Loft Conversion

[squelchy]

Hi

I have a few questions about converting my loft, I have read a few threads on this site and found out alot of very usefull information but alase no answers to the questions I have.

I live in a terrace house with a 9m by 4.5m loft space which is all open with no obtrusive w shape or v shapes to be seen. It has two part walls it does have 2 jiosts 8" by 2" on the floor and 4 (two each side) one quite low to the roof and the othere about 1.5 meters up from floor hight these rest on the party walls can some on please advice the best way to creat a workable floor space I will be useing the area as storage and possible office I am awair that to convert this I would need a 30 min fire barrier with stairs and loft insulation but I will be unable to fit the stairs as have no room due to other work done on the house before we bought it I have fitted a nice set of wooden loft ladders that retract and this will surfice for my needs and I will be fitting an excape velux after the flooring is done.

my neighbour has alrady converted his loft and terned it into a very nice space he has fitted stairs and done a proper job on the face value of it but I asked him for advice and he know nothing of the structual forces at work and had just guessed the best wood to use he has also not gone through building regs which I am not to chuffed about but dont want to start world war three with them as still got to live there.

My question is this what is the best configurational layout i.e jiost on hangers or pun through the party walls ? if so could some one let me know what the calculations are for the size of the wood or what wood I would be best to use.

by the way I also have 4"x2" lengths of wood running from the frount to the back of the house 9m split in to and resting on the centre wall of the property.

any help would be greatly recieved.

[Brian Sharrock]

Without sight of your space it's difficult to identify these ... but if they don't seem to be supporting something then they _might_ be 'ties' rather than 'joists'. Such ties are secured to the ceiling joists (for the room below) and stiffen the ceiling (below).

I've interpreted your statement to imply that your terraced property has a party wall at either end. These walls will tranmit forces down to the foundations ... and to mother Earth. The bits of timber, two each side, are 'purlins'. Supported by the party walls they provide support for the 'rafters' that go from the 'ridge beam' to the wall plates.

IANAL ! nor a BCO! The preferred jargon nowadays seems to be 'room in the roof' rather than 'loft conversion'. If you haven't got a 'proper stairway' {meeting Building Regs} than ipso-facto you _can't_ have a room in the roof. You seem to be be describing an 'attic' space. Such a space may be boarded and used for storage but not as a habitable room.

Ignore everything your neighbour has done. No Building Control means that it's illegal. From your post it doesn't seem to be 'a proper job' within in the meaning of the Act! No 'Sign -off' means that he'd have difficulty making any insurance claim.

The conventional method is to provide a new load bearing floor that is supported off both party walls. A 'web' of either steel or timbers is then surfaced. In principle the 'web' is not in contact with the existing attic floor/bedroom ceiling. A structural engineer's calcs will be needed

AIUI, the central wall is termed the spline wall, the 4"x2" 's provide something to fix the ceiling joists to. [The spline wall will be utised in the design of the 'web' to transmit the load of the new load-bearing floor.

Ask around for a 'Architectural Technician' to advise you. [IMHO, you don't require a fully fledged Architect. Your Councils Building Regulation department will help you ... if you ask them nicely.

[Phil Anthropist]

[Christian McArdle]

Sounds pretty much like mine.

The "joists" 1.5 metres up are called purlins. Are there any struts to these (i.e. in the middle) that require moving?

If not, and the space is suitably uncluttered, all you need are deeper floor joists, placed alongside the existing ceiling joists, resting on the front, middle and back as before.

When you do this, think about where the stairs would come in if you decided to convert properly and ensure you trim out a suitable hole. Also, you may be allowed to use shallower joists if you glue plywood down over the joists, rather than just using chipboard.

It will all need to be designed by a structural engineer.

Christian.

[squelchy]

Thank you both for your advice,

there is nothing obstructing the perlins in the middle they have been renforced with additional lenghs of equal size at some piont bolted to the existing ones and feed into the party walls they have also layed 4" by 2" lenths of wood the intire length of the roof jioned in the middle with bolts on top of the joists to stop the roof from splaying I have been told I can replace these with metal strapping as this is done on most houses instead of using wood as it is less obtrusive. I have obtained some metal banding from b & Q and was going to use this as at the moment there is no tension on any of the spans i.e there is 7 in total they are attached to the roof batons i think that the name for them.

so if I get this right I can put in new jiost raised above the old ones and two addtional ons in the middle and attach a web of timber to creat a floating floor.

If I was to go over comply with building regulations on this part of my build what size would the beams need to be out of timber and should I imbed then in the party walls or put them on jiost hangers ?

[John Rumm]

Where are these part walls?

As the others said, "purlins" by the sounds of it. These can be taken away if required but their function (stopping the rafters sagging in the middle from he weight of the tiles) needs to be taken up by something else like a dwarf wall.

The rules governing stairs are somewhat relaxed for lofts, so you may find that a "space saver" stair would be acceptable (i.e. one with alternate treads cut away and a much steeper pitch than normal)

If he did it a while ago there would be nothing much anyone could do about it anyway... (it would also give some reassurance that he got something right since it is still there!)

For joist sizing you can get some idea from the standard tables in the building regs documents, or you can calculate the required sizes to meet the loading, bending, and shear limits imposed both by building regs and the nature of the timber. If you read through the into on my page here I cover the information sources and the software that you can use to do this:

by the way I also have 4"x2" lengths of wood running from the frount

If you have a central supporting wall then it is quite common to use the same layout for the new floor. You place a extra 3/4" or 1" spacing plate on the wall plates between the existing joists on all the walls, and then sit the new beams on these. That ensures that the new floor does not make contact with the existing ceiling.

For an example, see how I did my floor structure here :

[squelchy]

thank you for your advice

I live in a terrace house with a plan as below

-----------------------------------

----------------------------------- party wall 9m meter lengths = / - / = = / - / = = / - / = = / - / =

------....------------------....---- party wall 9m meter lengths

------....------------------....----

/ is jiosts 8" by 2" 4.5m legths so the width is 4.5m in total from party wall to party wall

= roof with tiles on

- is wall through middle of house which goes to foundation single brick

..... where my and my neighbours jiost go through the party wall i.e his conversion and the one for his house and mine.

with limited ability to show drawings, below is where the tiled roof section is tyed together with 4" by 2" wood 4.5m lengths bolted together to form 9m lenth then bolted to the roof trusses

----------------------------------- party wall 9m lengths ='''''''''''''''''''''' -''''''''''''''''''''''''''''''''''''''= ='''''''''''''''''''''' -''''''''''''''''''''''''''''''''''''''= ='''''''''''''''''''''' -''''''''''''''''''''''''''''''''''''''= ='''''''''''''''''''''' -''''''''''''''''''''''''''''''''''''''=

------....------------------....--- party wall 9m lengths

------....------------------....---

would I be able to put four jiost into the party walls and tie these together with wood on jiost hangers to creat a grid like below and remove the above roof ties and replace with metal banding ?

-----------------------------------

----------------------------------- party wall 9m lengths = / / / - / / / = = / / / - / / / = = / / / - / / / = = / / / - / / / =

------....------------------....---- party wall 9m lengths

------....------------------....----

would you recomend putting the joists through the wall or putting them on heavey duty jiost hangers either by carving out the morter with a grinders and remortering in the jiost hangers or bolting them to the party walls with heavy duty wall bolts ?

the webbing would be made up of jiost hangers nailed to the main jiosts and then boarding over the top with chip board.

I do not rearly want to start romoving tiles from the roof as not looking to go the hole hog and fully convert just want a good solid floor so no chance of damaging the plastering below.

what size wood would be best and creat a solid floor i.e webbing and jiosts ?

I appreciate that I should get a structual engineer to look over everything but rearly do not want the expence. I cannot get building regs inviolved as they will not touch it as I only want to use my new wooden fold away ladders that I fitted just a week ago hense after looking on here realised that not a good idea to just board over the

4"x2"s that are holding up the cealing below I have already notices some cracks from me walking through the loft that where not there before lucky I have a plastering friend that is sorting out allot of work for me at the moment anyway.

[John Rumm]

Ah, sorry I may have been getting the wrong end of the stick - I take it when you said in your original post "it has two part walls", that was a just a typo and you meant it has two "party" walls. It sounded as if you has two bits of wall in the loft!

You may find switching to a mono spaced font helps when dooing this type of ASCII drawing - otherwise it is hard to make sense of unless we are using the same font and software as you!

There are two difficulties here. One is a practical one - have you got the headroom to do this since you will be raising the whole floor by about 10" minimum?

The second is an engineering one - if you used four cross beams as you suggest then you would in effect have to support three sets of 3m beams running front to back. This means that the front and back beams will be carrying upto 12 of these joist ends (assuming 400mm spacing), and the middle two will be carrying twice that. If you work on a loading of

0.8kN/m for your floor, that is 2.4kN per joist or 1.2kN downforce on each end of the joist. This will give 14.4kN uniformly distributed across the 4.5m span on your cross beams, and for the middle two this would rise to 28.8kN. This is way over what would be an acceptable load on a timber or even a flitch beam. So you would have to use heavy steel beams here.

The more practical alternative is to insert 8x3" beams in between and parallel to the existing ceiling joists and remove the tie beams altogether. You could then insert a row of noggings between the new floor beams which you fix to the ceiling joists.

Hangers are the favoured way these days. It saves creating a break in the fire protection between the buildings and is less likely that the ends will rot should the masonry get damp. You can get steel shoes designed to go flat on a wall, which you fix with rawl bolts.

Not sure why you think you would need to?

This is why you normally space the underside of the new floor joists off the existing ceiling level by a small amount. That way it is isolated (it also keeps noise transmission down)

Assuming you want a floor that would pass building regs for a habitable room:

You could use 4.5m beams that ran front to back on 400mm spacings, resting on the middle wall and the front and back walls. For this you eould need 8x3" C16 timber. (in fact you would probably make then a little bit longer so that they overlap at the middle wall, and nail em together with some builders band at the crosover.

Where the beams meet the front and rear walls you would need to cut the ends at an angle to prevent the top of the beam hitting the tiles.

Much depends on what you are planning to do with the space. Making a storeage area is easy enough, however if you try to create a habitable space it is also easy to build a deathtrap if you don't pay attention to the details. This is one of the areas where it is very worthwhile following the building regs. It will also make selling the place in the future possible.

It might be worth investigating if you could fit in a space saver stair.

[squelchy]

Hi John firstly thank you for your advice can you please answer these questions for me -

Can you please let me know how these calculations are done so I can calulate the correct loads with different materials and wood thickness as I like knowing these things for future referance?

The more practical alternative is to insert 8x3" beams in between and

sounds like a good plan as this would give me a higher cealing it is quite large already though with a middle hight of around 3m so quite allot to work with.

how would I go about this i.e how to support the cealing before the tie beams are removed I guess you need to remove these before you put in the 4.5m sections from the outer house walls to the center wall ? would I attach the cealing jiosts to the perlins with metal ties and cut the tie beams into sections to remove them ?

I am guessing this would pass the preasure to the middle wall and the frount and back walls of our house the floor tie beam looks quite large, is it not doing that much that it can be easyley removed as you sirjest? (sorry my spellings not very good)

I have been informed that I could also hang 8"x3" jiost on the party walls at 400mm centres using jiost hangers with the morter of the current brick walls grinded out and then placing the hangers in the wall and cementing them in is this a practical way of doing it ? and would it give me the same amount of structual stregth i.e is it better to hang jiost on the frount and back of the house or the two party walls ?

if I do hang the jiost over the fround and back walls how would I attach them to the walls just lay them on or what part should they be succed to ?

thank you for your help with this in advance.

John Rumm wrote:

[John Rumm]

The how (as in mathematically) is quite complex - not because the sums are that difficult in themselves, but because there are lots of different ones you need for dealing with point loads or uniform loads, bending, and shear forces etc. If you want that level of detail then I suggest some of the books I have linked to here:

practical answer (and the way that most architects etc are going to produce the calculations) is using a software package designed for the task.

You can download a demo version of Superbeam (the one my architect used) here:

however that software is not a total solution. If you don't know what data to feed into it, you will simply end up with a potentially dangerous design faster! With plenty of research and a book like:

can probably get a good deal of the source data you would need to produce unsable data for the program.

First let me state again I have not seen your loft - so there may be some fundamental deign point I am missing. Also while I am an engineer, I am not a structural one. In other words don't go hacking lumps out of your roof until you have verified what I am saying!

If you have a close look at the first two photo's on this page you can see me doing exactly this:

tie beams were in my case only 4x2" like the other loft joists. They ran across the loft and were skew nailed to each of the joists. Hence their main function was to prevent latteral movement of the joists, and provide a little coupling and load sharing to the adjacent joists. Note however they strength of this coupling was limited by the relatively low forces that would have been required to pull the nails out[1].

To allow the joist to be inserted, we prepared *three* lengths of 4x1" to use as temporary straps. You can nail these to the purlin[2], and the to the tie itself. We used one either side of the section to be removed. After these were in place a section of the tie was cut out. To insert the next joist you nail up the third one up to the side of the next cut, and then when that is in place, take out the middle one (that way you never "let go" of the tie beam and alow a bit of the ceiling to sag).

[1] You would need to look carfully at you larger cross beam and see how it is fixed to the joists to understand if it is simply doing the same as my ones were, or if it also performs some other function. [2] Leave the heads sticking out so you can pullem out and reuse the next time!

If it is doing as I susspect, then it can be removed in the way desribed. Once you have at least two new big joists in place you can stick a 2x2" noggin across between them and nail them to the old ceiling joist and the sides of the new floor joists. That will then duplicate the function of the tie beam.

You could, but it is harder work, takes longer, is more expensive, and you loose 4" extra headroom.

The tops of the load bearing walls will probably already have a "wall plate" on them (i.e. timber beam running along the inner course of brickwork). This will normally be fixed to top of the wall, and each of the rafters will be fixed to it by cutting a "birdsmouth joint" in the rafter, and nailing it to the wall plate. To add your floor, you first nail a packing piece to the wall plate between all the existing joists (this gives clearance between the underside of the new floor and the ceiling below). The new joists themselves are then skew nailed to the packer. There is not much structural purpose to this fixing - the beams are designed to carry the load simply resting on the wall plate, they are not held in tension - but it does serve to stop them sliding about!