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Suspended Timber Floor design

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Hey, I'm about to rebuild my downstairs extension floor and am after a few tips. It was a very badly constructed concrete floor and prior to that would have been suspended timber (it's a Victorian terrace).

My plan is to secure timber wall plates (are they called wall plates?) to two opposing walls with resin secured M12 threaded bar at 600mm centres (2.8m span) and then run C24 47x97 joists between the wall plates (2.1m span) at 400mm centres on joist hangers. I know that the more simple option would be to rest the 4x2s on top of the wall plate but this would not leave sufficient void under the floor for ventilation. If anyone could shine some light on these questions that would be great:

 

- The data sheet for my chemical fixings assumes a min compressive strength of 10N/mm^2 of masonry but bricks are of unknown standard, Victorian and over 120 years old. Do you think I'll be OK?

 

- One of the wall plates needs to rest on the brick foundation of one of the walls and this wall isn't level. For that reason, when the joist hangers are fitted to that wall plate they will be fitted closer to the edge of that wall plate, to the point where I might only be able to get 4 twist nails into the wall plate and quite near the edge of the wall plate. Is this risky? I can't find any information on what load a single twist nail can hold. 

 

- it's a small room, only 6m^2, but it will be a kitchen. I've done some calcs and the dead load for entire floor may be in the region of 1000kg, so 1.67kN/m^2 dead and potentially 2.5kN/m^2 live load. The regs tables I've seen don't allow for a dead load exceeding 1.25kN/m^2 and live load exceeding 1.5kN/m^2.

 

the more I'm thinking about this, the more more I think I might have to bite the bullet and build a sleeper wall at mid-span. I'm not familiar with regs on this but I've found some information proposing a 150mm x 300mm strip foundation, then a honeycomb sleeper on top. Is that foundation depth overkill for this application? The trench will have to be hand dug, so the shallower the better for me!

 

Really appreciate any advice and sorry for that information overload. 

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Why do you want to go timber, if you have to dig out the poor concrete floor, why not install one correctly, 

dpm, insulation, concrete 

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Posted (edited)
1 hour ago, Student said:

Hey, I'm about to rebuild my downstairs extension floor and am after a few tips. It was a very badly constructed concrete floor and prior to that would have been suspended timber (it's a Victorian terrace).

My plan is to secure timber wall plates (are they called wall plates?) to two opposing walls with resin secured M12 threaded bar at 600mm centres (2.8m span) and then run C24 47x97 joists between the wall plates (2.1m span) at 400mm centres on joist hangers. I know that the more simple option would be to rest the 4x2s on top of the wall plate but this would not leave sufficient void under the floor for ventilation. If anyone could shine some light on these questions that would be great:

 

- The data sheet for my chemical fixings assumes a min compressive strength of 10N/mm^2 of masonry but bricks are of unknown standard, Victorian and over 120 years old. Do you think I'll be OK?

 

- One of the wall plates needs to rest on the brick foundation of one of the walls and this wall isn't level. For that reason, when the joist hangers are fitted to that wall plate they will be fitted closer to the edge of that wall plate, to the point where I might only be able to get 4 twist nails into the wall plate and quite near the edge of the wall plate. Is this risky? I can't find any information on what load a single twist nail can hold. 

 

- it's a small room, only 6m^2, but it will be a kitchen. I've done some calcs and the dead load for entire floor may be in the region of 1000kg, so 1.67kN/m^2 dead and potentially 2.5kN/m^2 live load. The regs tables I've seen don't allow for a dead load exceeding 1.25kN/m^2 and live load exceeding 1.5kN/m^2.

 

the more I'm thinking about this, the more more I think I might have to bite the bullet and build a sleeper wall at mid-span. I'm not familiar with regs on this but I've found some information proposing a 150mm x 300mm strip foundation, then a honeycomb sleeper on top. Is that foundation depth overkill for this application? The trench will have to be hand dug, so the shallower the better for me!

 

Really appreciate any advice and sorry for that information overload. 

Wall plate would be the timber a wall is built upon, ledger board or runner board.

 

From the structural engineer sitting opposite me: I would use M16 threaded, M12 is a bit small. C24 on that span is overkill and not worth it, C16 will be more than adequate.

 

Bricks assuming not crumbling and poor will be of better quality than today's - they are also bigger so you can get a better depth. 

 

I would either sort the wall out so it can be done properly or remove some of it, level it, put in a wall plate and sit the joists over the wall. You may opt to put some forms in a pour a concrete pad along it to level it all - although 4 nails would probably hold it, it could fail under heavy load or a sudden impact etc. and it's not about situation now, it's about what happens in 10-20 years when something maybe gets damp or rots. Do this bit right.

 

You are over thinking a 6m^2 domestic floor - the calcs seem off.

 

A sleeper wall mid span for 2.1m is totally unnecessary.  

 

Can you post a sketch showing the section across the span - so it shows where the joists will sit - with some rough dim's on it. 

 

 

 

Edited by Carrerahill

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Thanks for the replies. In terms of spec, I used this table to spec the timbers and C24 47x97 at 400 centres came out at 2.24m permissible span. C16 came out at 2m permissible span. 6.4Atable2.gif

 

In addition, all the information I could find on typical loading for a kitchen was between 2 and 3kN/m^2 imposed and when I summed the weight of my kitchen, floor tiles, and allowed for 2 to 3 people to be in there, I found that figure to be about right. Could you clarify on how the calculations are out? I found worst case scenario to be 1500kg, spread over 6m^2 to be 2.5kN/m^2. Admittedly, the load might be highly concentrated in specific areas of the kitchen. 

 

In the image attached I've highlighted the area on which one of the ledger plates will sit. Actually on this wall, the plate will be fixed higher, it's the opposite wall where the plate will rest on the wall. I'm not sure at what degree I'm allowed to cut away at it to make it level as it makes up the foundation for the exterior wall of the extension?

 

Good point, a couple of reasons why I want suspended. The void is useful for running services, specifically the waste pipes from the kitchen. Also, as I've not constructed a floor before, I deemed suspended timber to be the simplest construction for matching the floor level with the adjacent floor. Also, access isn't great due to being a Victorian terrace with no rear access, so getting materials to the rear extension is quite cumbersome. 

 

Deflection of the floor is an issue as the plan is for a tiled floor. If deflection is unavoidable then I can rethink the floor surface. I'm starting to think a supporting wall mid-span would solve a lot of issues and would only be 20 bricks total. 

20190611_112525.jpg

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I don't think you have told us how much depth you have available and what sort of insulation you plan.

 

How bad is the existing concrete? Given you want to tile the floor I think I would consider going for ..

 

Tiles

Screed (and perimeter insulation)

Insulation

DPM

Sand blind

Existing concrete floor

 

No void so no need for air bricks to ventilate under it.

 

Tiling wood floors is possible but it's best to get the thing as rigid as possible starting with adequate joists (perhaps not bare minimum) and possibly  two layers of WBP on top screwed down (lots of screws). Then follow instructions for flexible tile adhesive.

 

 

 

 

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Posted (edited)
1 hour ago, Student said:

Thanks for the replies. In terms of spec, I used this table to spec the timbers and C24 47x97 at 400 centres came out at 2.24m permissible span. C16 came out at 2m permissible span. 

 

In addition, all the information I could find on typical loading for a kitchen was between 2 and 3kN/m^2 imposed and when I summed the weight of my kitchen, floor tiles, and allowed for 2 to 3 people to be in there, I found that figure to be about right. Could you clarify on how the calculations are out? I found worst case scenario to be 1500kg, spread over 6m^2 to be 2.5kN/m^2. Admittedly, the load might be highly concentrated in specific areas of the kitchen. 

 

In the image attached I've highlighted the area on which one of the ledger plates will sit. Actually on this wall, the plate will be fixed higher, it's the opposite wall where the plate will rest on the wall. I'm not sure at what degree I'm allowed to cut away at it to make it level as it makes up the foundation for the exterior wall of the extension?

 

Good point, a couple of reasons why I want suspended. The void is useful for running services, specifically the waste pipes from the kitchen. Also, as I've not constructed a floor before, I deemed suspended timber to be the simplest construction for matching the floor level with the adjacent floor. Also, access isn't great due to being a Victorian terrace with no rear access, so getting materials to the rear extension is quite cumbersome. 

 

Deflection of the floor is an issue as the plan is for a tiled floor. If deflection is unavoidable then I can rethink the floor surface. I'm starting to think a supporting wall mid-span would solve a lot of issues and would only be 20 bricks total. 

 

Sorry, I mis-read your post, I was sure you said 145mm and took it you were using 6*2's. Can you not beef these up a bit? 4x2's are only good for non-supporting stud walls and even at that they are a bit flimsy. 

 

With a proper makeup of flooring you are going to struggle to get a good solid tiled floor.

 

What is the distance from your solum to your finished floor height?

 

I think I would be adding some hardcore, sand blinding, DPC, concrete!

 

 

 

 

 

 

Edited by Carrerahill

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There's room for 150mm void below the 4x2 joists. There's certainly scope to go with concrete, it's just a bigger job than I'd anticipated and I'm faced with the issues of where to run services. Appreciate they can likely be ducted in the concrete?

 

Is the consensus that even with a sleeper at mid-span I'm going to struggle to get a solid floor? As I mentioned, I can live without tiles if needs be. I need the floor down asap as it's holding up the kitchen being fitted. I'd originally only planned on a single layer of wbp ply under tiles. This is my first house and hopefully a stepping stone into self build so happy to accept any advice you more experienced guys. My only source of information is the internet as my background is electrical and electronic engineering. 

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So quick and cost effective then

dig a small trench down the centre and put a small 100mm deep x 300wide concrete footing in

fix treated 4x2 to both walls with chem bolts and 12mm studs

level across from both of these and build up your sleeper wall with half bricks missing for cross ventilation 

lay a treated wall plate on top of the sleeper wall with a dpm under it

fit hangers to both pole plates on walls

fit treated joists across from side to side

screw down to centre wall

fit 100mm celotex between joists

fit vapour barrier on top of joists

screw and glue down 18mm t and g chipboard 

glue and screw 9-11 mm wbp ply on top for your tiling 

ensure everything is 100% bounce free

ensure a flexible tile adhesive compatible with timber floor. 

 

Good luck with your first house, 

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Thanks Russell, that's almost exactly what I have planned. I dug the trench last night and it didn't take very long at all thankfully. 

 

I must admit that I hadn't given much thought to insulation yet and would adding a vapour layer above the joists make gluing a pointless step? I was hoping to add heated matting below the tiles to add comfort during the winter months. Does this effect things at all?

 

I'd also been warned off of using chipboard, hence I've bought 18mm wbp ply for the bathroom floor (prior to heated matting and tiles as per the kitchen). Is 18mm sufficient or should I be adding a layer?

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15 minutes ago, Student said:

I dug the trench last night and it didn't take very long at all thankfully. 

 

Bloody hell that was quick!

 

I was about to suggest a new concrete floor, but since you have started, just carry on with what @Russell griffiths suggests. Run some DPC behind the wall timbers before you fix them - easiest to staple it to the back to hold it in place.

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36 minutes ago, Mr Punter said:

 

Bloody hell that was quick!

 

I was about to suggest a new concrete floor, but since you have started, just carry on with what @Russell griffiths suggests. Run some DPC behind the wall timbers before you fix them - easiest to staple it to the back to hold it in place.

Haha, well the delay is stressing me out so I thought I needed to take action! I have a long roll of DPC and plan on using it everywhere that wood contacts masonry (and on the sleeper wall) but didn't think of stapling to the timbers - good tip, thanks!

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A suspended timber ground floor needs decent subfloor ventilation to outside air and you need to make sure the sub floor area is well sealed from the room above. The Celotex suggested is ideal for this, with a generous application of expanding foam in the gaps to help it fit really tight.  You could screw or nail some pieces of batten at 600mm ctrs to the joists at the bottom to support the insulation level with the joist tops and it would also help resist twisting.

 

I would go with 22mm Egger Protect T & G boards (if they are reasonably available) glued and screwed.  Seal the edges to the walls with mastic.  Overlay with VCL (if you really want to) then ply screwed at 200 ctrs then heat mat and tile.

 

Work out the buildup height carefully so you don't have a step into the adjacent areas.

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Thanks for the info guys, how long will I have to leave this before it can be weighted by the joists from above? Approximate of course! Cheers. It's 300mm x 100mm x 2450. 1:2:6 sand, cement, gravel. 

20190615_133357.jpg

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Quite a small room!

 

You can do the brickwork tomorrow but leave a day before the plate on the sleeper wall. 

 

When you do the wall plates you may find it easier to nail the hangers on first. The joists do not need to go right to the walls, you can leave 100mm and pack those gaps with insulation / foam.

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Great, thanks for the info. 

 

Is there any value of adding a layer of insulation between the ledger plate and wall?

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