Loft Conversion structure and insulating party wall

[jonifife]

Hi

I have put a building regs app in for doing our loft conversion and have had a list of questions back from the plan check. Most I have dealt with but any thoughts/advice on the following welcome as really can't see the sense in some of it.

Main issues are structural-

1. We have a load bearing masonry wall down the middle of the house which makes domestic floor joist spans relatively easy as spans less than 4m @400mm ctrs either way- the problem is there is an existing knock though on ground floor which Mr Inspector has asked for a calc.

I know there is a steel beam as we checked when moved into property . Is there a beam size table I can use or do I really need to get a struct eng to calculate and charge a day rate for a 5 min job . My gut feeling is its really not an issue as 1st floor is sound as a pound.

2. BC have asked for confirmation of lintels over 1st floor windows- Rather than mess about with putting new ones in and messing up plasterwork on 1st floor (as think they are just 90 year old timbers so not man enough), is it acceptable practice to double up the new floor joists either side of window and run a trimmer across ( window width vary 1.2-1.5m joists are 200x50mm).


3. There are some diagonal struts at mid purlin span which we will be taking out they don't appear to be doing much as they are flappy BC have asked to confirm support for the puriln (275x50mm spans about 5.1m). Are there any tried and tested means of doing this- vertical strut onto a doubled up joist?

4. BC have also asked that we insulate the party wall under part L- this doesn't make sense as doesn't this create a problem with interstitial condensation? and the party walls will be the best insulated walls in the house. On stair side it will run to first floor and eat up space on what is a fairly small house.


Cheers

J

 

[freddiemercurystwin]

Employ an SE and stop being a tight git.

You need to insulate the party wall to keep the heat in.

Rocket science loft conversions are not!

 

[tkmax]

Hi, just my 2p worth

1. Any additional loading on ground floor beam would need to be justified by calculation, i.e. engineer. They're not all robbers though, shop around.

2. Yes, generally ok.

3. This will probably need to be designed/justified by an engineer, a double joist may not be sufficient to support the prop (and a horizontal tie back to the rafters may be needed if the purlin is in the plane of the roof, which it probably is).

4. Gable wall insulation probably more to do with reducing heat loss through a new 'thermal element' rather than condensation, assuming there is no insulation there at present. I'd suggest negotiation with the BCO if tight for room, over 5% reduction in room dimension would be unreasonable.

 

[mointainwalker]

I don't understand why they require the party-wall to be insulated.

Heat loss depends on differential temperatures and since the temp differential between two inhabited spaces is likely to be minimal, any heat loss/gain will be miniscule.

 

[RonnyRaygun]

1. We have a load bearing masonry wall down the middle of the house which makes domestic floor joist spans relatively easy as spans less than 4m @400mm ctrs either way- the problem is there is an existing knock though on ground floor which Mr Inspector has asked for a calc.

I know there is a steel beam as we checked when moved into property . Is there a beam size table I can use or do I really need to get a struct eng to calculate and charge a day rate for a 5 min job . My gut feeling is its really not an issue as 1st floor is sound as a pound.

An SE shouldn't charge you more than a few hours for such a job. A proper job would include checking bearings, piers, and foundations for the increase in load, but probably not worth worrying about that unless BC specifically ask for it as it could lead to a lot of disruption!

2. BC have asked for confirmation of lintels over 1st floor windows- Rather than mess about with putting new ones in and messing up plasterwork on 1st floor (as think they are just 90 year old timbers so not man enough), is it acceptable practice to double up the new floor joists either side of window and run a trimmer across ( window width vary 1.2-1.5m joists are 200x50mm).

Can't see any reason why you shouldn't trim around the lintels. Doubling up the joists would probably be fine but your SE would calc that for you.

3. There are some diagonal struts at mid purlin span which we will be taking out they don't appear to be doing much as they are flappy BC have asked to confirm support for the puriln (275x50mm spans about 5.1m). Are there any tried and tested means of doing this- vertical strut onto a doubled up joist?

You might need a new purlin. Otherwise, are you having a dwarf wall under the purlin? Then this could carry the roof load down onto new steelwork...designed by an SE!

I don't understand why they require the party-wall to be insulated.

Heat loss depends on differential temperatures and since the temp differential between two inhabited spaces is likely to be minimal, any heat loss/gain will be miniscule.

Doesn't he mean insulating the party wall in the loft conversion, as a well insulated attic shouldn't be much warmer than outside?

 

[tkmax]

If the lofts either side haven't already been converted, or if the party walls have a cavity which isn't fully sealed, then they're effectively external and the party walls need thermal insulation.

If its a habitable space each side then I'd be more worried about sound insulation depending on the party wall construction

 

[jonifife]

Thanks for the info

I will have to dig deep for a s eng then, I have just come across a few I would like to burry in my time Want to avoid adding steel though as into pary wall and all - will be intresting to know if my gut instinct is right about the beam - lets wait and see!

Re insulation

The loft to one side has already be converted, the other hasn't. Its a bit of a pain to insulate as the stairs will run up the one wall meaning taking the finish line down to first floor or along the sting of the stairs.

What I don't quite get is if I insulate the cold side will be a condesation risk? and will be difficult to vent and I think the 5% rule will aplly sop will mean about 40mm of insulation. Quite right plaster boaridng over will be good for sound proofing though.

Cheers

 

[jeds]

The new Part L does require party walls to meet certain u-values under certain conditions but this is to compensate for convected heat loss through cavity structures and has nothing to do with heat passing through the wall. the question is; is it a cavity wall? If not then heat loss insulation may well be worthless and BC have misunderstood the requirement. Which wouldn't surprise me as the way Part L is written is very challenging.

Bear in mind that English and Scottish Part Ls are not the same. There are subtle differences in requirements.

 

[mointainwalker]

What I don't quite get is if I insulate the cold side will be a condesation risk

]

I am actually finding it difficult to understand exactly what you are saying, but if i have understood correctly, the answer is no.

Condensation happens when warm, moist air meets cooler air/material and the moisture condenses out. If you prevent the moisture from meeting the cool surface , then no condensation.

You do this by putting a vapour-barrier ( plastic-sheet, foil backed plaster-board) on th warm side of the insulation to keep the moisture that side so it does not pass through to the cold side and condense.

Since this is a loft area and the other side may/may not be heated, then you should insulate it, whether they require it or not ( logically I think they should). In this instance I think single-brick or cavity-wall becomes irrelevant.

 

[jonifife]

Beam was fine as thought- but only just and have past plan check! Hardly like i am going to have 20 people partying up there. Have some build issues now but that's another post!