Sistering Joists

[Pappa]

Hi Guys

My Structural Engineer has designed a new attic floor for me. The diagram shows only 1 joist for brevity (structural walls in red):

 

 

 

 

He has chosen 4m, 4.8m 3m lengths as they can be picked up off the shelf cheaply.

He said 6m is also available at timber merchants but substantially more expensive.

 

This is a side view as to how the joists are to be bolted together:

 

I'm not disputing the structural side of it. But not sure about the practicality of fastening two lengths of timber together while floating in mid air.

Do you have any tips or tricks that I can employ to do this?

 

If no, I might go back to SE and ask for a 6m length timber as the main beam so atleast thats supported by the the masonry wall on either side of the long span, the sister joists will then be easier to screw together.

 

Thanks in advance

 

P

Edited May 5 by Pappa

[Mr Punter]

Lay them down on some trestles to do the drilling and bolting.

[Pappa]

52 minutes ago, Mr Punter said:

Lay them down on some trestles to do the drilling and bolting.

Thanks for the reply.

How do I ensure I'm bolting them together straight?

Its hard to find straight timber, by the time I've bolted the 4th timber on, I could be way off. 

[BotusBuild]

I feel your pain

2 hours ago, Pappa said:

hard to find straight timber

 

My local timber merchant allowed me to sort through the pile to select - in fact he helped and rejected pieces I was OK with 😁

 

Bolt them together as soon as you get them onsite, as they warp as they dry out. I would start with the two larger overlaps first and then do the middle overlap last. With the larger overlaps do the two pairs of bolts at each end of the overlap and work inwards (I presume you are putting in more than just two pairs of bolts on each overlap).

[Roundtuit]

I'm guessing the sistering is to gain a bit of head room rather than use deeper joists?  What size joists are we talking? I have no expertise, but it doesn't look like a very elegant solution; any chance of getting a steel joist in there to split the span?

[Pappa]

1 hour ago, Roundtuit said:

I'm guessing the sistering is to gain a bit of head room rather than use deeper joists?  What size joists are we talking? I have no expertise, but it doesn't look like a very elegant solution; any chance of getting a steel joist in there to split the span?

 

Reasons for sistering as per SE:

1) Steel beams just aren't possible.

2) 6m lengths are premium priced, 4.8m etc are more reasonably priced and easier to handle / not as heavy.

3) Sistering joists (2x8inch) like this and bolting them together with steel screws gives more strength than what a 4x8 inch timber beam would have provided. This is important as it is a long (6m) span.

4) Deeper joists aren't an option because it messes with the staircase/attic headheight + cost of deeper joists increases exponentially.

 

 

Let me know if you've got any more advice about how to easily fasten these together.

I can do it on a trestle table and then lift the beam up, however, by the time they're all fastened together, the weight will be significantly greater and difficult to maneuvre, it will be easier if fastened in situ.

 

Cheers

Edited Tuesday at 16:11 by Pappa

[Redbeard]

Take careful measurements, take them outside and cut to size. Clamp together, with a straight-edge to supplement the evidence of your eyes. Mark sections, drill, bolt up. Check again. Unbolt, take up to the roof, lay sections out and bolt up in situ. At this stage I'd use spiked washers.

[Roundtuit]

Maybe invest in one or two 6m lengths and install them mid span at the same height as the structural walls, just as a temporary measure to sit the joists on whilst you bolt them? 

[Spinny]

When you have fitted the first joist in situ and confirmed that it is the correct length and fits well, one possibility might be:

- move that joist across the area as a tester to help establish whether the walls are square i.e. all joists can be made to the same length

- take it apart and remove it. Then take it outside and use it as a template to cut and drill the remaining lengths of wood to make up the other joists to the same length (if it is square) 

 

(However I am not sure whether it is best to have all joists with identical overlaps and identical bolt positions. It might be arguable that different length/position of overlaps and bolts on different joists would give a more robust structure less likely to fail in the same place across all joists. But I guess your SE would say it is designed so that any one joist will never fail anyway.)

 

PS I am not a carpenter and probably don't know what I am talking about.

[Spinny]

PS Are the joists 'at the top' on your sketch done the same way with a span of 9.6m ? I guess the 2.9 wide section top-right could perhaps be joisted at right angles, instead of all joists being 9.6m long.

 

Also maybe consider what apertures/fixings will be going into the ceiling and consider whether the joist positions will be compatible with these e.g. loft hatch/ladder, downlights etc.

(My carpenter was good enough to adjust a couple of joists above a kitchen to make room for specific feature downlights). It is only something you learn too late - that the structural drawing should be overlayed with the lighting plan and the room layout to resolve any clashes.

[SteamyTea]

Has the SE specified adhesive as well as bolting?

 

As for getting them straight and level, much will depend with what you start with.

Even a relatively short but of timber will be unlikely to be 'flat' in both planes. Once clamped together (bolts clamp, screws don't), they are highly likely to twist.

 

Are Posi-Joist type joist an option, they are usually made in a jig and the metal webs are more stable.

[Nickfromwales]

Timbers like these always have a ‘cup’ to them and you need to have these noted. 
 

Lay the wood down, lift up one end, and look down the length. You’ll note a depression and these need to be marked so all the depressions are ‘down’. 
 

Once installed these then need to be turned to be upside down so they deflect upwards, as these will then settle when weight is applied; weight from intersecting joists and flooring, plus stud walls etc.

 

Use crown washers, upon final assembly only, and it’ll be fine. Remember to only tighten these bolts initially just enough to get a firm purchase, then jack and level with vertical batters off the floor at the mid-point of the span (one on each side so it doesn’t cause the timber to rock) to get it all ship-shape. Once happy with the level, tighten these all up snugly. I then go down the timbers each side with 90mm ring shank nails (NOT bright nails) and nail the shit out of them. 3 nails per row and repeated every 450mm from both sides, all down the entire run. Be sure of things being in place, because once you nail this, any thought of further tweaking goes out the window. 

 

Getting these in is probably a well educated 2-man job or a 3-man job, if you wish to retain any sanity and avoid getting smashed in the head by a timber.  Usually 2 men on muscle, one running for fixings and stuff you drop when you least need it to (tools / nuts / bolts etc) so don’t be brave, be clever, get as much help as you can.

 

My 2 cents is stop being tight and buy the 6m timbers obvs, as they can’t be THAT expensive?? Compared to the grief vs results, plus the cost of fixings and logistics / manpower, that has to be the sensible option surely?

[saveasteading]

6m is more likely to be stocked at a specialist timber merchant rather than a general BM.

Phone around.

8 x 2 are not at every BM anyway.

 

But I think you are over-thinking.

 

This is heavy stuff.

 

I would splice the lhs 4.8 and 3.0 and lift to place. Then the rhs 4.8. Then add the 3.6. 

Tweak for the best fit you can manage. Structurally any twists won't matter.

in fact 2 timberz bolted could be stronger than one very wide timber. 

 

Levels for floor and ceiling could be a bit wonky but would with any solution.

You will need some clamps and perhaps props.

 

If you want to whack in some screws for control then just do it. That might make the drilling easier.

 

[Nickfromwales]

I'd also tie the left end into the wall with both timbers, not as shown with one stopping short?! That will prevent the timber from naturally wanting to roll, and this arrangement will need less 'help' from noggins then. Very odd to see that detail tbh.

[saveasteading]

2 hours ago, Nickfromwales said:

not as shown with one stopping short?! 

I don't think it reaches. And the other end isn't detailed into the wall. So is is on a hanger?

 

But yes, add another bit of wood at the left so it is the same section all the way. Not needed but tidy.

 

 

[Temp]

On 06/05/2025 at 17:09, Pappa said:

Sistering joists (2x8inch) like this and bolting them together with steel screws gives more strength than what a 4x8 inch timber beam would have provided. This is important as it is a long (6m) span.

 

Make sure you understand this. He's saying 4*8 made from 2qty of 2*8 is stronger than just one large 4*8.

 

You might need something bigger than 8" deep? I think it's to do with knots?

 

Id go with his suggestion and follow the advice above about taking it apart to get into place. If working short-handed Id look at making some "clothes props" wuth a tripod base to support one end. While you bolt the other.

[Nickfromwales]

59 minutes ago, saveasteading said:

I don't think it reaches. And the other end isn't detailed into the wall. So is is on a hanger?

 

But yes, add another bit of wood at the left so it is the same section all the way. Not needed but tidy.

Yes, add a piece, but would defo be a simpler way to finish the intersecting timbers to a non-staggered surface and it would prevent any rotational movement as that’s creating a pivot. 

 

Also odd that the 4m and the 3m weren’t swapped, to further stagger the joints.