House Refurb - Flat Roof Timber Span

Discussion in 'Builders' Talk' started by Gary22gs, Aug 4, 2019.

  1. Gary22gs

    Gary22gs New Member

    In the process of refurbishing the downstairs of the house.

    On the rear of the property, there are 2 extensions (kitchen and dining room) which were both added at different points but, both rooms are joined by a wide opening.

    Upon ripping down the ceiling and re-building new pillars, we've found that there is a timber spanning that gap.

    The timber is 10x2 C24, it is holding the dining room flat roof timbers, which are at 18" spacings.

    By my calculations, a timber of that size is capable of spanning the total 2.9m, with an evenly distributed load not exceeding 500kg across the joists which it supports. It is further supported by 3 noggings in the mid-section (which I have not accounted for).

    I estimate the flat roof to weigh approximately 350kg, which has it's weight spread across the exterior walls and this timber - total load across the timber is therefore likely to be around 150kg (accounting for the timbers otherwise supported elsewhere).

    I do wonder though, whether I should have a 15mm steel plate manufactured, which would increase the support well above 1.5 tonnes and can be lost inside the ceiling.

    Thoughts or, un-necessary?
  2. Severntrent

    Severntrent Screwfix Select

    So you have a joist that can support 3 times the weight it is actually taking, why would you want to strengthen it?? By the way normally you would multiply the dead load of the roof by a factor of 1.4 to incorporate a factor of safety, I also note that you have omitted any mention of live loading ( i.e. snow) which would be in the order of 75 to 100kg/m2 depending where in the country you are, this loading would also be multiplied by a FoS of 1.6. Hopefully when you redo the calculation with these FoS incorporated the loading will be less than 500kg
  3. stevie22

    stevie22 Screwfix Select

    Normally allow 1.2kN/sqm all in for flat roof.

    It's also very common in domestic building to design timber to BS5268 rather than more up to date but more complex Eurocodes. Here everything is dome with allowable working stresses and factors of safety are not added. (They are present but already accounted for).

    Deflection rather than stress generally determines member size for timber in domestic use.

    A 10x2 fully restrained against buckling is going to be very strong and if it has been in place for some time it's not likely to suddenly go wrong.

    A steel plate fixed to the bottom of the timber would only have a limited effect unless it was fixed VERY securely so as to allow full load sharing between it and the timber

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