How do I joint very large, very long beams?

[SlowSteve]

Hello.

I am in the process of sorting out my loft. Officially this is purely out of love and affection for Mrs SlowSteve, but it does mean that vast amounts of tat and junk get removed from my workshop, so it's not all self-less.

I live in a semi and it's an old house with a "proper" roof - all the rafters are slow growth 4x2's and there's a huge 12 x 6 collar beams all the way around - an amazing piece of 100 year old joinery.

However, the actual joists which make up the "floor" of the loft are only 3x2's, and so when I walk on them I get ceiling cracks in the upstairs room as the plasterboard is nailed on directly ( maybe a sign I should eat less cake?)

I am looking to run a set of five 8 x 3's into the loft, bolt them onto the shared will with joist hangers and cantilever them over the load bearing internal walls - which are solid brick. Then build a floor over this. This means that I don't put any load onto the floor joists, and it gives me the height to keep all the insultation in the loft as well.

My issue is that I need runs of 19 foot for the joists, but I can only maneuver beams of 10 foot into the loft as I have to get them upstairs, up the loft hatch and not hit the tiles. So - I need to take the joists up in sections of about 10 foot and join them up there.

I was wondering how you guys would go about doing this? I need strength, but also that strength needs to last a long time. I was thinking about cutting half lap joints ( although I have no idea how big these should be ) on the ground, then gluing them in the loft, and then either doweling them or using fishplates for extra strength. I don't know if this is massively over-engineered or not engineered enough?

I have no plans to use the loft as a room - it's a junk store - so will only see moderate use.

Any idea's, thoughts or experiences you have would be hugely appreciated.

Many thanks

Steve

 

[Zeddedhed]

A simple butt joint reinforced by a 1.5metre long plate either side of the joint would do it. Through bolted with those spiky washers (Not sure what they're actually called - we always called them dog washers).

I've also used a couple of layers of 18mm ply either side - surprisingly strong.

 

[mseries]

traditionally, I believe a scarfing joint would be used for this application

 

[Droogs]

Use a scraf joint.
Quick vid here: https://www.youtube.com/watch?v=m9Wh2cgDL6E

or picture here: http://www.aisf.or.jp/~jaanus/deta/i/isukatsugi.htm

 

[condeesteso]

The lap and plates or similar alternatives will surely do it, but foir pleasure and entertainment take a look at how the Japanese do it - there are several examples on youtube - fine work indeed: https://www.youtube.com/watch?v=m9Wh2cgDL6E

 

[Beau]

There are all sorts of beautiful Japanese joints for making up long beams but for your use I would go with what Zeddedhed said. These are the spiky washers http://www.screwfix.com/p/sabrefix-m12- ... pk50/99094 Bolt through with 12mm stud with plate washers on the outside and the double sided timber conector sandwhiched in the middle. The minute you want to make joints and start gluing you need dry and planed flat wood. Sadly most construction grade timber available is neither. It's still wet and has moved post planing.

 

[SlowSteve]

Gents,

Thanks for the information

Just to check I have this correct:

I lay the 10 foot and the 9 foot section butt to butt. I use 18mm ply either side ( 1.5m long ) as a fish plate and then just bolt them up with big washers?

Have to say that works for me - very simple (which is important when balancing in a loft with no boards in it!) but I'm suprised that this is considered strong enough. Am I over thinking it or have I missed something critical?

(Also - I can't see now to use the double sided dog washers if I go with Steel rather than plywood fishplates?)

Sorry - I think I have missed something here...

Steve

 

[Beau]

I would go with 8"x1 1/2" timber each side but ply might work just never done it myself.

 

[Zeddedhed]

Steve, the toothed washers are there to prevent any turning motion. If you go with Steel plates then you don't need them. The ply WILL be strong enough for normal flooring loads as long as you don't put a grand piano and bath full of water over the joint.

If the ply doesn't fill you confidence then double it up - two plates either side.

You could always try it out in the garden. Make up a beam as described and put it between two garden chairs, then stand on it. You'll be surprised. What you must do is drill holes exactly the same size as your bolts - oversize them and you could get some shear.

 

[SlowSteve]

Many many thanks for this.

M8 bolts and big washers OK? Or do I need to be thinking about M12 beasties?

 

[Zeddedhed]

M8s should be fine. Use about three or four each side of the joint. Don't put them in a straight line - one on the top third of the joist, the next on the bottom third, then back to the top etc etc

 

[Beau]

If going down the ply and M8 route I would put some glue between the faces. Get the dryest beams you can and if you can't some PU glue would probably set OK. The reason I suggested the M12 bolts/stud is not for the shear load but the amount of force it takes to make the double sided timber connectors get pushed fully home.

 

[RobinBHM]

For a really strong joint, use 8 x 2 and double up. For example 1 at 9'0 with 1 at 10'0 then glue and screw 1 at 10'0 to it mid way and finish with a 4 1/2' board at each end. That way you have maximum overlap of the joints.

However whichever method you choose be careful to avoid wet boards or cupped boards. Treated timber is often both!

 

[SlowSteve]

Thing is though, I haven't found a wood yard anywhere near me which doesn't keep all it's structural timber outside.

Thats a point - am I OK using C16, or do I need to spend the money on c24/c32?

 

[Zeddedhed]

C16 will be fine. It's only a loft floor for storage. It's already well over engineered at 8 x 2 or whatever. Honestly 6 x 2 would be plenty. It's not a normal domestic floor - just a loft.

 

[dexter]

Whichever method you go with, if you have the space, make up each joist away from the loft first then break it down for fitting. It won't be any fun trying to create those joints in a confined space.
Remember to stagger the joints when you fit the joists.
Good luck.

Dex

 

[Eric The Viking]

Steve, I had a similar problem here but chose a different solution - Glulam beams.

Weight for weight, they're stronger and less expensive than steel, and you can specify the dimensions you want (thicknesses and length). Happily for me, they're actually manufactured locally (Avonmouth), so delivery was very cheap.

I asked a structural engineer friend to help me with the calculations (after a lot of head scratching), and went for a solution that partially reduced the unsupported span of the ceilings underneath whilst still using their joists (4x3") to support the loft floor (not for domestic use, definitely!).

Basically, we have two pairs of Glulam beams going across from the party to the loadbearing wall in the middle of the house, across the centre of the ceiling joists's span, at right angles to them. It's about 4.5m span.

The two halves of each pair are held a precise distance apart by big rectangular 12mm ply plates, with bolts through the centre of each. Imagine a long thin sandwich, up on edge, with a 12mm 'filling'.

Between the ply plates short slots are left, through which we dropped 10mm studding. The studding has big, heavy plate washers on top, and picks up clamps on the joists (2" web steel angle, bolted through to each side of each joist), effectively 'hanging' the ceiling from the Glulam beams. Because the Glulam is right above the clamps, you have to do everything in the right order (or you can't get at the joists to fit the clamps!), but given careful planning it's fairly easy.

They came in from a scaffold (35ft up, approx.!), across a dormer roof, through a temporary hole in the tiles and felt. They were light enough for me and the roofer to manage together up in the roof (and I'm semi-disabled!). No powered lifting was involved, even off the back of the lorry. They even came up via a gin wheel, although I really don't recommend this bit!

Once positioned, setting everything up was easy although time consuming (I made up several jigs for accurate drilling of beams and ceiling joists). I've been careful not to over-tension the studs lifting the ceiling joists, but even so, after a few years now the middle ones are slightly loose, implying the ceiling is under very little stress, even though the load on it has theoretically increased a bit. I have Nyloc nuts at the top, so I can adjust the support tension if I want to, but so far I've left well alone. The ends of the Glulam have 'pads' made (inexpensively) from pieces of reinforced concrete lintel.

I now have a floored area about 4.5m square in the middle of the roof, with two 'light load' areas, 2.5m x 4.5m outboard of the Glulam (towards the dormers front & back of the house). The latter just have 5.5mm ply over double thickness fibreglass over the ceilings, and are an ideal place to store empty cardboard boxes, Christmas decorations, spare rolls of wallpaper, etc. The middle, stronger bit has a load-bearing wall across the middle (at floor level), and is the "decent wood" store. It gets hot enough in the summer to effectively "kiln dry" anything up there

All the ceiling joists were cross battened (standard heavy-duty roofing battens), with fibreglass laid across and then ply on top. The middle section has T+G chipboard flooring, by Wickes*, which was the only stuff I could find in narrow enough pieces to go through the loft hatch, which is very small. In retrospect T+G floorboards would probably have been a better solution, but much more expensive.

I heartily recommend Glulam as a cost-effective and easy to handle solution.

E.

Yes I know - don't get me started about hte metal inclusions, etc.