Build Foundation Walls
Foundation walls support your entire addition and connect it to the existing structure. Whether you choose poured concrete or concrete block, proper construction and waterproofing are essential for a dry, stable foundation that will last decades.
Quick Summary
Time needed
3-7 days
Cost
$5,000-$15,000
Professional help
Foundation contractor
Why This Step Matters
Foundation walls transfer the weight of your addition from the floor system to the footings below. They also resist lateral soil pressure and provide a barrier against water infiltration. A poorly constructed foundation wall can lead to water intrusion, structural cracks, and expensive repairs.
Critical Connection Point
The tie-in between your new foundation and existing foundation is the most critical junction. Improper connection can cause differential settlement, leading to cracks where the addition meets the house.
Poured Concrete vs. Concrete Block
Both methods create solid foundation walls, but they have different characteristics, costs, and construction timelines.
Poured Concrete
- Stronger—monolithic structure
- More water-resistant (fewer joints)
- Faster construction (1-2 days)
- Requires formwork rental
- Higher initial cost in some areas
Concrete Block (CMU)
- Lower material cost
- Easier for DIY (with experience)
- No formwork needed
- More mortar joints (leak paths)
- Longer construction time (3-5 days)
Our recommendation: For home additions, poured concrete is usually the better choice. It's stronger, more water-resistant, and provides a better tie-in to existing poured foundations. Block walls are more common in the South and for above-grade applications.
How to Build Foundation Walls
Plan the Tie-In to Existing Foundation
The connection between new and existing foundations is critical. Your engineer will specify one of these methods:
- Drilled dowels: Drill into existing foundation, epoxy in rebar
- Keyway joint: Cut a groove in existing wall for new concrete to key into
- Isolation joint: Separate structures for different soil conditions
Important: Never just butt new concrete against old without a mechanical connection. The joint will crack and separate.
Set Up Wall Forms (Poured Concrete)
For poured concrete walls, formwork creates the mold. Forms are typically rented from a concrete contractor or form rental company.
- • Forms must be plumb, level, and braced securely
- • Wall thickness typically 8-10 inches
- • Leave openings for windows, doors, utility penetrations
- • Install horizontal and vertical rebar per engineering
- • Set anchor bolts at top before pour
Pour the Walls (or Lay Block)
For poured walls, use the same 3000-4000 PSI concrete as footings. Pour in lifts of 4 feet maximum, vibrating each lift to remove air pockets.
For block walls, lay CMU (Concrete Masonry Units) in running bond pattern. Fill cells with grout at rebar locations. Cap with bond beam.
Pro tip: For poured walls, don't remove forms too early. Wait at least 24-48 hours depending on temperature. Stripping too soon can damage the concrete surface.
Apply Waterproofing
All below-grade foundation walls need waterproofing to prevent water infiltration. Options include:
- Tar/asphalt coating: Basic dampproofing, code minimum
- Rubber membrane: True waterproofing, bridges cracks
- Dimple board: Drainage plane + protection
Best practice: Use spray-applied rubber membrane plus dimple board for complete protection. Costs $2-4 more per square foot but prevents expensive water damage later.
Install Drainage System
A perimeter drain tile system collects water and directs it away from the foundation before it can penetrate.
- • 4" perforated pipe at footing level
- • Wrap pipe in filter fabric
- • Cover with 6-12" washed gravel
- • Connect to sump pit or daylight drain
Schedule Foundation Inspection
Before backfilling, the inspector needs to verify:
- • Wall dimensions match approved plans
- • Rebar and anchor bolts properly placed
- • Waterproofing complete and properly applied
- • Drain tile installed correctly
- • Tie-in to existing foundation acceptable
Warning: Never backfill before inspection. Once covered, the inspector cannot verify waterproofing and may require excavation.
Waterproofing Options Comparison
| Method | Cost/SF | Protection Level | Lifespan |
|---|---|---|---|
| Tar/Asphalt | $0.50-$1 | Dampproofing only | 5-10 years |
| Rubber Membrane | $3-$6 | True waterproofing | 25+ years |
| Membrane + Dimple Board | $5-$8 | Best protection | 30+ years |
Frequently Asked Questions
How thick do foundation walls need to be?
Standard residential foundation walls are 8 inches thick for poured concrete or 8-12 inches for block. Deeper foundations or those retaining significant soil loads may require 10-12 inch walls. Your structural engineer will specify the required thickness based on soil conditions and wall height.
Can I match the new foundation to my existing block foundation?
Yes, you can use block for consistency, but poured concrete often provides a better tie-in even to existing block foundations. The connection method is more important than matching materials. Discuss with your structural engineer.
Do I need waterproofing if I'm on a hill with good drainage?
Yes—waterproofing is required by code for all below-grade walls regardless of drainage conditions. Even in dry areas, moisture can migrate through concrete causing mold, efflorescence, and damage to interior finishes.
How long before I can backfill after waterproofing?
For spray-applied membranes, typically 24 hours for the coating to cure. For self-adhering sheet membranes, you can backfill immediately after installation and inspection. Always get inspector approval before backfilling.
Ready for the Next Step?
Once your foundation walls are complete, waterproofed, and inspected, you're ready to begin framing. The next step is installing the sill plate and floor system, then framing the walls and beams.