floor joists

[plasmaman949]

hi, after some advice to understand loft conversions better. i understand that at a span of 16ft would need 9"x2" joists at 400 centres. can these joists be exchanged for 8"x3" joists.? also i have an area of 16ftx26ft. just wondered if a double header 9"x2" bolted together to make a 9"x4" spanning across the width set at 8ft6" and 8ft6" from other end to make 3 sections with the in between joists sistered with the celling joists be adequate.? thanks for any advice.

 

[alfiembra]

Extra width of timber doesn't really help you, you need to maintain depth to have a good solid floor, structural calculations are based on a span to depth ratio so 8x3's won't do you any favours. For 16 feet I would go so far as to suggest you need 10"x2" joists also don't forget to dwang the floor at mid span.

HTH

Alan

 

[Bowness72]

8 x 3 joists are actually better than 9 x 2 but I dont think either works for that span and spacing (theoretically). Assuming a grade C16 timber, I think you will be looking at 225 x 63 (which I believe are difficult to obtain at the moment) or 225 x 75.
If you wanted to span the joists the other way onto intermediate beams you would need to use much higher quality and bigger timbers and realistically steel beams would be needed.

 

[plasmaman949]

thank you so much for your info. it is appreciated. i have an engineer coming out in march, at least now ill have sum idea. can i then, if i use 9"x3" or 10"x2" go across the span of 16ft.? or if i fitted with what i think they call a double header ie: 2 bolted together, 1 at 8ft6" in from the back wall and 1 at 8ft6" in from the front wall going across the 16ft span and then joists spanning from the walls to these double headers connected via joist hangers.? thank you.

 

[Bowness72]

Dont think the "double headers" as you call them will be strong enough (at least not for building reg standard anyway). You will need steel.

 

[alfiembra]

Yep agree with Bowness steel is the way to go, why not just split the 26 ft span into two spans of 13 ft and then you can reduce your joists to something like 7x2's a lot cheaper and easier to handle. If you are going steel though you have to consider the supports at each end of the beam are you on solid walls i.e brick or concrete block, may need bearing plates or padstones depending on the materials.

Alan

 

[plasmaman949]

if i split it in half do i have to use steel.? also if i split in half, do i put 2 joists bolted together across the 16ft span to support the the joists coming from the end walls which are brickwork with sole plates on. thank all of you for your help.

 

[alfiembra]

A steel beam is your best option, only drawback is if you are planning to run services through the floor space (central heating, cabling etc.) and need to drill through the beam.

You will also have to fire proof the steel beam but this is not as difficult as it sounds it's just an additional layer of plasterboard to the soffit, and to the sides if they are lower than the general ceiling level.

To do it all in timber probably isn't the most practical as you would need a very deep beam to support the floor. Your getting into glu-lam or kerto beam territory now as the really big solid timber sections are no longer available and would be very expensive. There are however some composite timber I section beams which are very strong e.g http://www.benfieldatt.co.uk/components/joist_frames/i_beams

Only other alternative is a "flitched beam" which is two timber joists with a steel plate sandwiched between them all bolted together. You need to speak to a structural or timber engineer and get them to work out the most efficient design for you.

HTH

Alan