As a professional who ,during my many many years as a NHBC registered housebuilder and also carried out numerous renovation, improvement and extension projects on listed and other buildings I l salute your decision to involve a professional, horses for courses 
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If you create a floor and with access to walk on it then it should be of a sufficient strength for walking on or make it useful for storage. Instead of rough sawn timber you may want to consider
I Beams, as strong as timber but a lot lighter for you to handle and instead of relying on the existing structure you could look at 8x2 beams along the wall with upright posts under them for support and the I Beams sitting on the posts and don’t forget to brace the I Beams so they can’t twist in the middle, and with the I Beams there are knock-out holes if you should want to run some cables through for lighting etc but don’t drill holes in them. This is only what I would do not a recommendation.
Those things are terrible in the case of fire, I wouldn’t enertain them.
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lovely beams dont spoil them with steel. as stated already 250mm x 75mm at 400mm centres will do it no problem. Thats for 100kg per M2!
When bearing onto a load bearing wall maybe, but when bearing on a wide span existing beam it’s a different story.
Plenty of sky hooks might be the answer.
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250mm x 75mm at 400mm centres is a very strong floor for a 5.5m span.
Thats 10" by 3" timbers at 1 foot 4" centres- Anything more than that is overkill.
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You can get all sorts of grades direct from saw mills: green, seasoned, seasoned and treated…
A decent saw mill will also grade the wood according to the presence, or not, of knots & shakes etc.
Douglas is very good as it is naturally resistent to insects and rot, just make sure you buy the right quality.
When re-roofing our barn we bought all our timber, poutres, chevrons, voliges, OBS… from sudbois.fr. Even with having to get it delivered from the Gard to the Aude we made big savings by working round the standard sizes they use. Also keep an eye open for their offers - these are cancelled orders and the like and can save you even more money.
If you have the space and tools consider buying large cross-sections and ripping them down to the smaller sections for joists.
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John, interested why you say that I beams are terrible in case of fire, can you explain please?
Karem
glad you found my comments useful. I wouldn’t have known about all that if a French friend hadn’t shared the tips in the first place.
Grahame all you have to do is google i-beams fire hazard. The following article is just one example;
https://www.firehouse.com/safety-health/article/10492760/engineered-wooden-ibeams
Sue,
The link you have provided refers to a US post about engineered wooden I beams (and is some 13 years old) with no fire-rated protection.
I beams are steel beams in the shape of an upper case I.
I beams are not a fire hazard, it may be that engineered wooden I beams are if made to the same standard as the US beams referred to in the article you have linked to.
Composite I beams (made of wood and steel) are often used here in France but are completely different to the engineered wooden I beams you refer to.
The article talks about the lack of fire resistance for “wooden I-beams used as floor joists and open-web gang plate floor trusses. There was no fire-rated thermal barrier to the underside of the unprotected engineered I-beam joists or the open-gang plate floor trusses.”
Do you have information/links to the risks of wooden I beams (whether composite or not) that are made to EU standards? We all know that many US standards are well below those required in the EU or even in the UK.
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Steel I beams are of course not of themselves combustible. However, they do very rapidly transmit large amounts of heat along their length in the event of a fire, and can often spread the fire to other areas of the building not as yet touched by the original fire seat flames. Also, once heated by the fire, the steel beams lose their strength and become soft and easily bent, which in turn can lead to a catastrophic structural collapse.
Because of these factors it is necessary to heat shield all sides of a structural steel beam in dwelling premises.
I watched a london fire brigade video of the beams being tested against traditional beams. The I beam OSB laminated failed very quickly.
I suppose they are highly flammable due to the glue content like ply.
As demonstrated tragically in New York in 2001.
However, a fire in a barn such as David’s burning hot enough to soften steel would destroy the whole building (and pose a danger to any buildings in proximity) whether or not steel I beams were used to support an upper floor.
Its an interesting point however on a BRE fire test for building insurance purposes on substantial structural beams the outside chars but the oak maintained its integrity far in excess of required time.
We actually own a barn that was damaged by fire prior to our taking ownership. The fire started on the ground floor, burnt through the oak planking first floor, reached up some 8 metres to the roof, and burnt through that as well. The fire raged for some time before the local volunteer fire brigade were alerted, and they then had to come the 8km from their premises to our place.
The fire was so intense that two of the thick stone walls were weakened to such an extent that they had to subsequently be replaced with blockwork.
The upshot of all this is that the single massive oak beam (14 x 16 inches) which spans the 23 foot center of the building, and from which the central main support pillar for the roof rises, was heavily burnt and charred for a quarter of its length but was still structurally sound, and is still in place and doing its job 40 years later.
My understanding is that what happens is that the burning wood forms a layer of charcoal over its surface which actually acts as an insulator against heat penetration into the heart of the timber, and thus protects its structural integrity for a very long period of time indeed.
The total amount of heat generated will of course depend on the type of fuel available to the fire. While a bonfire of wood can easily reach 1100 C (and a barn roof is just a neatly arranged bonfire), if there are plastics such as garden furniture or vehicle parts, or other oil based fuels available, the temperature can reach up to 1500 C in a confined area in a very sort space of time.
Bearing in mind that steel becomes a molten liquid at around 1510 C, and starts to bend under load from around 600 C, then I think that I prefer the use of solid wood beams over steel ones, unless the latter are very well heat insulated indeed. The wood will hold it’s structural integrity for longer.
As a side note; It’s the non burnt end of the beam that I have to keep treating against woodworm. The little critters don’t seem to like the taste of charcoal.
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