Joist Strengthening 1860's house.

[pic_whizz]

Guys,

I'm looking to strengthen the floor joist in my 1860's house. The timbers vary between 5 and 5 3/4 inch depth and 3 inches wide. The bedroom floors on either side of the RSJ are very bouncy, hence I dropped all the plasterboard to expose the joists to investigate.

Span is 3.7m on one side of the RSJ and 3.4m on the other side.

No herringbone or blocking (a bad thing i suspect), but no apparent sag to floors, just very bouncy.

Also noticed that some of packers are missing (or disintegrated) across the RSJ.

Any ideas most welcome. Attached some photo's as descriptions often fail

Cheers,

Matt

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[TheVictorian]

Pack under every joist by hammering slate in, this should support them well as it doesn't compress and is easy to insert as it is thin.

I'd also add some noggins at 1/3 and 2/3 of the span using 2x4, this should help tighten it all up.

You could also get your fattest mate to move around upstairs while you look at the ceiling downstairs to see if you can spot any obvious areas of weakness.

 

[pic_whizz]

Thanks Victorian!

As the ceiling is out of level by 3inches (with some joists not level with each other) I was considering using 7x2's to sister the joists with M12 bolts and dog tooth connectors to create a new ceiling level with the added bonus of adding strength to the 5x3's. (Not a true sister as the ends would not be sat on the RSJ or in the wall, unless I can remove some joists material to sit them on the RSJ?

Other thoughts are to screw 4x1's to the 5x3's to create a new level and add some support?

Cheers,

Matt

BTW I have got a mate up there to create some bounce and it looks like the whole floor is deflecting!

 

[ree]

If the ceiling is out 3" then how "out" is the floor or floors above?

Is the RSJ original, or has a knock through taken place? Is there a wall above?

The softwood packings will all fail in time (depending on compression), and the gaps are too thick for totally packing with slate (slate is for wedging, not packing).

If you can get some hardwood off cuts, rip or split them, and use them for packing, you'll be better off.

Fix diagonal strings, over say 2" by blocks, from corner to corner each side of the RSJ. With another 2" block you can now determine precisely how much, and where, your joists are off.
Its no use sistering the joists until you know precisely how it will all work out.

Be very cautious of driving wedges in or jacking up at any point - you could raise unpleasant consequences.

 

[pic_whizz]

Hi Ree,

The RSJ I believe was installed in the late 70's early 80's. The wall above the RSJ goes up to roof and does provide support for the large Victorian purlins in the loft space. The house is stone built and was 3 cottages in its early life before being made into one house.

The floor above is out of level, although some effort must have been made prior using ply packing etc on top the joists.

I do have some hardwood to make the initial pack to make the gap smaller. Do you suggest I leave up to 15mm packing space for the slate?

Cheers,

Matt

 

[ree]

You really need an Acrow to slightly lift (while supporting) each joist in turn. Stand the Acrow on a two by or three by length of plate positioned across the lower floor joists. Only lift enough to enable you to knock out the softwood packing, and drive in a tight fit hardwood packing piece.

Stay away from slate: floor bounce, or compression movement, could split and crumble it. You could cut hardwood wedges on a table saw or a mitre saw. Use two drive them in from opposite sides. But a single packing piece is best. Leave it long and cut to suit after setting it in place.

You might need two Acrows, one on each side of overlapping joist tails.

I still dont get the hang of the floor being 3" out. Sometimes its best to leave old floors alone, given that its not moving and causing door problems etc.

But i've suggested above what to do to achieve a respectable ceiling level.

 

[tony1851]

Look at the top, middle pic. A lot of the bounce is due to the layout of the joists. You have a considerable area of floor carried by two of the joists (the ones with the tennon joints).
I suspect you may need to do a lot of sistering to reduce the bounce - say 7" deep joists. They wouldn't have to go full length to the bearings, just as long as they are very firmly fixed.

 

[pic_whizz]

Thanks Guys.

The 7x2's have arrived today, so i'm going to sister the old joists initially where the two tenon joints are first and take it from there.

A couple of questions i have:-

1. I'm using M12 threaded rod with 50mm Dogtooth connectors with 50mm square washers - what is the recommended bolt distances, centerline or staggered onto the 5x3 old joists?

2. These 7x2's are heavy!! Will all this extra load across all 13 joists be a problem as the sisters will not be sat on any bearing/support?

 

[tony1851]

To ensure a firm connection, consider putting them at no more than 450 centres.

If using dog-tooth washers, you may find they won't bite into the old wood so easily. Sometimes you can strip the threaded bar before the two joists are tight together. If this happens,you may have to get a short piece of high-tensile threaded bar, just to tighten each one up. Then you remove this and insert the normal threaded bar and tighten that up.

As long as the two joists are firmly fixed together, the 7x2 doesn't have to go end-to-end. Stop it a foot short each end and that will be OK.

Try to stagger the fixings so they are not in a straight line - reduces the tendency to split the wood.

 

[pinenot]

Wow! that's a complicated/involved way to sort a common problem, propping the ceiling in the middle with an Acrow Prop on an 8' foot batten across the joists, lifting slowly (allow time for them to settle) until an approximation resembling flat (old buildings seldom have flat floors [try for bounce at this stage]) measure and cut full depth dwangs (noggins) and insert at 1/3 & 2/3rd spacing from ends i.e. equidistant measurements. and chap them in place, they should be full in length but not over tight to the joists. Stagger each proceeding dwang to allow full through joist fixing with nails or skew nail and when you release the prop on completion, you will find the joists stay flat (flattish) because they are racked, a torsion box of kind...pinenot