Pier and Post vs Perimeter foundation
My sister is planning a small addition to her house 9′ wide by 22′ long. (property line issues are dictating the size) The existing floor joints run parallel to the planned 9′ wide section of the addition. My question is about the feasibility of using a pier and post foundation rather than a full perimeter foundation. Cost is an issue, and my labor is free on the project. (big smile) I think I’d be able to lay out and do the work to set up a pier and post foundation, but don’t think I’d be able to do the full foundation. I thought is is use a glulam on top of five piers and posts, each with a 2’X2′ by 10″ footing, on the outside edge of the 22′ long section of the addition, and run the floor joist from the existing perimeter foundation to the glulam and attach them with joist hangers. I’d havea 9′ span with 2″X10″ floor joists. The would be at probably be 18′ to 24″ from the bottom of the joists to the old concrete patio and we’re in California, so cold wouldn’t be an issue, although I plan to insulate the floor joist bays. I’d be appreciative of any comments on my ideas.
Thanks,
Jeffrey
Replies
If you call the TrusJoist people and send them your floorplan, they will give you a materals list and floor plan at no cost.
Thanks Mike. I'll give them a call.
Jeffrey
Your plan will likely work from a structural point of view, particularly since cold and frost are not big issues (The glulam might even be overkill.). My question is one of style. Do other houses in the neighborhood have pier foundations? If not, yours may look out of place. Since you don't have to go deep to get below the frost line, a full foundation may not be as hard as it now seems.
Al Mollitor, Sharon, MA
Hi Al, Thanks fro the thought that it would work from aa structural perspective. The addition will not be visible except in the back yard (blocked in the front by the garage) and I thought I'd wrap lattice on the bottom where it is visible.
Jeffrey
Where in California? Here in LA, I don't think DBS would let you do anything but a perimeter footing with lotsa bolts sticking up and the cripple wall shearwalled with plywood. Instead of seventeen 2x10's for floor joists, code here allows 2x6's for spans up to 9'-1". That should save you a little.
-- J.S.
John, I live in Glendale. We have a city arborist, more aptly called the tree n*zi. This guy has absolute power. My friend was adding on to his house with the standard perimeter foundation. Everything was done to code and had passed design review. Then the tree n*zi comes out to the site and says that the addition is too close to some oak trees. With a stroke of the pen he makes them change from perimeter to pier and post foundation. The engineering and changes added 20k to the job.
Hi John, we're just south of San Francisco. Hadn't thought about the earthquake angle, and will have to give it some thought even if it meets code here. Thanks for the input!
Jeffrey
Jeffrey,
First, I admit that I don't have a clue about earthquake prone areas vs. construction details, but from an Insurance Agent's point of view, I'd say approach with caution. Here in Georgia, almost industry-wide, the property insurance requirement is for a solid masonry foundation. Two very important reasons - fire hazard and damage from animals. A grass fire ( and I know you have those in California ) can be a real problem with pier construction. And we've seen a lot of damage over the years from dogs re: insulation, plumbing, and ductwork.
A pier addition is grounds for non-renewal with many companies. Again, I don't know what is the norm in California.
Greg.
Hi Greg, another angle I hadn't thought about. Just assumed that if the addition met code, there wouldn't be a problem with insurance. Will have my sister give her agent a call. Thanks.
Jeffrey
Pier and post is a good way to go. Check with an architect or maybe code officials about size and depth of piers. You could save more money if you use pressure treated 2x12's instead of gluelam, I would use pressure treated for joists if you have termites in your area. Pressure treated will shrink so glue and either screw or use screw nails.This should take care of squeaks.Fiberglass lattice looks good under the floor. You could put this in over the piers or cut in between piers if you want the piers to show.
Mike
Hi Mike, Glad to see that my plan was completely off base. I'll go with the PT on the joists. Thanks for the advice.
Jeffrey
I would advise you to keep the same foundation system as the rest of the house if possible. Unless properly designed, the piers/post foundations may settle differently and cause some problems. Generally, it is good practice to keep structural systems similar. If there are settlement problems, they will usually appear in the first year. After that, the soil consolidates. If you do decide to go with pier/post, make sure they are adequately sized. Your building official may be able to help you with that, or you can hire an engineer.