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I have a client who wants to add a 3 seasons room to the rear of their house. It would be difficult to get heavy equipment to the rear of the site to dig the foundation. I have been toying with the idea of pouring 1′ in diameter piers to support the structure. I’m not sure if this is acceptable with it being attached to the house. Has anyone tried this? Any feedback would be appreciated. By the way, the room is 16′ x 14′.
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If you use piers make sure to use an apprpriate sized grade beam around he perimiter. If you don't the piers could move independently. My mothers home had a concrete porch added this way and I have never seen any problems with movement.
What is your frost line depth? How is it going to be attached to the house? What type of foundation does the House have? These are questions to answer. I would also look at some other additions in the area.
*I've done several room additions on piers when cost was a factor. I pour in 12 " sonotubes frost line here is 3'.Just make sure you use enough piers to prevent them from sinking. I roughly figure the weight of the structure, all live and dead loads, and then figure out how many piers are needed based on the soil bearing capacity. I don't agree that you would have to tie the piers together to prevent them from shifting.
*A foundation Engineer once told me that it's not what's under the pier that counts, but what's around it. The resistance to movement comes from pressure of the soil around the sides, not the measely 1' circle on the bottom. He also specifies piers that are much deeper than many of the local builders use with stipulations that the concrete hit the hole the same day as the drilling (otherwise the previously undisturbed deeper levels have time to de-compress).Now maybe that's just engineer overkill, but I've seen quite a few decks and add-ons that have done some unsightly moving over the years, mostly (I think) because the piers weren't drilled much deeper than the frost line. So I'm inclined to take him at his advice but we also have to build in gumbo clay, so there's probably some room for regional differences.As with any addition, DO try to determine if there's any regular or seasonal movement of the original house foundation first. It will be the new piers that will take the blame if you find them holding rock solid while the rest of the house floats and sinks.
*......have you thought about pole-barn construction?PT 4x6 posts treated to .060 retention...if the addition is at grade, your bottom horizontal can be PT 2x10..with an insulated slabif you have a crawl space you can enclose the space with PT 3/4 ply....we've done about 10 additions using modified pole-barn constructionThe addition on this 24' x 24' cement block building was 20' x 26', all pole-barn.
*Scott,I think you're talking about a friction piling. I'm no expert in this either but I was under the impression that for friction pilings to work, they had to be very, very deep. I've heard of driving pilings 40' deep in soil.I know that it's also pretty common to pour 1' diameter footings in sonotubes on top of 2'x2' footings. I've not had trouble with just the 1'D footings if I use enough. We have alot of clay here but there's alot of sand too.
*...piers....piles.....posts....each can be designed to do the job.....most of our soils here will support 2000 psf.. so you can get a lot of bearing on a 12" diameter footing......friction piles are driven to point of refusal... they know the driving force of the hammer.. and can convert blows per inch of drive to bearing capacity....some of the steel h-piles we used to drive were driven to 120'
*Foth,In my eyes, a permanent addition like this attached to someone's home is worth the added expense of a foundation to match the house. There is a lot of equipment or persons to hire with smaller equipment that could get back to the site, more than likely. I've seen a lot of these types of rooms built on glorified decks, Patio Enclosures comes to mind, that don't look quite right, if you know what I mean.They don't add the value to the house like the conventionally built rooms do. If you really can't go that way, it is acceptable to do it the way you asked about. I would probably put 4 piers along the 16' width and another line of them 1/3 to 1/2 way away from the house and go from there.Airborne,Is a grade beam just a term for the beam between the piers that the joists set on? Or is it something else? Thanks.Red dog
*RedDod, A grade beam is a concrete beam that spans between piers. It is Usually the same size square as the piers diameter. It is tied to the piers with turned bar and also tied to the wire mat of a slab, as well as having bar running its length.Most of the time it is used in a monolithic pour.Picture the underside of a parking garage. The beams you see are what we term Grade Beams. It is that the structure is set at "grade", hense the name.I first learned about them in the Army, while attending The Engineer School at Ft. Leonard Wood, MO. As I was taught they are ideal for loose, clay like, or heaving soil. Provided your piers are rated for the load and are sunk to the proper depth the soil can shrink, erode or wash away and the structure is solid.I built a garage several years back on sandy soil near the water. It was 24 x 18 and I speced out the sizes based on my handy dandy Engineer Bible and sent it to the Permits Office. The County Engineer checked it out and approved. Since then I know of three other garages the county had don the same way.Incidently VERY LOW impact on the area.As a side note The Engineer Bible is a Field Manual for the ARMY. the number is FM 5-34. If you have a way of getting this put it in your tool box. Its Water Proof and has more raw data in it then you could ever use.
*May be just regional termonology but I thought a grade beam was a thickened portion of a slab like a beam made of concrete that was used as a footing for a load bearing wall within a slab. It doesn't necessarily have to go below frost line because it is under a heated space but it does need to sit on undisturbed soil.
*Grade beam here means pretty much the same as Airborne described. Typically 2' high by 8" wide with bar in the top and bottom sitting on void form. Unlike a footing it literally becomes a solid beam made of concrete and is totally self supporting. The void provides a crush zone for ground heaving and the weight of the beam and structure over it us directed onto the piers, not the ground. Keeps the walls rock solid. The floor is another matter....