Joist span regs

[Orsum]

Hello,

Bit of an amateur DIY'er so forgive lack of terminology or knowledge.

I bought a 1750's stone house earlier this year as a lifelong project, and mainly looking at doing a few projects to improve the longevity of the structure itself. The one I am currently trying to get my head around, is replacing floor joists that have been historically attacked by woodworm.

Below is the rough setup of the house, with the timber structure shown, being the floor that separates the two floors of the house. There are 2 main beams that run between a supporting wall, and the external wall of the house. The bit where my research has hit a dead end is the for the recommended span of the joists between the two beams. They seem to be unconventional sizes as every sizing chart I find online does not cater for what I actually have. These middle joists (oak) are generally 75mm height x 95mm width x 1910mm length, and connect to the beams using tenon & pocket joints. I am trying to figure out whether this is safe, or whether I need more support if they span too far.

 

[Cubist]

In these old places the centres for timbers forming joists were crudely based on 18 or even 24 inches and loosely based on the dimensions of the joist timbers. At 4x5 inches your extant joists conform to the lower end of the scale so I would expect them to be laid at around the 18 inch mark. Presumably you are proposing to have any new joists milled to the same dimensions? Hopefully so, but in the event you elect to use one or other of the standard dimensioned timbers commonly available from the market, its worth noting that there would be a great deal of work involved in adjusting the width and depth of these to ensure they do not interfere with the ceilings below. Also, many of the circular saws commonly available do not have a cutting depth capable of handling timbers over 75-100mm.

Mortice pockets and tenons, sometimes dovetailed, were a common fitting technique - the latter also helped to avoid problems resulting from timber shrinkage. Regarding the spans of the joists; at 1.9m there will be a little deflection, may be as much as an inch, caused by a point load of a mans weight over the centre of a four inch joist and some may consider this alarming. However, assuming you plan to either re-lay the old floorboards or replace them, these will spread any such load over a very much larger area and thus reduce the deflection to negiigible levels. In addition, but only if you are particularly worried, you could consider laying a sub-floor over the joists before laying the floorboards. (I did just this with one of the floors here due to the very odd 500 year old joist pattern that would otherwise have prevented me laying the boards in just one direction).

 

[Toby Newell]

no need to remove the old joists unless crumbling to dust, you can fix new ones along the side.

 

[Orsum]

@Cubist
We are planning to put in exactly what we are taking out, so it will be the same dimensions. Also, originally, the floor upstairs was old OSB boarding which we are eager to remove, and replace with floorboards. So all this seems to marry up with what you are saying which is a nice relief. Thanks for that!

@Toby Newell
The beams are in pretty bad shape, I would not really trust them to hold my weight on their own. I know it's more work, but I think it will be worth it to replace them.

 

[Toby Newell]

@Cubist
We are planning to put in exactly what we are taking out, so it will be the same dimensions. Also, originally, the floor upstairs was old OSB boarding which we are eager to remove, and replace with floorboards. So all this seems to marry up with what you are saying which is a nice relief. Thanks for that!

@Toby Newell
The beams are in pretty bad shape, I would not really trust them to hold my weight on their own. I know it's more work, but I think it will be worth it to replace them.

I think he meant something like plywood underneath any boards. The top floor adds considerably to the integrity of the floor.

A good 3x6in C24 timber would do well.

 

[Cubist]

Replacing those beams would be a very challenging job for even a very experienced DIYer - and many pros would be sucking their teeth too. How sure are you that they are in a poor condition and need to go? Also, are the oak joists exposed in the ceiling below or are they covered by the plaster? If the latter, or alternatively you intend to install a new flat ceiling below then C24 as Mr Newell recommends is a good choice.

One thing to watch out for - OSBs and chipboards were sometimes used as an initial element above the joists as their flexibility allowed them to more closely conform to to the height variations and disparate surface levels often found with old wooden floors. Best to check the levels now to understand the potential scale and extent of any disparities.

Oops, nearly forgot - if you do find it necessary to replace some of the joists be sure NOT to use 'green' oak' it can be quite amazing just how much it can shrink, warp and twist as it dries.

 

[Orsum]

In some beams, over a third (if not half) of certain cross-sections has been turned to dust/missing. This made it flex so much I could not stand on it without the fear of it snapping in two. The joists are exposed in the ceiling, and we would want to keep it that way for the character aspect. The levels of the joists are not too bad between the two beams, so that should be manageable.

I will have to look into the green oak thing, new term for me, will have to check with the builder's merchants about the options.

 

[Cubist]

Are we talking about the major beams (bressummers) spanning the stone walls? Is so, and unless you can verify that the full extent of the damage is restricted to very shallow areas of the timber, then most likely they will need to come out, as will the joists and the ceilings below. What are the approximate lengths and dimensions? Is there sufficient space on the floor below to get new timbers into position or are there obstructions like other internal walls that would prevent this.

The term 'green' means that the timber has not been seasoned (allowed to dry) after the tree was cut down and milled to the required dimensions. Whilst many framers like to use it because its very easy to work, as it dries the shrinkage and stresses associated with a given piece of timber can induce stresses that produce warps/bends/twists and fractures. Whilst seasoned oak is more difficult, in some respects, to work the finished product is considerably less likely to incur serious harm from its continued drying in the new location. I would advise that you talk to a respected timber merchant in your area rather than a general builders merchant.

 

[Orsum]

Sorry, I did mean the joists. When buying the property, we did get a structural engineers report which said the major beams only has surface damage, so luckily we shouldn't need to touch them. There is an internal wall that splits the room shown in the image in half horizontally. We have spoken to several builders that have confirmed it is not load bearing, so we are taking it down and replacing it with a new stud wall.

Cheers for the info on the green oak, big help!

 

[Toby Newell]

I would never use OSB, chipboard or mdf if I had a gun to my head, use ply.

You can add noggins to spread the load, the extra dead weight they add is negligible compared to the extra 'strength' they give. Point loads both live and static can be spread from one member to three, for example.

Unless the members deflect heavily under load and are rotten all the way through it is always better to keep them and augment. Very often there is 25% of useless flaky pastry cream sapwood with solid, hard heartwood underneath. Deflection may also be caused by loose tenons, you can apply some high modulus hybrid adhesive into the gaps and noggins bracing the joint either side to firm up the joint, just be careful not to screw into the tenon as this can split it.

Very rarely do I need to remove and throw away joists and totally replace, but it is very obvious when this is the case!

Tricky if you want to keep the beams exposed, I have never had to source waney edged beams.

You haven't given us dimensions or any photos. Dropping the ceiling and cutting in new waney edge timbers isn't really a DIY job, especially allowing for shrinkage, I would want a good timber framer to tell me what to do.