ceiling joists: doing the maths

[newinvention]

We're finally in the process of renovating our old cottage in Shropshire. We're going to need to get hold of some ceiling joists shortly as building work progresses and we are trying to find out how many (and what size) we need? We're hoping to use oak.

The internal dimensions of the cottage are 19 feet x 13 feet. Joists will be spanning the 13 foot space. Our builder reckons we'd need fifteen 7" x 3" joists.

The cottage is built directly on to the soil - no foundations. So we have not been able to go down very far to lower the floor, so if we can get away with joists that are less than 7" deep, we may wish to consider that.

The joists will support a first floor bedroom and bathroom which will be floorboarded.

1) Does anyone know the maths on this?

2) Can anyone recommend a supplier of joists in the West Mids/East Wales area?

3) Would people recommend new or salvaged joists - we'd prefer the latter, but expect that de-nailing etc will make them more pricey (if we can't do that ourselves). NB: the original woodwork in the cottage was mostly shot - they'd used softwood which had suffered badly with damp due to leaky roof. So no chance of re-using those.

We're new to all this so any thoughts would be much appreciated.

 

[inwood]

Try this calulcator, but I would use a structural engineer personally, especially as building control will want a copy of the calcs for their records;
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

 

[FamilyWiggs]

I would definitely recommend getting a structural engineer to do the calcs. It cost us around 300 quid, and we had some fairly tricky t-joined main beams as well as floor joists to calculate, across 2 separate rooms.

Expect the engineer to over-engineer what you need - but I'd rather be on the safe side.

Kiln-dried new oak is more expensive, but cracks/warps less than green oak. In terms of historical integrity, there is no reason to replace with reclaimed timber; better to do an "honest" repair and make clear for future generations what you have done.

Although, unless you're a purist, this latter point is really a question of personal taste I guess.

 

[Penners]

3) I suggest that you should use new, green oak.

If you introduce reclaimed oak into the building, you're confusing the chronology of the building. You're also foregoing the immense satisfaction of seeing your own new work as your contribution to the reclamation of the building.

Yes, as green oak dries out it will sometimes warp or crack a bit, but neither of these should be much of a bother, and are what people call "character"!

 

[FamilyWiggs]

Kiln-dried new oak is more expensive, but cracks/warps less than green oak.

I meant "air-dried" of course... d'oh. It's why mine cost so bl00dy much!

 

[chuckey]

I would not touch reclaimed joist unless you actually test them before any cash changes hands. I recovered a 10" X 4" x 17' long purlin from my pigsty. I was hoping to use it in my barn 13' 6 wide. Being a cynical old b****r, I thought that I should compare its strength with that of a C16 9" X 2". So this is what I did, propped it on a pair of bricks, 13' 6" apart with a scrap of hard board laying on the floor at its middle. Got a piece of cardboard, put one end on the hardboard, marked the top of the purlin on it. Sat on joist repeat the experiment. looked at cardboard, the two marks were about 3mm apart. So with my weight ~13stone the purlin had bent 3mm. Repeated with one of the 9" X 2"s, the pencil marks overlapped, so the deflection was .3mm or less. Sawed the purlin in half and gave it to a mate for decorative mantlepieces. In the cottage I have 5 1/2" X 2 1/4" X 13' joists at 18" centres. Original "beams", Douglas fir I think, with 1" thick floor boards. Floor is on the verge of acceptability/nonacceptability. i.e. it springs noticeably when walked on and occasionally ornaments fall down. worst part is the en-suite enclosure creaks as you walk about. All part of the charm of an old place?
Frank