Ground Floor Joist Replacement

[cpower]

HI. I was hoping to get some opinions on a few things relating to replacing my kitchen floor joists. I had recently bought a house from 1906 and whilst stripping the kitchen I had came across some rot when my foot went through the floor. On investigation most of the joists were rotten at one end. I decided to take up the entire floor and make good.

Basically the floor was divided into three sections. Two were suspended timber floor separated by a sleeper wall and the third was concrete. For reasons I won't go into I removed the concrete section to have the same type of subfloor throughout the room.

Due to the poor quality of the sleeper walls I have removed all but the first course of brick with the intention of rebuilding with honeycomb sleeper walls. My initial question is whether a wall plate is required. The previous joists were resting on the sleeper wall directly and had no issues apart from at one end of the joists where the DPC was missing and I also noticed that the air bricks were blocked in that section. All others were ventilated well with good dpc and sound joists. I had intended on just installing DPC on top of each sleeper wall and have the joists just rest on that. The bricks are old pre war commons and are very very hard so I can not see the need of the wall plate in spreading load. What thoughts do people have?

I will be replacing the air bricks and adding adding extra to increase the ventilation but my other question is what size of gap is required in the honeycomb walls. I was hoping to get away with 25mm, understand 50mm to be the norm but read that there is no real exact measurement.

I can post a few pics if required. Thanks in advance.

 

[^woody^]

A honeycomb wall would normally have a timber plate across the top to spread the load and allow for joist positioning.

It's not imperative, as you could in theory just run a solid course of bricks, but in practical terms I can't see any reason not to fit a plate as part of the work

 

[urbanite]

You don't really want timber under the floor, the best way to do it is to pack out the joists over the sleeper wall with slate. Traditionally these would be bed on mortar although the modern alternative is adhesive, not the foam filled type but an epoxy. This method also acts as a damp proofing.
You should be asking yourself why did the timbers rot in the first place. is there adequate ventilation to the underfloor. Check your vents, they could be blocked. The air should be flowing under the floor too, sometimes the floor void is only vented on one side not really giving cross flow, consider using low voltage mechanical trickle vents if you feel you need to increase the ventilation.

 

[cpower]

Thanks for the contributions.

I had mentioned they were rotten due to the the previous owner having blocked the air bricks and the lack of dpc on the sleeper wall where the rot was located. Both issues I am resolving by adding pdc where required, not only clearing the current air bricks but also adding additional vents and also changing the sleeper wall from solid to honeycomb. I was also thinking of treating the joists with a preservative but not decided on that yet. I expect all of these measures to increase the flow of air thorughout the lower ground floor to prevent future rot.

As it happens I do have some slate left over from my roof repair so will use that as packing as suggested.

As I understand it the benefit of a wall plate is distributing load but has the con of being another potential point of failure if it rots. The benefit of laying joists directly on the sleeper with dpc/slate/etc would be no additonal failure points, less material to buy and install but has the con of potentially not spreading the load across the wall. I think given those options and the fact that the brick used in the sleeper wall is very hard I might go for the joist on wall option.

Can anyone give any more advice on why a wall plate would be required?

 

[tony1851]

The purpose of a timber wall plate on a sleeper wall is not to spread the load, as the loads are very small relative to the strength of the bricks.
There are 2 main reasons; one is to enable the joists to be spaced evenly; sods law usually means that some joists may come over a gap in your honecomb brickwork, and you would then have to re-position some of the joists to make sure they sit on bricks.
The other reason is that nailing all the joists to a common wall plate makes the whole floor more rigid. If the joists are levelled separately by packing, then the individual joists can flex more easily. |Nailing them all to a continuous piece of timber helps a little in keeping the floor as a whole more stable.

 

[^woody^]

The purpose of a timber wall plate on a sleeper wall is not to spread the load

Talk us through point load distribution of a weak quarter-bond honey comb wall

 

[cpower]

I was going to add structural stability by adding noggins between the joists and adding a strap between each joist and brick but take on board your point. Regarding the wall plate is there a minimum size or would piece of tanalised 2x4 do the job?

 

[cotswoldbuilders]

Lay the dpc across the wall then bed the treated plate on, wrap the ends if they are touching any outer walls.
As said, nothing wrong with timbers sat on the wall, but by using a plate you have more chance of getting it level, plus fixing the joists to avoid any movement.
If you start using bits of slate for packers, how do you know they wont fall out if the joist spring a bit when your walking on the floor.

 

[stuart45]

Lay the dpc across the wall then bed the treated plate on, wrap the ends if they are touching any outer walls.
As said, nothing wrong with timbers sat on the wall, but by using a plate you have more chance of getting it level, plus fixing the joists to avoid any movement.
If you start using bits of slate for packers, how do you know they wont fall out if the joist spring a bit when your walking on the floor.

This is the correct method.

 

[urbanite]

Looks like there's some division here. However, lets add something to help you choose your preferred method.

My personal experience with timber under floors, in old and refurbished buildings is that they twist, all those nicely spaced joists lift, you could strap it down to the sleeper wall, that should do the job, straps at 1500mm cntrs & fingers crossed.

That's an alternative to a traditional method of the slate packers. As i mentioned before you epoxy the slates to the sleeper wall, they wont budge with vibration, they use epoxy to bolt steel beams down.

The point loads on a honey combed wall act exactly the same way as any other wall, they are spread at 45degrees from the point. if you don't have a decent foundation, as this is what is spreading the load to the ground, then consider distributing the weight across the wall by using timber or even a concrete lintel, This will only give you a better load spread to take the burden off weaker parts of the foundation. If you have foundation issues then prepare for some movement, whichever method you choose.

Use a lean mix in your wall mortar if using the slate packers, consider using a bonding additive. It gets damp and cold down there and you don't want the mortar breaking up from frost damage.

As you will also know what centres your joists are going in at then you will know what spacings to make your honey combs, unless you decide to spread the load then this won't matter.

Timber can be unpredictable, definitely put noggins in-between the joists, the galvanised 'x' type (Herringbone struts) are easier to install and you can buy the right size to help you space the joists, which makes it a little easier.
Consider a Posi-Joist or I-Joist as they tend to give a more stable flooring option.

Hope this gives you something that helps.