Joist size for 5 metre span

ads1 · 17 Feb 2021

Hi All,
Just moved in to a house with a 5.2m x 5m Garage.
I would like to board the loft for storage of mainly lightweight items, although I would like to store 4 spare alloy wheels, no tyres, of my classic car up there too.

Garage is 5.2m long by 5m wide, the roof is made of trussed rafters at 600mm centres running from a double garage door at the front to the wall at the back. The bottom rafter measures 95mm x 38mm. There is a RSJ running above the garage door. Hopefully the picture below gives you an idea of what I have.

I did think about putting supports under the bottom rafter and perpendicular to the rafter from wall to wall but due to the electric garage door I am limited on the height there.

Any advise on how I can strengthen the loft? Or would I be ok to board it with say 22mm chipboard flooring as it is?

Many thanks
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ads1 · 19 Feb 2021

Just been looking at it a bit more, would I be better putting new joists running from RSJ to Wall plate. Or is the span too big? I guess I would need 8x2 joists which may not fit on top of the wall plate in any case.

ads1 · 26 Feb 2021

Has anyone got any advise please?

JohnD · 26 Feb 2021

I don't know how to calculate it, but, if you were to add a intermediate support or pillar at the far end from the door, you could halve the span, making it much easier to support a storage platform or gallery. You would still be able to put two large cars in as each bay would still be quite wide. It could even be a gallows-like support.

The gable walls appear to be quite high so there might be scope to add support from there.

I did once have an Edwardian house with a trussed partition wall over a large room, supported off the walls with nothing underneath it. It's quite a rare construction, and during rebuilding work, the carpenter showed it off to his buddies when it was exposed by removing plaster.

HERTS P&D · 26 Feb 2021

Nice size garage, well done.

Andy

tony1851 · 26 Feb 2021

@op; you can chamfer the top corners of 8" x 2" joists to get them under the felting.

ads1 · 27 Feb 2021

Thanks for the input. I'm now thinking of putting joists between the gable walls above the truss bottom chords. I have lots of height to play with and then it will be independent from the roof trusses.
My only concern is how to attach joist hangers to the gable walls. It is only single skin brick one side and concrete blocks the other, and the mortar bed doesn't line up all the way around.
I was thinking of using the face fix hangers but not sure what they would be like mounting to the single skin brick.

Deleted member 174758 · 27 Feb 2021

If your original trusses sit onto a timber wall plate which is fixed to the top of the masonry I'd possibly toe nail the joists to that or maybe use 90 x 90 angle plates screwed to joists and wall plates instead to connect the two.

The type of hangers which connect directly into the masonry often require 2 to 3 courses of masonry above them or at the very least a well made top course of wall. They are almost all designed to hook over the top of the masonry (or be built into a mortar joint) and need a level mortar line to work - face fixed hangers (jiffy hangers) are really designed to be nailed to the face of a timber ledger with twist nails (the holes are therefore too small for adequate screws and plugs) and would be a PIA to fix to a masonry wall IMHO.

BTW, is it me or are there no straps to connect the wall plate to the masonry in this building?

Edit : Just saw your photo above and realised that you have a wall plate above the masonry

ads1 · 1 Mar 2021

JobAndKnock said:
If your original trusses sit onto a timber wall plate which is fixed to the top of the masonry I'd possibly toe nail the joists to that or maybe use 90 x 90 angle plates screwed to joists and wall plates instead to connect the two.

The type of hangers which connect directly into the masonry often require 2 to 3 courses of masonry above them or at the very least a well made top course of wall. They are almost all designed to hook over the top of the masonry (or be built into a mortar joint) and need a level mortar line to work - face fixed hangers (jiffy hangers) are really designed to be nailed to the face of a timber ledger with twist nails (the holes are therefore too small for adequate screws and plugs) and would be a PIA to fix to a masonry wall IMHO.

BTW, is it me or are there no straps to connect the wall plate to the masonry in this building?

Edit : Just saw your photo above and realised that you have a wall plate above the masonry

Correct, there doesn't look to be any straps, built 1983 / 4 so guess that it wasn't part of building regs then?

So I could put joists from wall plate to above garage door on the RSJ, but I'm concerned about getting them in without taking tiles off etc.
Or I could fix a wall plate to the gable walls and then hang joist hangers on those having joists perpendicular to and above the trusses to create my own floor. I have 2.4m from the truss bottom chords to the ridge.

Deleted member 174758 · 1 Mar 2021

OK. First off you can saw a taper on the ends of the joist to get them to fit under the roof slope. Secondly if the joists won't go in in a single piece it is possible to install them in two parts sistered together with a minimum 1 metre overlap (preferably more). The two parts are connected together with a minimum of three M10 coach bolts each with a dog (star) washer on the bolt between the two halves of the joists. Use a heavy M10 washer under the nuts - the bolts we get rarely come with washers in my experience and they need to be bought in.

diynot

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Joist size for 5 metre span

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