Loft conversion - technical questions

[hansen]

Hi all,

this is not really a DIY question but I'd like to be a bit knowledgable when I talk to the Council/Architect/structural engineer/builder or whoever...:

I'm looking at the feasibility of converting my loft space into usable (not really "liveable" but I know the BR doesn't make much of a difference) space for a study and sadly missing storage, with a permanent staircase above my existing ground-to-first floor staircase (same size and shape). I've read the building regulations about insulation, fire escapes, party walls etc., and I believe I have a relatively clear picture of what's required.

My house is end-terrace, around 100 years old, of very solid stone construction. The party wall is actually an old external wall, my house is two stories high, and the neighbour's three stories high.

The width (span) of the area I want to convert is around 2.8m, length around 7m. There are no modern "purlins/props" that would obstruct a conversion, however as the roof pitch is around 40 degrees (good slate/felt/chipboard construction) the max height on one side (towards the party wall) is only around 2m, i.e. not really suitable as it is.

However, the existing loft floor joists (very old wooden beams) are around 300mm high, and the ceiling height in the room below is around

3.2m. This would, in my opinion, give me the opportunity to do the following:

- Reduce the ceiling height below to around 3.0m, gaining 20cm ceiling height in the converted loft from this alone.

- Replace the 300mm loft floor joists with laminated wood or steel beams, gaining an additional 10-20cm in addition to the above.

A space 5m long, with a 2.3m reducing to 1.9m ceiling height in the "corridor", and using the perimeter for cupboard/workdesk space (and

2.3m lost to the staircase) would be extremely acceptable indeed.

The questions I have really are the following:

- My previous house in France had glued laminated joists for the attic floor, around 15cm thick with spans of 4m on one side and 4.5m on the other. This floor was actually far more stable than my next livingroom floor, by Barratt in the UK. I believe this was similar to the "Glulam" products on

but I'm not sure. For a less than 3m span, and say 40cm between the joists, would this be a suitable solution and how "thin" could the joists potentially be?

- I've also seen references to "LVL" which is laminated wood, with thinner wood layers running vertically instead of horizontally, and which is supposed to be stronger. However I can't find any examples of how this is used in a loft conversion.

- I looked at one conversion example a while ago where thinner (~100mm) I-shaped steel beams (RSJ I think they were called) were used, for a longer span than mine. Again, would this be suitable for my situation and how thin could beams for a 2.8m span be?

- I also saw one newsgroup posting talking about "open core" steel beams (~3mm steel bent in a long 70mm square beam), again, would this be an option?

Note: I know that the above are questions for the structural engineer but I'm looking at "ballpark" figures before spending the first money on the next-step feasibility calculations. I've tried to use "SuperBeam" but obviously you need to know well what numbers to feed into it to get a sensible output ;-)

With regards to the material cost - I don't really care. I believe the cost of the steel or wood joists would be small compared to the overall conversion cost anyway. What I *really* want is to gain every centimeter ceiling height possible.

With regards to the roof itself, the rafters that hold the roof are

150mm thick. I believe the building regulations stipulate 50mm ventilated space between the underside of the roof and the insulation material, and that the use of a high-quality insulant (rigid phenolic/urethane) with conductivity 0.022 W/m.K between the rafters plus possibly insulated plasterboard will be enough to achieve a U-value of 0.20 W/m².K as specified by the BR. However the following two information sources don't add up:

(parts E1/E2)

(Sigh, if "Masterbuilders" and the Building Regulations can't agree what hope is it for people like me ;-) ? )

So the question is really: with 150mm rafters, spaced around 600mm apart, how much insulation can I stuff inbetween the rafters, what type of material is the best to use, and how much ceiling space would I lose apart from the obvious 12.5mm to the plaster board due to any insulation material that I would have to put *under* the rafters?

With regards to fire regulations, I already have a top-hung large Velux around 60cm above the existing loft floor, which means even if I gain

30-40cm ceiling height it will be less than the max. 110cm in the building regulations. The new staircase would be linked with the existing staircase, looking like a "natural" continuation of the staircase from the ground floor. What confuses me a bit is the BR that appear to stipulate a 30min fire resistance, and the BR example of 9.5mm plasterboard plus 60mm mineral wool plus 12.5mm chipboard that might achieve this. My ex 2002-built Barratt house did not incorporate the 60mm mineral wool between the floors, only the plasterboard and wonky chipboard, so I wonder how Barratt homes stand up compared to the BR ;-) ... My thought would have been to make the loft an open gallery area to the stairway, so would the 30min fire resistance be irrelevant, or would it force me to scrap the "open gallery" idea???

Anyway that's it for now I guess, any feedback to any of the above would be immensely appreciated...!

Thanks in advance!!!

[John Rumm]

It is DIYable, BTDTGTTS ;-)

Well habbitable rooms are treated a little differently from non habitable. Not sure what a study counts as - habitable I would guess.

Oddly there is no minimum height specified for ceiling height in the regs (there is over stairs - but not rooms). However it you are like me then building regs become a secondary considderation with being able to stand up taking first place ;-)

Which way do the span the house? (i.e. the 7m or the 2.8m span)

Are there any supporting walls in the middle - or do the have to carry the full length span.

If they are across the short length then they are well over specced!

Tad messy - but doable.

Here is where superbeam will help...

A reasonable floor loading on a joist would be 0.8kN/m as a uniform load. Since there are lots acting together you can turn on either the "load sharing", or "4 members acting together" options and this will increase the statistical strength of the joists and may let you specify lighter ones.

Sorry, not familier with those.

You may come accross use of "flitch" beams however (steel plate sandwitched between two joists, all bolted together).

It might be worth getting an architect to simply look at the layout and having a feasability chat before going much futher - that way you can get a feel for how "doable" the whole project is before you actually commision anyone to do anying.

The whole floor on mine (bigger area) was about 2K all in for wood, steel, joist hangers etc.

By thick, do you mean deep?

No supprise there then ;-)

If you have 150mm of depth on your rafters then you don't have much to worry about. Typically 90mm between, and 25mm underneth would be plenty when using PIR foam (foiled). You may be able to get away with just insualtion between the rafters, although this allows a certain amount of cold bridging from the wood itself. Note also that many of the figures you see are based on a 400mm c/c rafter spacing. 600mm is actually a little mote efficent do to a bigger insulation to wood ratio.

It also needs to be within 1.7m of the eves (so somone working from a ladder could reach someone at the window).

Knock the "compared to the BR" bit off the end of that last sentance and you have the more pertinent question! ;-)

The regs get more complex in this respect when you add a third storey (or above). Fire regs rquire 30 min protection to the new floor (but no changes made to the existing ones). As you say 100mm rockwool/isowoll plus 12.5mm PB will do the trick. You would need firedoors on the new storey on the habitable rooms, and any doors in the rest of the house that open onto a potential escape route would need self closers added. You also will need mains powered interlinked smoke detectors on each floor.

Don't know... sounds susspect.

HTH

[bigcat]

usable

(towards

ceiling

why not do the obvious and convert the floor below to 2.4m high. Gives you more usable room upstairs then.

The thinnest floor achievable is solid wood. There are no separate joists, just one thick wood floor, typically around 3" thick. Need to PB line it underneath for fire regs.

NT

[hansen]

I've got tall sash windows in the room below - I don't want to ruin the look of that room by covering the top of the windows and not leaving any space for cornicing above them. It should still have its "period" look after this and lowered ceilings in a period house don't always look good ;-) ...

Thanks for the tip - will put that down as an additional option!

[Hugo Nebula]

On Sun, 10 Apr 2005 00:14:37 +0000 (UTC), a particular chimpanzee named hansen randomly hit the keyboard and produced:

The FMB site is based on the English & Welsh requirements, which are different from the Scottish Regulations.

[Owain]

Without looking, I suspect Masterbuilder is referring to the ENGLISH Building Regulatiosn whereas scotland.gov.uk is referring to the SCOTTISH regulations. Are you in England or Scotland?

Your Barrat house was probably two-storey. Additional fire precautions come into effect when you go three-storey, as you are finding out.

AIUI You can have the loft open gallery to the stair leading up to it, but it must still have a fire door between the foot of the loft stair and the first floor landing. IE the staircase becomes part of the loft rather than part of the landing, but there still has to be separation between the two.

If the house next to you is a storey higher anyway you stand a very good chance of getting PP for dormers (especially at the rear).

Owain

[John Rumm]

Probably won't need PP for dormers to the rear (unless it is a conservation area etc).

[hansen]

I'm in Scotland, I know the BRs are different with regards to the required U-value for roofs etc. but I would have thought the formula for how to arrive at a target U-value of 0.20 W/m².K with a given conductivity would yield the same result whether one is north or south of the border ;-) ?

Ouch, it's worse actually - it was four stories, identical to the following:

broke the BRs with regards to the amount of thermal insulation between my garage and first floor livingroom, 25mm mineral wool instead of the 50mm required by the BR (and the Architect had actually specified

150mm!!!) After freezing through the first winter and reading through the BRs, and a stream of letters to Barratt, the Architect, NHBC and Edinburgh Council Building Control, I finally managed to get Barratt to tear down the ceiling in the garage and replace the insulation.

Oh, a moron from NHBC came aroud, but refused to get up on the ladder to check the thickness of the insulation, just stating "well there's wool there, you have to realise that insulation settles over time"...

So basically Barratt appears to have broken the BRs with regards to fire regulations as well, what a surprise ;-) ...

[Owain]

That'll be the new devolved physics then.

Owain

[dmc]

Hijacking the thread somewhat...

We are considering a loft conversion (probably summer 2006) but have not idea of the likely cost. We have a nice open loft that looks ripe for conversion (a couple of the houses on the street have already). I know there are companies that specialise in loft convs but we have a few things that we would want to get sorted. We want our existing backboiler in the front room removed and replaced with a decent boiler (in the loft pref). The roof is currently slate with no felt or anything and needs a bit of work. I just have a feeling that a loft conv company are not the best to sort out the roof and design, spec and fit a new heating system...

I like the idea of getting an architect to draw up plans and come up with ideas. Preferably I would want someone to run the whole project for me - I can't be arsed trying to coordinate builders, plumbers, inspectors etc etc but we are worried about what sort of cost an architect and a project manager (maybe the same person?) would add to the conversion.

So, any one who has been through this got any advice? Is 30k a reasonable stab at a budget or is that too tight? Also, if anyone has any recommendations for tradesmen in the southeast (folkestone area) who would be able to take on this work then please let me know! The people in the street who have ha conversions claim they wouldn't recommend the companies that did theirs anyway! (I don't think either of the companies currently exist anyway. At least, not under the name they were using...).

Should I just approach an architect? You mention having a "feasability chat" - is this likely to be something architects offer usually?

When should I start looking at this seriously? We are locked into our current mortgage till may 2006 so would not be able to free up cash till then at the earliest.

Would this rule out the escape window being above a large full width conservatory? If so then we would need the escape window in the front of the house - something we would rather avoid.

Any doors on existing floors that open onto the escape route need self closers fine but I assume they would have to be firedoors as well? We currently have glass paneled doors to get some light into our dark hall way - replacing those with fire doors would be shame (but obviously if it is deemed the "safe" way to do it then fine. Is the idea of all this that occupants of the new 3rd floor can always excape down the stairs and out or is the idea that they stay in the fireproof 3rd floor waiting for the fire brigade to rescue you out of the window?

I guess both options would be the best solution :)

It seems like a big project and even though I'm not planning to do much of it myself we are wondering were to even start :-/

Cheers,

Darren

[Dave Plowman (News)]

A decent local architect will likely have all the connections to provide a turnkey solution.

[Hugo Nebula]

On Sun, 10 Apr 2005 16:46:12 +0000 (UTC), a particular chimpanzee named hansen randomly hit the keyboard and produced:

You would have thought so, but not necessarily.

In 2002, the English (and Welsh) guidance changed the calculation method to take greater account of bridging elements, such as joists, mortar beds in insulating blockwork, etc., and correction factors for wall ties, gaps between insulation boards, etc. The upshot of which is that the amount of insulation required to achieve the same U-value increased. I don't know if Scotland is behind or ahead of the rest of the UK in this.

He who pays the piper...

Baratts virtually own the NHBC, and I've heard anecdotes of how NHBC inspectors were threatened with the sack for picking up too many faults on Baratt houses.

Just be grateful that you don't live in England where the NHBC can carry out the Building Control inspections as well. The contraventions on your house may have been missed by the BCO, but it's very unlikely to be deliberate, and who knows what they _did_ pick up and make the builders put right.