Getting ready to finalize plans on our new house in Maryland. Basement will be walkout/daylight that has 5 full size windows, 3 french doors and lots of corners. Total footprint of basement is about 3900 sf, garage 1200 sq ft. About 50% lin ft of walls will have 7' to 8' of backfill (includes garage wall), 30% walkout area and 20% stepped. Still deciding between 9' or 10' walls but leaning towards 10'. If I go with poured I plan on 10" walls with a 4" brick ledge and with block 12" and 8" block. Entire exterior will be brick veneer. I will be GC on this. Have not been in this business recently but built quite a few homes back in the 70's in Michigan before interest rates and the auto industry sent me in a different direction. Back then everything I did was block. Now it seems as though all non custom developments around here are poured while custom homes are around 50/50 (with a few Superior thrown in). Costs certainly need to factor into the equation but beyond that I want to have a quality product. Any suggestions or direction on either side of the fence would be greatly appreciated. Jeff
Builders that I work with like block foundations, because as they build them, they can corect minor mislignments and end up with an accurately square foundation.
With concrete, often when the mix is poured into the forms, the forms shift slightly, I've seen concrete foundations end up as much as 6 inches out of square because of this.
For my taste, a properly parged block foundation looks best also, compared to concrete, no matter how they finish it.
In many areas, builders are accustomed to one method and not the other. In Rocjland County, New York, where I practice, block foundations are almost universal. In Long Island, New York, less than 50 miles away, one seldom sees block, and concrete is nearly universal.
Whwn both are properly built, there is essentially no difference, but as "homebild" pointed out, a block foundation will more easily permit future modifications or repairs.
Architect (NY) and Home Designer (PA)
Posts: 2546 | Location: Tobyhanna, PA | Registered: 24 October 2005
Your prior participation in the construction of homes in the 70's with CMU Foundations (concrete masonry units) will allow you to oversee the quality control of your masonry contractor throughout the entire project with a certain level of comfort.
Proper construction technique is the key to either method used. Both methods being considered have equal advantages as well as disadvantages.
As a Structural Engineer I also like the use of block in foundation applications however my designs over the past 30 years have leaned towards cast in place concrete foundations.
This was a direct result related to the soil conditions in our area. Other factors included avaliable skilled labor, labor cost per sq ft., material cost and avaliability, ground water levels, soil bearing capacities and lateral pressures on the foundation. (just a few of the considerations)
YOUR CONSIDERATIONS SHOULD ICLUDE: size of block, footing size, lateral and vertical reinforcing, solid grouting or bond beam installation, foundation insulation, water proofing, foundation drainage systems, compaction of backfill to 95% relative density with satisfactory materials (spending a little additional time on the backfill phase is important to help insure that no damage occures to the waterproofing, foundation insulation and drainage system), striking of exterior joints (reduces moisture infiltration at all joint locations)
Quality and personal satisfaction may also offset any additional cost that may be occurred over the cost of a cast in place foundation, if any.
A properly constructed CMU foundation will perform very well for your new custom home and is a method that you are famililar with.
Our custom home in Colorado has a CMU foundation , constructed 20 years ago without any failures noted.
Good Luck on the construction of your new residence.
Homebild, Richard, and Lee, thank you for the replies. It's very kind of you to take the time to monitor and reply to folks asking you questions. It was good to hear from each of you on your take on CMU's. It certainly is a direction I prefer given my previous experience with it. I do have another question. I am going to end up with approximately 8'-3" of unbalanced fill. I know I will need to get an engineers approval/stamp because of this but can anyone give me an idea of what I should be looking at for horizontal and vertical reinforcing. My preference would be to use solid grouting as opposed to installing a bond beam but would certainly appreciate any thoughts on that topic as well.
By unbalanced fill, do you mean the dimension from the outside grade to the basement floor? Usually you need no special arrangements such as reinforcing or bond beams, if you use the correct thickness of block, with masonry reinforcing at least every second course.
The floor joists will brace two of the walls directly, and the other (presumably shorter) walls would be braced by the diaphragm action of the subfloor.
Make sure the floor joists and subfloor are in place BEFORE you backfill, and read Licensed Waterproofer's many posts about how to and with what to backfill, and with what to waterproof the walls.
Architect (NY) and Home Designer (PA)
Posts: 2546 | Location: Tobyhanna, PA | Registered: 24 October 2005
Richard, yes that is what I meant by unbalanced fill. In Baltimore county typical reinforcing only covers unbalanced fill up to a maximum of 7 feet. I will definitely not be backfilling before the floor system is in place and thank you for mentioning License Waterproofers's posts.
International Residential Code, which Maryland seems to use statewide, does not proscriptively deal with foundation walls taller than 9'.
Even if the wall was 9' tall with 8' of unbalanced fill, it is not possible to tell you whether you need reinforcement, what size the wall thickness should be, or how to deal with reinforcement (if any) without first knowing the soil conditions.
But as an example, a 9' high foundation with 8' of backfill in 'good' soil can use hollow 10 inch block with no reinforcement or grouting whatsoever.
If the soil is the extremely poor, you could use 8" block with #6 rebar at 24" on center; 10" block using #4 rebar at 16" on center or 12" block using #6 rebar at 48" on center.
There are also provisions for alternate reinforcement and spacing.
Without knowing your exact wall heights and soil types, however, it's anyone's guess.
To all, another question. Have been thinking of doing 2x6 frame walls for the walkout portion of the basement instead of block (App 129 lin ft.) The frame walls would be on top of 8" CMU's over 12" CMU's to form a brick ledge. As a note, there will be 2 stories above the walkout basement. My thinking on this is 1) there are a lot of windows and doors in this area and I would prefer to have standard 6 9/16" jambs versus what I end up with 8" block and 2X4 walls. 2) Beefed up insulation. 3) Applied house wrap to the framed wall and good flashing techniques should help me as well. 4) Slightly lower cost. If anyone has any downsides (or upsides) that you would like to share I would appreciate it. Jeff
Jeff, been out of town this week as well as next week on one of our state projects. Thought I'd provide a few items for consideration prior to leaving this morning.
CONSIDERATIONS: 1.0 Maximum active earth preassure with an equivalent fluid weight of 30 pounds per cubic foot.
2.0 1,500 psf maximum soil bearing value.
3.0 Footing Width 32 inches.
4.0 Footing thicknes 12 inches
5.0 Top of footing key way at centerline of CMU.
6.0 Minimum distance from face of CMU to face of footing (exterior face) 6 inches. 7.0 Footing reinforcement 3 #5 bar continouos
8.0 Vertical steel #5 bar placed at 24 inches on center. Cold bend 90 degree from footing to CMU, project minimum of 30 inches
9.0 Reinforcing steel lap splices not less than 40 bar diameters all locations.
10.0 CMU 12 inch.
11.0 Horizontal steel in CMU starting at second row of block and continued at every other row above. Use of bond beam block at all horizontal reinforcing locations.
12.0 Type S mortar is satisfactory if entire CMU wall is grouted. If not Type(s) M & N mortar is requires.
13.0 Pre-inspection prior to CMU grouting. Grout should be placed in a continuous pour to not less than the middle of the seventh row to no greater than 2 inches from the top of block of this row. Balance of grouting to top of wall in final pour. (not to exceed 6 feet) NOTE: Full height groutiing if cleanout holes have been provided in the first row of block.
14.0 Sill plate anchors 1/2 x 12 @ 32" o/c typical. (not to exceed 48") 15.0 Footing concrete 3000 psi, 4 in. slump, 6 % entrained air.
16.0 Grout mix 2000 psi minimum, 1 part Portland Cement, 3 parts sand, 2 parts 3/8-inch aggregate (pea-gravel), If hydrated lime is used should be at a ratio of 1/4 part. Average of 7 gallons of water per sack of portland cement. Water shall not allow segregation of grout constituents.
17.0 Install Simpson HD10A holdown anchor bolts(or equal) at all required locations as per your local building dept. Due to type of grouting the anchor bolt length requirements will be longer or structural concrete will be used at the holdown locations.
18.0 Footing and CMU elevations will change at walk out area that is to receive 2x6 wood framing above.
19.0 Additional holdowns will be required at this location. Install Simpson MSTI strap ties (or equal) at and above holdown locations connecting lower framing to main level framing.NOTE: Additional studs will be required at these locations. Layout of doors and windows above will have to be adjusted to allow for these connections to be continouous.
20.0 Lower framing should receive 1/2 inch plywood over framing to receive brick ties.
21.0 Install approved house wrap over framing prior to installation of brick.
22.0 Window and door openings should be installed no closer than 6 feet of the end of the lower wall section that connects to the CMU allowing for a shear wall section. Plywood application at this location to receive nailing at 2 inches o/c at all edges and 4 inches o/c in the field. Block and nail all edges (blocking required every 4 foot of vertical wall) RE: your architect, structural engineer or building dept for required design in your area.
23.0 Wall frame connection to CMU shall be multi stud and bolted. Stud adjecent to CMU shall be treated. Install caulking at this location.
Aforementioned information provided for consideration and for your review and comment.
