Loft boarding - Truss, Brace, Kings & Queens

[brennan2470]

From your pictures there is a pile of insulation, what s the ceiling now?

They wouldn’t be my pictures. I currently don’t have a ceiling just open trusses 6x7m. No insulation, it’s detached garage.

 

[noseall]

They wouldn’t be my pictures.

It would be lovely is you shared some with us.

Do you have an image

 

[brennan2470]

It would be lovely is you shared some with us.

@catlad @noseall Here (easier to show you it before I filled it with crap!) Trusses are 35x72mm spaced at 600mm centres and sit atop a 3x4 wall plate at either end. The room is 7m long by 6m wide.

How would you approach this? The way I see it is I have several options, unless anybody thinks I am missing something:

Method	Pros	Cons
Board existing truss chords	Easier & cheaper Can be DIY	My roof may fall down(!) Cannot accommodate storage needs+flooring sheets+plasterboard (if I wanted to PB)
Install new joists parallel to chords and install several perpendicular cross-members	Leaves existing trusses untouched Can be DIY	Joist spans still far too long so questionable loading tolerances Excessive deflection
Install metal web joists parallel to chords	Can span entire 7m No cross members needed Can be DIY	Expensive Minimum beam height for span at 600 centres is 373mm so new finished ceiling height would be 2100mm which is not tolerable for a garage/workshop
Install a suitably designed RSJ at centre point to cut joist spans to <3.5m	Only one cross beam needed Will allow shorter lengths of timber to be used Will allow for 7x2 to be used instead of 8x2 Piers for RSJ can be built for free ex-materials (family)	Still quite expensive Structural engineer required to size RSJ Installing an RSJ will be complex and cannot be DIY Building control sign off required

 

[catlad]

The easiest way for storage would be to glue and screw 18mm ply wood sheets to the bottom of the cords with say 75mm decking screws, after that you can add additional strengthening beams where needed.

 

[brennan2470]

The easiest way for storage would be to glue and screw 18mm ply wood sheets to the bottom of the cords with say 75mm decking screws, after that you can add additional strengthening beams where needed.

For a smaller garage I’d tend to agree as you can access pretty much everything from your ladder hatch, but with such a large space I would need to be able to walk around up there to get from item to item. I don’t particularly like the idea of walking on sheet material that’s suspended rather than on top of the trusses. This also doesn’t solve the issue of weight which I was trying to address by building an independent floor.

The strengthening beams would also still be spanning a considerable distance of 6m unsupported which would presumably put more downward load on the centre of my spans and potentially case the chords to deflect even further.

I’m not storing a grand piano up there, but I have a lot of stuff, and the plywood alone is nearly half the 0.25kn/m2 tested strain.

So unless I’m missing something I’m not sure just boarding it will be sufficient

 

[catlad]

I don’t particularly like the idea of walking on sheet material that’s suspended rather than on top of the trusses.

Glued and screwed with lumberjack wood glue will be very strong, You could add J bolts with 2" washers if you want to go over the top. Are you planning of parking your car in the garage of could you prop your beams in the middle.

 

[brennan2470]

Glued and screwed with lumberjack wood glue will be very strong, You could add J bolts with 2" washers if you want to go over the top. Are you planning of parking your car in the garage of could you prop your beams in the middle.

Not planning on parking my car in there but I will be using part of the space as a workshop and as I work with sheet material or lengths of timber, having props would get in the way (I did consider props previously). I was also averse to doing anything that relied on the existing trusses which ideally would be left alone given they’re already spanning quite a large area. This seems to be the general consensus.

More than happy to put in 3 91x250 beams underneath as intended but my concern (apart from placing reliance on the chords for storage) was still with the span of these beams over 6m. I was worried they could actually make the situation worse given that each of these beams would weigh around 150kg each and when tied to the newly added plywood, that is tied to the chords, could actually in turn strain the chords further when they deflect.

 

[catlad]

You could ply gusset the trusses near the gutter area with triangular 11mm osb for added strength, as you wouldn't really use that space. I wouldn't use the beams your suggesting as they are over the top, have you considered engineered joists?

 

[brennan2470]

You could ply gusset the trusses near the gutter area with triangular 11mm osb for added strength, as you wouldn't really use that space. I wouldn't use the beams your suggesting as they are over the top, have you considered engineered joists?

Do you mean like a metal web joist? I did look at those however whilst they can span greater lengths they are very deep (375mm deep for a 7m span and around 325mm deep for a 6m span) and therefore lower the ceiling height considerably. They also need to be placed at say 600 centres, so if placed perpendicular to the trusses to support the floor I would need around 10 of them which even if not plasterboarded would seriously affect the ceiling height effect.

These joists would have to be hung from a hefty timber bolted to the gable walls presumably. One of those walls is single skin (with 2 double skin piers along its length).

 

[catlad]

You could have doubled up 6x2 joists instead of your 10x4 if you could prop them in the centre, even if they where just friction fit say 4x4 posts. another option is a stud wall.

 

[brennan2470]

You could have doubled up 6x2 joists instead of your 10x4 if you could prop them in the centre, even if they where just friction fit say 4x4 posts. another option is a stud wall.

Currently, having any form of upright is out of the question because of the reasons previously outlined. Stud wall would be even worse