joist design underneath a hearth?

[JanJonWestWales]

Hi, I'm just about to start putting back a new floor in our (one day) library.
I've taken up the old floorboards and old joists and repaired the mainbeam.
Now I want to start putting the new Oak joists back in.

However I want to put in a Hearth (probably slate or tiles) in front of the woodburner.
This will be used actively, so need a solid and fairly well sized hearth.

My question is...does anyone know how to design the joists underneath the hearth to support it, without it being a fire hazard?

Originally there were 4 simple straight joists into the wall, with a brick&lime hearth fitted between them, but I cannot work out how it was supported underneath...if at all!!

Some photo's here....
http://blaencwmbychan.blogspot.co.uk/2014/12/first-christmas-at-bcb-and-start-of.html

 

[Penners]

I want to put in a Hearth (probably slate or tiles) in front of the woodburner

When you say "in front", I presume you are talking about an apron hearth that sticks out into the room, and does not have to support the weight or accommodate the direct heat of the woodburner itself. Is this correct?

If so, then I think the answer depends very much on the size of the joists that you're planning to install. If they're good, stout, oak members, fairly close together, then there's no reason why they shouldn't be able to support a masonry hearth, particularly if you introduce some strong noggins between them.

If I were doing the job, I think I would use some sort of precast base as a constructional hearth substrate to the slate or tiles - something like concrete paving slabs. I would lay these on to the joists - perhaps notching the top faces of the joists slightly to ensure that the slabs can't move. This would give you a solid base upon which to construct your decorative hearth.

 

[JanJonWestWales]

yes an apron Hearth, probably 18inches into the room

yes they will be 4inch square oak joists...so will take your advice and lay a slab base into them, then tile onto it.
oh and set in some "noggins" between the joists too...never knew they were called that...thanks !!

 

[Penners]

Hmmm... 4in square is not that massive, so other factors become significant - like how far from the supported ends of the joists your hearth will be.

As a lighter alternative to my suggestion above, you might perhaps consider introducing a supporting panel of something like plywood between the joists. On top of this you could lay some lightweight concrete blocks*, glued together with polyurethane glue. Then lay your slate or tiles on top of this base.

I hasten to add that this suggestion is not something I've tried myself - but in your situation I think I would. However, your decision - your risk!

* In case you're not familiar with lightweight concrete blocks, they are made with air-entrained (ie: foamed) concrete. So they have most of the properties of concrete but are very much lighter and can be cut with a saw. You can buy quite thin ones - certainly 100mm thickness, possibly less.

Here's one brand - others are available: http://tarmacbuildingproducts.co.uk/products_and_services/blocks_and_mortar/blocks/lightweight_aircrete_blocks/toplite_7_73_n_mm2.aspx.

 

[agbagb]

Hearth construction comes under part J of the building regs (2.22), and its part of the solid fuel appliance installation so it'll need signing off (Hetas or LA).

The "guidance" given, in my opinion, is ready for a review and upgrade. At the moment it says: If under the stove doesn't get hotter than 100 degrees then you only need a 12mm thick hearth and it can be straight on to timber. However anything over 100 degrees and it needs a full constructional hearth at least 125mm thick and an air gap of 50mm before any timber, or be 250mm thick. The full constructional hearth is specified for open fires and appliances up to 50kw, there is no guidance for anything in-between.

The minimum area of the hearth is also given, 500mm in front of the jamb of the opening even though you only need 300mm in front of a stove opening.

The guidance is often treated as the absolute requirement by the inspector or installer rather than just "guidance". So it could be worth talking to whoever will be signing it off first to find out if they are amenable to common sense and the physics of thermal dynamics and protection against combustion.

This is something I've had to deal with in the Whitby cottage where the original hearth was resting on a single beam and the underneath clad in timber. I cast a new one in to the old timber with some steel reinforcement so the weight is cantilevered from the opening rather than resting on the beam. To keep the weight down for the bulk of it I used a vermiculite concrete. Along with the stone hearth surface it is close enough to the guidance (250mm) to be passed off.

 

[bennoa1]

Some good info on hearth's here:
http://www.stovesonline.co.uk/stove-hearth-size.html

 

[JanJonWestWales]

Thanks for all the advice...
As you can see there were originally 4 joists into the chimney breast, under the original hearth (which was old bricks and mortar...I still have no idea how it stayed up)

My approach is to put in the two joists either side of the Fireplace...I think they will be the requisite 150mm away from the fire.
Take out the fire, and pull up the tiles I had layed previously
Make a slab 150mm thick (probably of vermiculite cement as suggested)
Fit it into the fireplace and at least 300mm out in front of the fire.
Fit a Joist Noggin in front of the slab, that should ensure it cant move
Then add in the remianing two joists back to the beam from the Noggin
something like this....

Then lay the floor, tile slab, and re-fit the fire
I will be insulating and bording the ceiling in the cellar below, so will leave 50mm air gap under the slab

does that sound ok??

 

[Penners]

If under the stove doesn't get hotter than 100 degrees then you only need a 12mm thick hearth and it can be straight on to timber. However anything over 100 degrees and it needs a full constructional hearth at least 125mm thick and an air gap of 50mm before any timber, or be 250mm thick. The full constructional hearth is specified for open fires and appliances up to 50kw, there is no guidance for anything in-between.

All useful information, agbagb, but it's worth noting that JJWW is not talking about the hearth that the stove stands on, but an apron hearth that projects out from the chimney breast. I therefore feel that the kind of temperatures experienced immediately beneath the stove are unlikely to be relevant.

 

[Penners]

does that sound ok??

I reckon that looks pretty good, JJWW. The only detail that your drawing doesn't make clear is how you will handle the thickness of floorboards, in relation to the thickness of the hearth. If your plan is to create the construction that you have drawn and then lay a slab of slate (?) over the whole lot, thicker than the floorboards, then I think that, in your shoes, I would be happy with it... except that I would extend the slate to some 500mm in front of the front face of the stove.

PS: I am unqualified, thus my views are based purely on a degree of amateur experience.

 

[agbagb]

All useful information, agbagb, but it's worth noting that JJWW is not talking about the hearth that the stove stands on, but an apron hearth that projects out from the chimney breast. I therefore feel that the kind of temperatures experienced immediately beneath the stove are unlikely to be relevant.

You are quite right, I think the bit sticking out at the front doesn't need the same protection as immediately under the fire but the regs guidance doesn't allow for the compromise. It's crazy, unless the intention is to ensure safety if a future owner takes out the stove and has bonfire in there.

The solution with the trimmed back joists does seem like a "sensible" solution. But it would not be to the guidance as the hearth should also extend past the jamb by 160mm at both sides. Trimming another couple of joists back might be too much structurally ie. four joists on to two. Although being a cellar underneath some extra support could be arranged.

On the QT I would do it with just the two joists trimmed. If its going to need approval then I would check with whoever first and hope they have a brain.

 

[JanJonWestWales]

thanks Penners...
My intention is to make the tiled hearth (Apron) flush with the floorboards, so will have to calculate the thickness of tiles, grout etc, against the base floorboarding(chipboard) insultaion layer (underlay) and the new pine flooboards (inch thick).

I have some old Ruabon quarry tiles, quite thick(approx an inch) which I would like to use.


To Agbagb's point, I would be very-very nervous with four joists into a noggin, so will opt for two, and tell b-regs later
I could put in a supporting post or two in the basement below (there is a slate cold-shelf directly under the hearth), but would prefer not to!!