Beefing up garage roof trusses – how? do I need to?

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  • Beefing up garage roof trusses – how? do I need to?
  • slackalice

    Roof truss design is a science in itself. I’ve done a bit of design and build roof trusses using green oak, and also familiar with cutting and pitching, but I am by no means an expert on pre-fab stick frame trusses, which could be what you have in your garage.

    Much will depend upon their design as to how they give and take the loads and forces, so pictures will be helpful and probably necessary before anyone with any knowledge could say yay or nay.

    In general terms, if you are wanting to support a floor by the roof, your trusses will want one central or two vertical posts, which will transfer the resistance of the side walls wanting to spread due to the weight of the roof, upwards through the King (1 post) or Queen (2 posts) post/s.

    It is probable that the truss design in your garage was designed to resist external loading (e.g. snow) as well as the weight of the tiles.

    Also, bear in mind that a little knowledge is a dangerous thing and my knowledge is close to dangerous 🙂

    Bearing in mind that

    a little knowledge is a dangerous thing

    I would say you don’t have anywhere near the strength in a 2″ x 4″ joist for a 6m span. For that you want to be looking at 10″ or maybe even 12″ deep joists with a maximum of 18″ spacing between. The cost of those would likely be more than a suitable RSJ.

    Premier Icon Mat

    I’ve got a double garage roughly 6x6m and I’d like to take advantage of the roof space for storage. It’s got roof trusses made with 4×2 spaced every 0.5m (rough estimate – I’ve not measured them yet). I’m just not sure what sort of weight it could take- I’m not massively filled with confidence as the trusses aren’t a continuous length of timber as obviously you don’t get 6m lengths. They’re made up of 2 lengths joined with one of those spikey plates.

    It’d be great to board out and keep stuff up there but I don’t want the roof collapsing on me! Am I being over cautious? Any suggestions on how I’d go about beefing it up without putting pillars in? was thinking an RSJ but I suspect that’d get quite expensive. Another thought was more joists (perhaps)6×2 bolted together with a hefty overlap using threaded bar and washers. SWMBO has lots of other projects she wants doing so I’d prefere to DIY to keep costs down.


    Removed a load bearing wall supporting the trusses/joists
    3600 span, beams 203×102. 23kg/m weight of the beams. K9 lintol pad stones 300mm long beam rest 200mm

    Premier Icon tomaso

    If you are only looking at using sections to store light stuff and access it from steps it may be ok, but there is no way 4×2 across a 6m span is going to hold much weight, let alone someone walking around with heavy boxes.


    I agree, a 4×2 won’t span 6m – but it isn’t being asked to. It’s the bottom boom of a truss, so it’s in almost pure tension (unless there’s anything hung off it between the nodes). The capacity of a truss depends on all the members, and the connections, so very difficult to beef up without replacing the whole roof.

    You could put in an independent floor, with beams that span, in which case (at nominal 20:1) you’d be looking at 30cm deep. Or *possibly* put new trusses mid-way between the existing ones, but sharing the load gets complex. And they might need brick piers – are the walls single skin brick?

    Premier Icon martinhutch

    Must admit, when I stuck a plywood climbing wall up in my garage, I doubled up all the trusses and rafters, in 6×2 I think, to support it, which was a bigger job than building the wall itself. But that was only a 2.5m span.


    Roof trusses are ‘designed’ to take the roof materials plus snow load with a safety factor built in.

    anything other than soft furnishing I’d be inclined to store elsewhere or sell/dispose of

    Spreading the load would help naturally


    Even the weight of the chipboard flooring might be too much.
    I’ve just put an independent floor on joist hangers in a single garage with a 4m span…. But you may not have the height (I had 10′ in an internal garage).

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