Is Manchester and hearth pad too heavy for existing joists?

[Anthony W]

I'm entirely new to Hearth forum and to wood stoves.
Had a traditional, original fireplace in my previous house, no problems.
Bought a Hearthstone Manchester for our current home (hasn't been installed yet) and am beginning to lay out hearth pad. My first plan was Hardie board installed on top of existing LVT and then ceramic tile in the usual way. But, then I remembered that there is a good amount of flagstone left from the previous homeowner. I could piece it together with smaller stones or there is actually a slab large enough to be used in one piece to meet my requirements. This all got me thinking of the combined weight of the stove and pad (whichever material we end up using). The stove is 550 lbs and I'm not sure of the other weights. The joists in the basement have been reinforced to some degree (before we had thought of a wood stove) but I'm looking for oppions on if it's enough.

Some info;
Joists are old, true 2x8, 16" center to center, and have been sistered with 3/4 ply.
Span is 13'6"
They rest on exterior round stone wall on one side and are notched into 8x8 beam on other side, reinforced with Simpson strong tie brackets and steel angle. We'll be setting stove on stone wall end.


Any info/suggestions would be appreciated.

God be with all who read this and in charity lend some help

 

[begreen]

It's probably ok. The weight will be distributed across several joists. Do they have 16" or 24" spacing?

 

[Anthony W]

Stove will go in this location on floor above

 

[Anthony W]

It's probably ok. The weight will be distributed across several joists. Do they have 16" or 24" spacing?

16"

 

[TheBigIron]

They are probably going to be fine considering there 16” on center, you could add joists or bracing between existing joists if your worried. Maybe contact a builder ask his opinion, just my thoughts.

 

[begreen]

Our almost 600# stove + heavy hearth pad is on joists on 24" centers. (well sort of) The joists are true-dimension, doug fir 2x8's . Hasn't fallen through the floor yet.

 

[jalmondale]

How many square feet is the hearth? If it's 5x3, and you assume 100 lbs for the slate/hearth, that's going to be about 45 lbs/sq ft on the joists. That's cutting it pretty close with nominal 2x8s (according to https://www.southernpine.com/app/uploads/SPtable3_060113.pdf), but if these are true 2x8 plus plywood reinforcing, you're probably fine.

 

[Anthony W]

How many square feet is the hearth? If it's 5x3, and you assume 100 lbs for the slate/hearth, that's going to be about 45 lbs/sq ft on the joists. That's cutting it pretty close with nominal 2x8s (according to https://www.southernpine.com/app/uploads/SPtable3_060113.pdf), but if these are true 2x8 plus plywood reinforcing, you're probably fine.

Roughly; It'll be 57in x 40in. So by my math that'll be about 41 lbs/sq ft.
They are true 2x8 with 3/4 ply on both sides, totalling 3 1/2in thick (slightly more thick as doubled modern nominal 2x8) am I thinking about this correctly?

 

[Supersurvey]

Google is telling me residential floor code is 40 lb/sf.

 

[jalmondale]

Roughly; It'll be 57in x 40in. So by my math that'll be about 41 lbs/sq ft.
They are true 2x8 with 3/4 ply on both sides, totalling 3 1/2in thick (slightly more thick as doubled modern nominal 2x8) am I thinking about this correctly?

I'm not enough of a building engineer to say precisely how that affects the span, but I think it's reasonable to say that if a nominal 2x8 is a few inches under span (for a 50 psf live load, to leave wiggle room for someone to be on the hearth pad to load the stove), then your setup should be fine. At the very least you're almost half way to a 12" oc spacing, and nearly halfway to a nominal 2x10. If you're really worried about it, grabbing a couple lally posts to support the joists directly under the stove legs might be a cheap and easy peace of mind, but I don't think it's necessary.

 

[kennyp2339]

For piece of mind you could always run a few 2x4's or 2x6's down supporting the floor joists, pad it out and make a small closet for coats in the basement.

 

[Bad LP]

A closet or quick load bearing wall to the floor under the stove area is not a bad idea. It's not the load that bothers me but more the bouncing floor as you walk by.
If those plywood sisters are thru bolted with 3/8" hardware with washers running thru 3/8 drilled holes versus just screwed into the 2X8 joists it ain't going anywhere.

 

[begreen]

If the stove will be located on an outside perimeter wall, then floor deflection should be almost non-existent.

 

[rwhite]

Hopefully I'm not way off, but this is how I understand span and load limits. A 2x8, 16" oc can have a max span of 12'9" and be rated at 40lbs sqft live load (from span tables). With a true 2x8 and the sisters you're probably okay at 13'6" but that's my uneducated guess. You take the area you will be spanning 57x40 and that gives you 15.83 sqft. Sqft x rating= 633lbs that can be safely placed in that area. Without being an engineer, I assume that's the safe working load at the weakest point (probably the middle of the span).

 

[rwhite]

I would think you can also test deflection. IIRC floors should be a L/360 rated. Span in inches/360 =max deflection. At 13'6" that's around a 7/16" deflection. That seems like a lot to me. String a plumb bob from the joist down to a board on the floor, say 1/2" chunk of plywood as a gauge. When you get everything in place see if it has deflected at all and if the plywood will still slide under the bob. If it does not fit you probably need to brace your joists.

 

[Max W]

Looking at your floor supports again I have a few thoughts. There is a whole lot I don’t know about building and engineering but have worked a bit as a framer, designed and built two houses and leveled floors and added beams and posts to shorten spans on our present old farmhouse. First, I’d say you did a good job of after engineering the underbuilt floor. Ample fasteners in the ply is important. Using construction adhesive when laminating the ply would be a plus. Your challenge was more on the center beam end where the joists are let into the beam and where those joists can split at the notch. From what I can see the joist hangers provide support but the notch remains and the ply with the one screw showing in that area doesn’t look to gusset where it could split by that notch. I could be missing something there. The bolted plate is a good idea and I think really helps. Still I think that end may be the weak point with extra weight on those three joists. While the weight of the stove etc will be supported more on the concrete wall end there could be more weight including an unexpected number of people in front of the stove. While a failure seems to be unlikely, with a few 2Xs and hour or two’s work and you could put up a simple support under the three joists and front of your hearth and have two shorter spans, be sure of no joist deflection problem and and be confident in it for years to come.

 

[rwhite]

Looking at your floor supports again I have a few thoughts. There is a whole lot I don’t know about building and engineering but have worked a bit as a framer, designed and built two houses and leveled floors and added beams and posts to shorten spans on our present old farmhouse. First, I’d say you did a good job of after engineering the underbuilt floor. Ample fasteners in the ply is important. Using construction adhesive when laminating the ply would be a plus. Your challenge was more on the center beam end where the joists are let into the beam and where those joists can split at the notch. From what I can see the joist hangers provide support but the notch remains and the ply with the one screw showing in that area doesn’t look to gusset where it could split by that notch. I could be missing something there. The bolted plate is a good idea and I think really helps. Still I think that end may be the weak point with extra weight on those three joists. While the weight of the stove etc will be supported more on the concrete wall end there could be more weight including an unexpected number of people in front of the stove. While a failure seems to be unlikely, with a few 2Xs and hour or two’s work and you could put up a simple support under the three joists and front of your hearth and have two shorter spans, be sure of no joist deflection problem and and be confident in it for years to come.

I could not tell if they just notched the joist to allow the angle iron to lay flat, or the running beam was notched for the joist.

 

[gthomas785]

That floor is more solid than what my stove sits on, and mine is doing fine...

 

[rwhite]

What's going on at the red arrow? The blue arrow area appears to be a 13' span but it looks like the running beam stops there. Does the red arrow have a longer span?

 

[Max W]

I could not tell if they just notched the joist to allow the angle iron to lay flat, or the running beam was notched for the joist.

From the the location of the joist hanger and the joist being a bit taller than beam it has to be both.

Anthony, I think that I had it wrong on the laminated plywood not also acting as a gusset to counter any splitting. In the pic it looks like there is a second screw hidden by the electric cable. If so there are likely(?) two screws on the hidden side as well. Those along with the visible, heavier, construction lag screws further back would act to prevent splitting and be plenty strong. I should have picked up on the different sized screws, the pencil lines showing that care was taken and the irregularity in the plywood near where the other screw looks to be driven. As you undoubtedly knew with your reenforcement those joints don’t look like they could split and are not going anywhere. As far as deflection and stiffness maybe add the support afterward if needed.

Enjoy your new stove.