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what are the steps to strengthen pier and beam (sometimes called post and beam) foundation?

i will be installing a 3-5 ton Rocket heated cob bench mass heater in a rambler home in the living room over 8' length of floor space that has no beams beneath it.

i was told "If you have a suspended floor, the extra weight will probably need additional structural support so your floor doesn’t sag."

my research suggests that to reinforce beneath this heater, we would add 1 or 2 6' 4x4 beams going N to S beneath the joists (which go E to W. these would be nailed into posts nailed into pier blocks which would support the beam which would support joists.

how do i know where the spot i need to reinforce is when i'm in the crawl space?

how much weight can floor w/crawl space beneath, support ? the rocket heated cob bench mass heater shouldn't be more than 135 lbs/sf. i asked the local code officials what weight the floor itself is rated for but they didn't give me a clear answer .

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2 Answers 2

The new support structure with a beam that you are proposing sounds fine.

If you are concerned, then overdo it by doubling the materials for the beam, or placing a steel L-rod 3/8" thick around the beam. Using an L-rod will reduce the number of bolts that you need to put through it into the beam, and will add structural support due to the angle.

The more important thing to remember here is what the pier block sits on. If you have bedrock or a similar rocky aggregate under the house, then you may be able to safely rely on it not shifting or changing over time.

If it is just soil, you can expect that you will get some compacting due to the weight of the heater. Even if you are able to bring in a tamper and pack the soil, you can still expect that over time it will change through natural cycles in the ground. Instead, place the broad flagstones under the peers to distribute the weight.

Make sure that the flagstones are thick enough that the weight will not crack them – since the breadth of the flagstone is being used to distribute that weight across the soft soil. Do not use metal, as it will rust and deteriorate.

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A typical residential first floor would be built to a minimum live load of 40 PSF. A dead load of 135 (plus any live loads) will stress things just a bit. There's no particular guarantee that your floor meets this standard, it's just a typical one. The odds that it comes anywhere near supporting 135 PSF without help are essentially zero.

Fundamentally, you add more posts and piers to transfer the additional load to the ground. You may also add more joists in parallel with the joists you have. Beams are somewhat optional, but can be useful. You know where to put things by measuring. You know how many you need and the correct sizes by hiring a structural engineer. Or just cut the floor out and build your mass heater from a foundation on, or more typically in, the ground (aligning with typical practice for masonry heaters and removing the excess load from the wooden structure.)

Here's a link to a nice bit from a structural engineer about placing fishtanks (another high load object, though a bit smaller in scale) that might shed a bit of light on floor loads, what we might know about the loads a floor should hold, a great deal of things we don't know about residential floors as built, and even why nobody will tell you what yours will hold.

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