Considering a loft conversion in a shallow pitched roof, raising joists would impact useable space

[Martkimwat]

I am considering a loft conversion in my 70s built end of terrace property. It has a low pitched tiled roof, spanning 9M with a 2.6M ridge height above the 2"x4" rafters and purlins supported on vertical struts at approximately mid span with a horizontal brace between the purlins at approximately 2M spacing, and 1,5M high which would obviously need removal. I am concerned about trying to avoid increasing the depth of the 2"x4" ceiling joists as this would impact on the useable space available. Is there a way round this? the usable space between the purlins is 3.8M. The main issue apart from useable space in the loft, is the staircase which would need a right angle turn at the 9th/10th step to allow the top step to more or less coincide with the ridgeline, but also allow the main staircase to be located in the existing single bedroom below and still have room for a door into that room, albeit a 695mm door, not a 750mm door. any increase in height will create a similar increase in the length of the flight of stairs, (which I have already calculated using the maximum pitch of 42º). Increasing the number of steps after the landing will have a detrimental effect on ceiling height in that location in the adjoining room ( I can accept three steps but not four) and the landing would already require a two-step arrangement rather than a flat landing. Because this is a one bedroom flat in a conservation area, I am already concerned about getting planning permission, including permission for velux windows which may only be allowed, if at all, to the rear of the property. I am also hoping they will allow a window in the end wall as it is an end of terrace property, but those are concerns to be addressed at a later stage.
Additional information:
The main staircase will run parallel with the ridge line and then turn against the party wall for the last three steps. One last point, is a door required at the top of the stairs before entering the loft bedroom and if so, being at right angles to the last flight of steps, does the 400mm distance from the top of the stairs rule still apply?
(Which is another potential reason for restricting the top flight of stairs to three not four as it would push the door too far away from the centreline and meaning it would potentially have to have an angled cutout at the top which I don't want.) The walls appear to be prefabricated "egg box" style construction of only about 75mm thickness with extra thick plasterboard bonded to a cardboard matrix internally with approximately 2"x2" timber verticals at 400~500mm centres. The wall that the main part of the staircase will run alongside of, is about 500mm off the approximate centreline of the property.

 

[cdbe]

Have you considered lower the floor into the rooms below?

 

[Martkimwat]

The floor below are standard height ceilings (2.45M) and it would be massively expensive and disruptive to do that as well, it would basically mean completely boarding and re plastering the entire floor below along with coving in all the rooms. I'm not sure you are even allowed to reduce ceiling height below that figure if it is already extant, whereas older properties like my home which already have low ceilings are OK (1795 property with 2.2M ceilings reducing to 2.05M under the Oak beams). What is the minimum for floor joists, 2"x 6" or 2"x 8"? I would be more inclined to put in additional lateral support beams (steel) into the party wall and end walls to provide additional support for the joists, it would be less disruptive and if clad with plasterboard with additional coving, would blend in reasonably well. I don't know if that is an option though, I'm guessing it would need a corbled or bracketed support on the party wall as going into the party wall is probably not allowed (likelihood of breaking though into the neighbours side of the wall). My neighbour already has a loft conversion that was carried out by the previous owner, I might check with them and see what they have done, although their property is the original three bedroom house and they had the stairs on the other side of the wall and go straight up ,so they don't have an issue with the length of the stair run, it does make their loft conversion much smaller though and less adaptable. My property ( and the flat below, both of which I own) were converted by the original purchaser during the initial construction into the two flats from a three -bedroom property similar to my neighbours, so that he could live in one and house his aged mother in the other.

 

[cdbe]

I believe the generally accepted minimum is 2.3m - so a bit to gain and sounds like it would help with your stairs - personally, in the grand scheme of a loft conversion I don't see the ceilings and coving a big deal. For a 3.8m span you would need something like 6x3 or 7x2. If your house has no structural internal walls (I don't know if what you have are) you may be looking at much deeper joists spanning front to back or 9m!! steels from gable to gable.

 

[stevie888]

There is actually no laid down minimum room height but you need 2m in corridors and over stairs (except loft conversions with single room where reductions are possible.

Are you having a dormer?

If you don't want to drop the ceilings then a possible option is to use several small steels set between the joists: you need a sensible and sympathetic designer

 

[Martkimwat]

Thank you CDBE for your reply that’s useful information. Regarding the walls in the floor below, I don’t believe any of them are structural. As mentioned in my original text, I think all the walls are that ‘eggbox’ construction which I believe was quite popular in the 70’s although god knows why, they are a nightmare if you have to modify them, and I am assuming they would have to have been bespoke. I had a damp issue in the bathroom where water managed to get behind the tiles and soaked the plasterboard and subsequently the internal timbers, causing some of them to rot. I had a horrible job removing the plasterboard and replacing and adding additional timbers to add stiffness, without going right through the walls in to the adjoining hallway and bedroom because I had remove all the plasterboard on both internal walls which of course was originally bonded to the carboard eggbox arrangement. As it was a bathroom toilet I ended up plywood boarding it and them covering both walls in 10mm UPVC shower wall in a marble effect which ended up looking very smart. The idea of interjoist steels intrigues me, can you provide more information please Stevie 888.? I am already aware from my neigbours that the local council will not allow dormer windows (they tried for a pitched roof dormer and failed) so like they already have, I am hoping that I will allowed velux windows although I don’t believe they will even allow them along the front of the property. As mentioned it is a row of four linked terrace properties built in the 70s of which mine is on one end and surrounded by much older properties, typically three or four hundred years old, hence it being a conservation area of Horsham yet only 300M from the town centre. The local planning officer is very anti the properties claiming she would never have allowed them to be built if she had been around when planning was granted, hence my concern regarding whether planning permission is required ( and may be refused) although I would still have to look at losing headroom in the loft conversion, the issue with regards the stair run could be resolved if I were allowed to increase the pitch slightly beyond 42º. Is that set in stone? My old cottage has 45º pitch on both the original and loft conversion stairs although I believe the loft conversion was an ‘informal’ one. The stairs are one above the other so needed to follow the same or steeper pitch, especially with the low headroom to start with. it was carried out at least 40 years ago so the rules may have been different then.

I should add that the property is being run as an Airbnb so I don’t know if that has any impact on the issue. I have had to have 30 minute firedoors installed along with the obligatory linked smoke and fire alarms in every room. The reason for creating the extra room is to increase its appeal and short stay rental productivity. At the moment I can only take three people max ( one on a chair bed) whereas with the extra room I could increase that to four, all in proper beds and additionaly a child in a cot, in other words an average family.

 

[cdbe]

As mentioned above, their are various flexibilities with the stairs including headroom and pitch where access is to a single bedroom.

I don't know what the span is side wall to side wall but if you can avoid the 9m gable to gable and PWA you could consider telebeams.

Fairly thick layers of insulation fixed between and below the rafters are required to meet current regulations.

If the room is a "den" for children, I guess that within reason the headroom is slightly less important.

Probably time to get a local designer in now to have a look in person and guide you on the planning issues as well.

 

[Martkimwat]

Hi CDBE, I think you misinterpreted my dimensions, the 9M is the totalt span of the roof (along the joists) the actual unsupported span being 8M between the inner leaves of the front and rear walls, the gable to gable span is 5.65M unsupported span from party wall to end wall of my property. I am thinking now about a steel joist running from the party wall to the outside wall that could be set along the centreline of the building that could possibly even have it’s unsupported span reduced to 3.5M with a vertical steel support going down to a sub floor spanwise steel supported on the brick walls of the floor below(a separate flat that I presently own) that would not be seen in that ground floor flat. The span of that steel would only be 3.5M running spanwise, not parallel to the steel beam it is supporting. The vertical would line up with the dividing wall between the front room and the hallway so it could be relatively unobstrusive. the Mainsteel could be supported at one end by a wall bracket that would be up the party wall at one end and so hidden behind the end wall cladding in the loft space and theother end supported on the inner leaf of the gable end wall, the vertical support would break the span of that steel into 3.5M and around 2M and in turn would would support the joists at midspan reducing their unsupported spans to 4M or less. You mentioned 6”x 3” or 7”x 2” joist supporting a 3.8M span , is that the amximum fro those sizes? I could probably accept an extra 2” reduction in headroom in the loft space if I double up the existing joists with 6”x 3”s. I will eventually consultant an architect/structural engineer but only once I have thoroughly checked the feasibility of the project and that it meets with what I am trying achieve.

You mentioned that the rules can be relaxed for a single bedroom, there will also be an ensuite shower and toilet in that conversion, will that still be classed as one room or two?

Lastly, can the rafters, which are currently 2”x 4”s still be used or do they need increasing as well? I would be thinking of using 4” Kingspan foam insulation rather than rockwool/ FG. Though more expensive, it has a much better R value compared to the equivalent thickness of Rockwool/FG. The cost difference would be offset by the lack of having to increase the depth of the rafters to accommodate the thickness of Rockwool/FG., although not as good as I had been led to believe. I understand 4” Kingspan K5 has the same R value as 6.5” of rockwool, so to achieve the equivalent of 10” of Rockwool ,I would need to use 6” of Kingspan, however its possible that 80mm (3.5”) of K5 may be sufficient for a loft conversion, depending on local building laws. (something I would need to discuss with my local building control department)

 

[cdbe]

Yes. 5.7m gable to gable is more reasonable. Probably better with the conventional two steels below the purlins than an arrangement with posts; the PWA act allows you to put steels on the party wall and in a modern house you should only need to use the leaf on your side. Timber span tables are readily available online to play around with sizes. Relaxation on stairs would include an ensuite to one bedroom. Existing rafters probably ok but usually need doubling up around openings when veluxes are fitted. I presume you're aware of ventilation and cold bridging when it comes to insulating the rafters - so you'd be looking at 50mm PIR between and a fair thickness of continuous underneath - but BC might allow a bit less due to your headroom