are staggering, easily measured in tons per square foot at the bottom of 5′ tall 8″ thick wall. Bracing and forming blind side walls is much more involved than conventional wall work.
Form work for a typical residential stem wall
Bracing for a blind side basement pour
Monolithic Pours, Thickened Slabs and Staged Pours
A basic concrete wall might be poured in two or more stages. First the “horizontal” footing is poured, and later the vertical wall is poured on top of that. Sometimes the wall pour is broken into stages if the height of the wall and available resources make pouring a particularly tall wall in one event impractical.
The more concrete poured at a time, the lower the job cost. Typically, every individual pour requires a pump ($300 $400 per pour), and concrete, like most materials, is more expensive when ordered in smaller quantities. Consolidating labor, set-up, tear down and transportation costs can lower the overall job cost significantly.
It’s now common now for residential wall footings and the walls that go on top of them to be poured as a single event. The form work is a little more involved, but not so much that savings from a “mono pour” are off set.
It’s also possible to incorporate the footings under the walls into the concrete slab that is to be the floor of the new construction, or basement in our case. The slab and footing thicknesses are specified in the engineering process, but the thicknesses usually do not need to be added together if the footing and slab can be poured together. Combining the slab and footing (a “thickened slab”) means less earth has to come out of the basement and less concrete has to go in, not only in the floor assembly but the new poured walls are so much shorter as well.
Basements in Particular
Blind side pours, i.e. basement walls, and monolithic pours don’t really go together very well.
In-floor radiant heat requires a suspended or floating concrete slab floor, as opposed to a thickened slab. The radiant heated slab has to be separated from the earth and any footings by at least 2″ of rigid foam insulation in order to avoid losing all the heat gain from the system into the ground below.
Most importantly, until some concrete goes in, it’s very difficult to hold blind side or monolithic forms in place. No matter how elaborate and robust the form work is, forms can move out of line and they can float up as concrete flows under the forms and lifts them. If the walls are to be finished, it may not matter if the sections are slightly out of alignment. They will still be structurally sound.
A Thickened Slab
In this case the excavation was completed prior to our work. We were asked to poured a monolithic thickened slab with wall footings and then 6′ tall 8+″ thick retaining walls.
For this single side or “blind side” pour, we first built the interior stud walls and sheathed them with 1″ xps rigid foam board. After that we inserted brace panels and layered on walers and massive strongbacks. After we strip the forms, the exterior walls are already framed and ready for trade work.
Underpinning
- Done when a foundation wall (and so the house) is settling or when there is a concern a wall will settle, such as when a second story is to be added.
- The foundations of most older homes in Portland do not have a spread footing under the foundation wall, and a “knife edge” foundation wall can slowly work its way deeper over time.
- Shifting, wet soil and a shallow foundation also contribute to settling.
- A section of earth can be dug out under the old wall and a concrete footing or pad can be poured under the old wall.
- The new underpinning pad does not typically need to run the length of the wall.
You can see the difference between a foundation wall with a spread footing and without one on the bottom of page 4-4 in Chapter 4 - Foundations in the 2008 Oregon Residential Code.
Finishing an existing basement space
- This is a great way to add living space once a basement is sufficiently dry, meets building code requirements for living space, and you are sure that the soon to be concealed foundation walls will not need any work in the future, such as in the case of the situations described above.
- If you are finishing a basement now, but there is a chance you may build upwards in the future, you might consider underpinning before finishing the space.
- Wet basements are best dealt with from the outside by redirecting the damaging water, often with french drains.
- Literally waterproofing an existing basement foundation wall usually involves excavating the exterior perimeter.
- Sometimes, an interior drain system and a sump pump can be installed underneath a basement floor.
- Code compliance issues usually involve height clearance, egress and access (stairs). See BDS’s brochure on this.
- If you don’t have any code or water issues, finishing your basement is likely to be a home-run with a likely significant gain in property value.
- Talk to a few realtors first to confirm any expected appreciation.
Interior / Non Structural Example
- New 2×4 stud walls were framed into the existing space
- 1″ rigid foam insulation between the concrete exterior walls and the new wood framed walls
- The stud bays were filled with r13 batt insulation below grade and r21 above grade where space allowed
- Much of the finish trim was mdf, but we used finger-jointed wood trim baseboard and plywood against the windows and doors for added moisture resistance
- The client chose to use flor.com carpet tiles which are little more expensive than typical glue-down carpet tiles but still a great choice for basement flooring where you are going directly over the old concrete slab
- Rebuilt the basement stairs to meet code guard and hand rail requirements
- tapped into the existing HVAC system for forced air heat
- used existing recessed ceiling lights and wall outlets to meet code
Before:
After:
Egress Windows
If you want to turn your basement into pemitted living space, it needs easy and clear access in case of an emergency. Unless the stairs to the basment lead straight directly outside, the basement needs an Egress Window, and we can install these.