Struts Noggins Joists.

[Pancho]

I had a bulge in an upstairs floor and have removed some chipboard floor panels to rectify.

The joists below are split into two parallel sections. The divide being over a concrete lintel, about 30cm wide. The joist faces touch each other and have a 10cm overlap, but don't appear to be attached by nails, bots or screws. One joist span is 3m the other 1m. At this point the joists were separated by lightweight concrete wall blocks, cemented in. As the blocks were taller than the joists they caused the bulge in the floor. I presume their purpose was to hold the joists in place and prevent twisting.

These blocks appear pretty useless, the cement around the has crumbled and so they don't seem to serve any purpose, apart from preventing a catastrophic twist. I could just shave the top off the blocks and put the floor back down, but thought perhaps it would be better to use herring bone struts or noggins.

The joists are very slightly off vertical.

I only have access from above, which means I can't use a standard metal strut from the bottom face of one joist to the top of the next. So I figure I'll cut wooden ones. This will still be a little tricky to attach the bottom end of the strut, due to the ceiling panel below.

Any advice, or is it a reasonable plan?

I also thought I might buy a mitre saw to get the correct angles on the struts. For a custom angle these all seem to be electric. Is this an ok choice

[The Natural Philosopher]

Herring bone always a good plan and will reduce flexure as well. No need to be anal about the struts any old angle more or less right will do, even more so if you use a gap filling glue as well as... you can drive a nail or indeed a long screw through the joists at an angle to pick up up on the lower ends.

As I get older I prefer to wast money on screws as its less hassle than driving nails and does less damage and with a cordless driver, its relative piss to get them in.

[Fredxx]

That is a very small saw.

I've tended to use 'solid' noggins rather than struts. Much stronger.

[John Rumm]

I think I would get a nx2 joist (where n matches the depth of your current joists), and chop noggings off that. Make em a snug fit and hammer them into place between the joists, then either skew screw them to the sides of the existing joists, or stagger them, and screw through the side of the joists into the ends of the noggings.

[Andrew]

I suspect the bulge in the floor was caused by the existing poorly restrained joists drying out and twisting, pushing up the floor. I had the same problem but ended up with a crease in the ceiling below. The builders had used a pair or 9 by 2.5 joists sitting side by side and fixed together with three 3 inch round nails !. At one end they were built into the party wall and the other ends were floating free on top of the false chimney that hid the baxi back boiler.

I tried using a car scissor jack to push them back together but the gap at the top of the pair was about 1.5 inches when the bottom was in contact with its pair. I could slide the whole joist from side to side, so it wasn't even attached to the chipboard flooring and obviously carried no weight.

Once they have dried out you cannot remove the distortion without damaging the floor or ceiling below. Flattening a bulging floor is just a case of using a power plane to carefully reduce the part of the twisted joist where the bulge is.

[newshound]

• 1 on both

(also screws are easy to remove even long after).

[John Rumm]

That would be a possibility, bu the OP did not mention his joists actually being twisted, only that the blocks currently fitted would not work to prevent it if they did. Also if they are taller than the joists, they will result in a lift in the floor.

Yup once that far gone, there is no going back to straight. As you say, planing the top will fix it. Possibly still worth adding some (cut to shape) noggings to stop any further twist or movement.

[Harry Bloomfield, Esq.]

I suspect full height noggins would be easier to fit and secure, something like 30 to 40mm thick by what ever size would fit in the height available. Drill some large holes through the middle to allow some air flow. You can then either add long screws either through the joist, into the noggin, or from the noggin, at an angle down into the joist.

[Brian Gaff (Sofa)]

Kind of begs the question, why was it done that way? M;aye there is more to the flooring than is obvious. Has the building been modified in some way, or damaged by fire in the past? Brian

[tabbypurr]

The main thing that matters with mitre saws is that they have a metal base, otherwise they often cut off angle. And the saw should cut the max size of timber you're using.

I can't imagine anyone sane choosing a nonelectric one these days.

NT

[rick]

Use some 6x2 noggins, you can stagger so you can screw through to each one, good fixing screws will allow you to pull some of the lean out ..but once wood has twisted it don't like going back.

[Pancho]

Thanks everyone for the advice.

I removed the blocks and I went with 38mm x 63mm herringbone struts,

38mm x 63mm seemed to be cheap.

The joists were not just slightly off vertical they were warped too. I didn't think I would be have been able to get noggins in tight.

Even with the struts I used a bit of gripfill to make up for the warp/unaligned gaps. It made me wonder if smaller struts would have been better. However, I couldn't get 38mm x 38mm as it was sold out and was more expensive anyway.

[Pancho]

This is original design. The house is a bit wider at this point, maybe it was cheaper to add a little extra joist than to have two sizes of long joist.

[newshound]

Although I have got one stored away in the workshop "just in case"