A loft hatch, joist cutting challenge for you...

Hi - I'm after some structural joist cutting type advice and figured this would probably be a good starting point! :-)

First of all, I'm very confused by the structure of our 1900's terraced flat so I've taken some pictures to accompany this post which I hope will help!

Basically, I want to fit some (rather substantial) loft ladders however I'm a bit stuck when it comes to locations for them and am left with little alternative but to run them perpendicular to the joists. The opening required for the ladders is 120 x 70cm (47 1/4" x

Unfortunately this means cutting 3 joists in order to fit them in! (see

1. Our ceiling joists appear to be held up by 1x1" bits of wood nailed to 1x4" bits of wood which are somehow attached to the walls (see
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   - I'm sure I'm missing something here but see for yourself!)

1. There would appear to be a large king post (11' high) supported by two 2x7" joists held together in the middle somehow which look like they're resting on our ceiling joists (see
   formatting link
   are NO load bearing walls in our flat (other than the outside walls of course) so can anyone explain the structure of our house?

Anyway, my plan was to basically cut the three joists to create the opening and install some new 2 1/2" x 4" joists to support the cut joists - these would be morticed and screwed together for extra strength. This is much easier explained if you look at this:-

So basically, the profile of the new joists (shown in blue) would be like this:

Should I be concerned? Should I employ the services of a structural engineer? Should I sell my loft ladders?

As a side note to all this, does anyone know why we appear to have ready made joist holes in our walls above the ceiling joists? (see

Cheers,

Andy

(apologies for the double post - blame Google Groups!)

Andy

Personally, I would say that you would need to replace your "blue" beams with sufficiently strong ones hung from structural brickwork at both ends. Simply tying the cut joists together when they are already worse than marginal probably won't do. An alternative would be to strengthen the next uncut beam either side to hang your "blue" beams off, either using a very deep wooden beam or a steel beam sufficiently protected against fire, which may fit within the existing joist height.

Either get a structural engineer to design the thing, or have a play with SuperBeam, if you've got enough mechanical knowledge to make safe use of it.

Personally, if I was doing structural alterations of this sort, I would design a whole new structure based on standard floor loadings. That way you can be sure that (a) you're laughing if you decide to convert the loft and (b) you can use the loft for much heavier storage without breaking the ceiling.

Christian.

I'm confused by your structure, and feel that you need to get expert advice before doing anything.

If your joists are only a bit bigger than 2 x 2, they're only capable of holding themselves plus the ceiling up - and I certainly wouldn't want to store anything other than empty cardboard boxes on top of them!

Ceiling joists are often an integral part of the roof structure - and are in tension to stop the ends of the rafters from spreading. If you cut them, you can have problems with the roof. [Yours may not be like that, because they appear to be at right angles to the rafters - so I can't work out what's going on! - all the more reason for getting expert advice].

Some of your pictures seem to suggest that the joists are not all at the same level - implying that the ceilings slope. Is this the case?

Cheers for the replies! Comments below...

Bugger. A cinema room with sofas, people and various other heavy stuff would be out of the question then? :-(

No - they're all at the same level - must just be my dodgy pics. To summarise, the loft area is 20' x 30' and there are two beams (effectively splitting the loft into three 30' x 6.6' sections) with king posts running from back to front (30' stretches) - these are 2x7" beams joined together in the middle somehow (double sided sticky tape?). The ceiling joists run left to right (20' streches - broken up by the chimney in the middle) - these are 2.5 x 2.25" joists with a gap of 12" between each. The loft feels VERY sturdy to walk on (nothing really happens when you jump up and down on the joists) but I still don't really understand how?!?

Anyhoo... I've just spoken to a structural engineer and he reckons the ceiling joists will be held by the king post beams somehow - don't quite understand this. Anyway, getting him to come out and have a look, which is probably a good idea.

Any more advice would be greatly appreciated though! :-) Cheers again,

Andy

What depth are the current ceiling joists?

ceiling joists "fly past" each other would suggest there is a supporting wall or some other load bearing structure under there.

It is quite common to have a load bearing wall in the middle of the property which has the effect of splitting the ceiling joists into two sides (or front and back).

How do you know the wall are not load bearing? (a stud wall can be load bearing as well - it does not need to be brick / block).

If there really is no load bearing wall, is there a substantial joist at the apex of the roof? If so it may be that the king beam you see is actually under tension and carries the horizontal joists rather than being supported on them or a wall.

quite common to strengthen the ones at the end which are now taking the load of several joists (i.e. the black beams at the points marked A in your photo). This would often be done by running one or two extra beams beside the existing ones and bolting them together.

I doubt the king post comes into the equation directly here since it must either be supported from beneath, or is doing the supporting.

I would be more concerned with the depth of the existing ceiling joists. They look a little shallow from your photos. If there are less than 4" deep I would be wary of placing much load on them at all (in addition to their own weight and that of the ceilings).

supporting

Nope - nothing underneath that one - just empty space! The other one (next to the chimney breast) might be supported by part of the chimney breast - haven't checked that yet...

There are certainly no internal brick / block walls other than the chimney breast and all the stud walls seem to run in directions which would offer little or no support to the ceiling joists... I could be wrong though - no expert on such things. :-)

Not really - the joist spanning the apex looks pretty small - I've taken some more pics which I hope will help:

This one gives a better 'overall view' of the loft:

Still don't really understand how my ceiling's held up...

...and anyone have any ideas on the existing 'joist sized holes' in the walls??

Cheers again!

Andy

I would have thought that is not enough to use for storage - it is likely to be close to the limit just with the weight of the ceiling at those spans.

more substantial rafters each side than all the others. It also has a pair of stays from those rafters to the base of the beam. creating a shape like:-

| /|\ / | \ / | \ /\ | /\ / \ | / \ /----\|/----\

A B C

My guess would be that points A and C sit on the load bearing walls at the sides of the house. Point B is actually under tension and holds up the main ceiling span. This being repeated twice as you have two of these.

(in effect you have an old and very solid version of a modern roof truss!)

Could we have a photo of how those stay beams (i.e. the diaglonal ones that run between A abd B or B and C in my diagram above) attach to the rafters?

I would expect to see them well jointed or bolted together - rather than just butted up to the side of the rafter, since if my guess is right they will be under tension rather than compression.

Floor to ceiling height on the landing for the loft ladder?

Nope - might have been used for temporary support of timbers while the place was being built?

Don't know if it makes any odds, but those 'big rafters' seem to be supporting larger beams (spanning the full width of the loft and held in the brickwork at either side) which in turn support the main rafters.

Don't have a photo of that bit (not yet anyway), but they're basically diagonally notched in, similar to how they're attached to the king post:

Yup.

That's the only thing I can think of - very conveniently placed for a brand new floor me thinks... :-)

Andy

It could be the other way round. You have beams that run the full width of the loft that are fixed into the brickwork at either end. These support the pair of "A" frames, the A frames susspend the central floor joists. These floor joists in turn carry the other floor joists.

(its times like this one of those 3D walkaround pictures would come in handy ;-)

I would not worry about the existing holes - compared to the task of adding a new floor, having two holes available is not going to make much impact on the total scale of the work. If you need to fix beams to the wall these can be hung from a heavy shoe that is bolted to the wall:-

(In true Blue Peter tradition - here is one I made earlier)

Er, my guess is that the "King Post" and the other timbers joining it make a truss which supports the roof - the rafters resting on wall plate and ridge with one or two purlins in between. Where the horizontal truss timbers overlap they are almost certainly firmly bolted together. So the whole roof+truss are independant of the ceiling joists which may be removed however you want. The 2 ceiling joists which cross the end of the proposed opening would need strengthening as they would carry the extra load of the ceiling no longer supported by the removed joists. Perhaps double up with a new timber on top bolted through in several places.

cheers

Jacob

Yup, sounds likely to me.

Independant in the sense that the foor structure will not be affected by what you do to the floor, however related in that the truss structure also supports the floor.

I think something will need to be done to strengthen the whole area if much use is going to made of it for storage, since 2 1/2" timber is not going to be that much for se other than holding itself and the ceiling up.

OK, cheers again for the replies. I spent another day in there clearing crap out over the weekend (is 28kg of lead piping worth anything? :-) ). Having hoovered out 11 bags of dust in the area of the proposed opening, it's all making a lot more sense now, although I can't say I've ever seen anything like it before! I've taken a couple more pics to explain...

For starters, here's a scale plan and elevation of the loft (Visio is really starting to do my head in for this sort of stuff - can anyone recommend a more 'appropriate' package for plans, elevations and (if pos) 3D views?):-

- I haven't bothered to show the joists, but needless to say they span left-to-right (perpendicular to the main beams) and, as mentioned, are

The main beams spanning the full loft are indeed bolted together in two places, as can be seen here:-

I was planning on using joist hangers mortared into the brickwork as opposed to the 'bolt in' type since I've tried drilling these bricks and they're like solid steel. Anything to avoid drilling 80 x 10mm (or whatever) holes would be a bonus! Anyone have any experience of using these? I was planning on using the Catnic 50x150 ones from Screwfix, although (as yet) I have no real clue of how many joists I'm putting in, how far they'll be apart or how big they'll be...

I'm also trying to decide whether to...

OR

OR

Option 1 would obviously be better for noise since the loft floor would be totally isolated from the ceiling, however it would add quite a bit of height to the floor.

Option 2 would save a couple of inches of floor height and would share the load between the walls and beams, thereby reducing bounce in the middle (I presume) - however noise isolation wouldn't be as good.

Option 3 would probably be the easiest but places much more load on the main beams which (although seem to be incredibly sturdy) probably aren't designed to support a floor.

???

Any other ideas?

A structural engineer is kindly popping 'round to check the situation out in a couple of hours, so doubtless I'll come back with a whole different plan... :-)

Cheers again,

Andy

A quick play with SuperBeam in my unskilled hands suggests that a 6.3m span would require either (assuming that you want a proper floor later suitable for conversion). Assuming 30cm joist spacing:

a) 30mm x 300mm wooden joists

or

b) various steel sections such as i) 100x100x8 square hollow section (thinnest) ii) 160x80x4 rectangular hollow section (lightest) iii) 178x102x19 universal beam (easiest to find)

The taller steel sections would have much less deflection than the wood, whilst the 100 SHS solution is similar.

Your building control officer or structural engineer can make more accurate calculations.

Christian.

What a fascinating structure! Thanks for sharing it with us.

I agree with you that, the way the joist are supported, they are clearly only designed to hold up the ceiling - which appears to be lath and plaster, which is heavier than plasterboard, anyway.

With regard to bolting joist hangers to the walls, have you tried using a good SDS drill - that should go through hard brick quite easily!

Good luck with your structural engineer.

First may I congratulate you on a very well put together posting and synopsis! Makes it so much simpler to work out what is going on.

Only if you take it to a scrap yard yourself... and then not that much.

I often ask myself that - there must be a package that is more oriented toward building plans and layouts without going to full blown CAD. When you find it let me know!

right then. The two trusses hold the two big beams, they carry the ceiling.

than plasterboard, but less likely to have acceptable fire performance if doing a conversion. Not a big problem though.

The truss design and beams it carries do indeed look strong. If they are upto carrying a complete floor however is another matter. The ceiling beams however look far less sturdy.

The "mortar in" types are really designed to be built into the wall rather than added afterwards. Treat yourself to a nice SDS drill and the bricks will loose any abilty to fight back!

Usual spacing for a floor is 400mm centre to centre.

The trickey bit is that you have no central supporting wall (hence the unusual design of the roof). So the spans in either direction are rather long. I would expect the simplest way would be one substantial beam (probably a RSJ) right through the middle of the loft to which you can then attach new floor beams (ther other end resting on the wall plate).

So in plan you get:-

##################### be totally isolated from the ceiling, however it would add quite a bit

You look like you have a fair bit of height available. If converting (and say adding a rear dormer) that would make for a decent ceiling height even with the raised floor level.

Let us know what he says - it will be interesting to get his take on it.

Was that a typo? seems like a skinny joist!

For visual representations, I use a program called Floorplan 3D - which came on the cover disc of a recent PC Plus Magazine [Issue 213 - March 2004]. It's nothing like as dimensionally accurate as a proper CAD program, but it's very much quicker to knock up a reasonable 3D rendered picture of the outside of a building or of its internal structure. It does walk-throughs and also lets you selectively display layers so that, for example, you can see what it looks like with the roof removed.

Well spotted! That should be 50mm.

Christian.