4" roof joists @ 30cm spacing - planning to board out loft

[David Hearn]

I'm planning on boarding out our loft so we can store some boxes up there easily. The joists are 4"x2" with a 30cm gap between them. It's a 1930's semi BTW. The joists appear to be half the length of the house, with them meeting (and joined I assume) over the middle wall of the house. This has a (measured from outside wall to middle wall) 11' span at the front of the house and a 13' span at the back of the house.

The way the loft and roof is layed out, there's a large centre part which would be best for storage due to height of roof and ease of access into the loft. This centre of this area is over the middle wall.

Any idea what sort of loading may be possible? I would prefer not to have to add anything extra to the joists if I can help it. It shouldn't be used for anything more than storage of boxes, and not terribly heavy ones at that (as I have to carry them up the ladder!).

I've heard that 3x2 flex a lot and therefore aren't really suitable for boarding directly out onto, but does the extra 1" make much difference with my 4x2's? Obviously, thicker joists would allow for more insulation - but at the moment, I'm really looking for doing it with as minimal work (ie. cost) as possible, and therefore really want to know about the loading of what I've already got.

Thanks

D

[David W.E. Roberts]

contains the tables which calculate acceptable loadings.

My loft has 4*2s on 14" (36cm) spacings and longest span of about 22' - there is a stud wall under this but I don't know if it is load bearing at all - certainly the lounge runs front to back without any load bearing wall.

There is a beam above the joists and at right angles but I don't know if this is connected to each joist and carrying part of the load.

The house is 1930's and the loft was part boarded when we bought it. I have added more boarding for more storage.

The loft seems fine with boards - no sign of the ceiling sagging etc. - but on the other hand the next door neighbour had someone in to do engineering drawings for a loft conversion and he had an attack of the "Never seen one like that before - I suggest you move all your stuff round the edges to reduce the loading."

I am pretty sure the beam sizes are well outside current building regs. for flooring, but then again the regs. allow for cast iron baths etc.

Seems O.K. for us but then we could be lving on borrowed time.

As for increasing the thickness of the joists, I suspect you can't get the extra thickness over the side walls and under the existing roof. This is why loft conversions tend to install a new, independant load bearing structure.

I presume that if you halved the spacing i.e. inserted a 2*4 between each current joist to go down to 15cm spacing you would be O.K. but this is a very expensive option given the price of timber nowadays.

HTH

Dave R

[David Hemmings]

But the best way to quickly add that etra load bearing capability with the least amount of effort, so assuming 7.2m front to back that should only be approx £7.50 per extra joist run. A usual semi should be somewhere around 15 runs with this spacing. ANd remember if you can to place the heaviest items near the supporting wall and the wall where you semis meet - this is where my storage tanks are.

Also bear in mind that usual spacings are now 16" centres (14" spacing) which would affect the required size of joist.

by inserting another joist the spacings would be reduced to 13cm not

15 (remember the joists are 47mm wide)

My loft also has approx 12" spacings but also has various places that are cross joisted (3x2) and also some with more joists sitting on top of existing joists (4x2) although i think the latter sitting over the main supportiung wall had the origianl cold water tank on it -which i need to cut up to remove at some point.

My gas installation people sat the pipework in the loft on top of the

3x2 cross joists so boarding out my loft is going to be an absolute nightmare - i will try to make it at two levels, which will be an infinite improvement over the current state of affairs.

[Andy Farrall]

Might be a silly question but ... how would you get the new timber joists into the loft ? Im assuming here that the joist must run the whole width of the house. Or can they be split on an internal loadbearing wall ? Even so I think I would stuggle in my house.

Andy F

engineering

[gaz]

Take the opportunity to fit a skylight.

[David Hearn]

Kind of takes away from the original point of doing it as cheaply and easily as possible. So, to board out the loft I need to actually add 15 4x2's @ £7.50 each (£112), plus boarding costs, and also add a skylight... ;) Sounds more like a proper conversion every minute.

Thanks for all the advice - I'll bear it in mind once I get round to doing it (and figuring out how to fit in a ladder which needs < 50-60cm height above it - probably have to use a folding wooden ladder which uses less height, but a larger access hole which is a problem due to the T&G cladding someone put on the ceiling below which introduces more problems...)

Thanks

D

[Roger Mills]

Alternatively, if you can't wangle them through the trap door, you could temporarily remove a few roof tiles just above the eaves and peel back the felt. Should be easy enough to reinstate once the joists are safely inside.

Roger

[David Hemmings]

Yes they can be split but allow them to overlap over the whole of the loadbearing wall if possible.

I have just started actuallly clearing out the crap in the loft in preparation of insulatiing, crossbattening, more isulation and boarding out. For some reason in my mind the joist were 4x2, as these are always 4x2, but i took a look when i removed some of the old crap fibreglass ....... 3x2 ! hmmm i can see why 7 years in nick for the person responsible for these 1930s houses was given in court (main reason nowhere near enough lime/cement in the mortar - now = sand

[BigWallop]

This site gives a lot of information on the subject.

[Rick Hughes]

4"x2" and 30cm (assuming your mix of units is correct)

Maximum unsupported span for standard loading would be 1.98m .... so your

13' span already exceeds the loading that is allowed nowadays on joists.

OK .. your house is safe as dead load is probably just ceiling boards - hopefully any water tanks have supports over a loadbearing wall.

But the joists ain't going to take any load from above.

Even boarding out with sheet material is high risk, the dead load of the sheets would be too high.

If it were my place as the spacing is low at 300mm ... I would be happy to deck out in t&g 18mm wheyrock, screwed at 150mm centres. This would spread the load well enough for storage of light weight boxes.

A thought for you ... if joists are only 4" deep and you want to board over what about the insulation ? presumably you still have the 1930's cold roof construction and have rolled out insulation between joists - and this must be no more than 4" (or you couldn't board)

This is VERY poor insulation, and you would be well advised to send your money on increasing depth of that instead of boarding out.

I have 450mm depth in my place.

Rick

[David Hearn]

Thanks for your advice. I may well add some more joists (possibly at 90 degrees to current ones) to increase the depth of insulation. In places, there are already some 4x2 at 90 degrees (to stop spread I think) and it would be good to try and board out at a level height. 4" of insulation is poor, and I'd like to improve it.

What is wheyrock? I was thinking about 18mm chipboard (I have some packs from our old place which we didn't use). A quick google says its a composite floor decking - but I've never seen it or know what its like. Is it much lighter/stronger than chipboard? What's the sort of cost of it?

Thanks

D

[Rick Hughes]

sorry, should have been clear - Wheyrock is a trade name like Thermos for vacuum flasks ... I refer to 18mm waterproof T&G flooring grade chipboard.

Rick

[David Hemmings]

ummm not waterproof, but water resistant EN312-5 or P5, but in a loft P4 should be adequate and should be about a pound a sheet cheaper (£5 vs £6 per 8x2 board)

[Rick Hughes]

I agree I used the wrong word, it's made with waterproof glue - but water resistant is the term I should have used... on new floors I use phenlioc coated wheyrock, that will take a lot more water than water resistant board.

Rick

[David Hearn]

Ahh, so the £3.90 + VAT (£4.58 inc) is a good price for P4 boarding (quote given on Friday - with them not trying to beat another price). And £3.96 + VAT for P5 is also a good price then (quoted on an leaflet advertising their new store I've got from the same local merchants, valid until 31st Dec).

Wondering whether to use P5 seeing as the roof is unlined.... just in case....

Total number of boards I need for the main section is 11. Need another 10 for the 2 sided portions, so even if £1 extra per board, that's only an extra £21 - better that than having problems should there be a small drip or something.

D

[David Hemmings]

Yes good price if it includes delivery, make sure you get an en312 stamp as some boards are not up to spec.

for such a small price extra the peace of mind you will receive should definitely make it worth it...

[David Hearn]

Yep - free delivery on orders over £30. Finally we've got a wood yard in Guildford!! Don't have to rely on B&Q now. :)

D