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Strengthening loft joists

Discussion in 'Floors, Stairs and Lofts' started by alderjas, 25 Apr 2021.

  1. alderjas

    alderjas

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    Hello, apologies if this question is asked a lot, but I find mixed answers in my searches. I want to board the loft of my 1870-ish mid-terrace house for storage, not a living space, but I also don't want to worry about the weight of the things I'm storing so might want to strengthen my joists. Rather safe than sorry, and better now than later... These are the different ways I've read about doing so, but it's unclear what might be best.

    1) Sistering by attaching timber along the length next to the existing joists
    2) Attaching timber along the length on top of the existing joists
    3) Cross-battening timber 90º on top of the existing joists
    4) Struts 90º between existing joists

    Is there a method which is the best? What are the pros and cons in terms of strength, added weight, difficulty, time, and cost? Are there other considerations I should be thinking about?

    Thanks in advance
     
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  3. JobAndKnock

    JobAndKnock

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    1. Look at the span. As a very rough approximation for a floor, for every 2ft (600mm) of span you add 1in (25mm) to your depth of joist (assuming C16 structurally rated softwood 2in/50mm thick). So a 13ft span would require at least 7 x 2in C16 softwood joist

    2. Sistering to the existing joists is often easier as you can take advantage of any existing walls to support joist ends, thus reducing the required depth of joist (e.g. a 24 foot/7.2 metre span might have a wall at 12ft/3.6 metres from the outside wall which would require approximately 6 x 2in/150 x 50 C16 joists. Sistering often also allows you to use existing wall plates.

    This would be my preferred option

    3. Attaching to the top of the existing joists does not give the same strength as single full sized joists (e.g a 4 x 2 joist + 2 x 2 add on = 6 x 2 is weaker and less stiff than a 6 x 2 C16 timber even if glued and peppered with screws). This is the cheapest but least effective option IMHO

    4. In most situations cross battening will require the same depth of timber as sistering but can create problems when it comes to installing insulation. This is the second cheapest option and can be effevtibebif loading os more limited

    5. "90° struts" or as we call them "solid strutting" will stiffen a floor structure but will not increase the load bearing capacity. Solid strutting by its' nature must be the same thickness as the existing joists and at least 80% of its' depth to be effective (e.g. solid strutting in a 5 x 2 joist floor would need to be 80% as tall but the same thickness, or 4 x 2in). Solid strutting isn't normally required on joists less than about 15 or 16ft long in normal usage

    6. The strongest stiffest floor structure I can think of is one with appropriately sized joists at 16in (not 400mm) centres, with solid strutting at 16in centres, and with a properly laid, double 9mm diaphragm floor skin glued onto the tops of the joists. This forms a stressed skin which is very stiff and has high loading capacity, but is a PIA to install (believe me, I've not long completed about 1500 square metres of the stuff, albeit onto 3in C24 joists and with a bit less solid strutting)

    So failing that I suggest that when you floor your loft you screw the flooring down at 100 to 150mm centres and glue the boards on top of the joists. In other words treat it as a normal floor. With this technique you could reduce your flooring thickness to 12mm or even 9mm (if plywood and more limited loading is envisaged)
     
    Last edited: 26 Apr 2021
  4. cdbe

    cdbe

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    A lot depends on what sort of access you have for getting long lengths of timber into the loft.
     
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