Building a wall inside...

January 1, 2009 5:14 AM

Victorian house construction/DIY: building a wall in a shared loft (attic). 'What's this thing?' and 'Am I doing it right?'

Our loft (attic) space runs straight through into our neighbour's house and we need to build a dividing wall. We've been trying to get a builder in to do this for years but (unsurprisingly) no-one wants the job, so it's DIY time. According to the local council there are no specific building regulations to comply with beyond creating a barrier that will be fireproof for around an hour. I think we can handle it, but there are a few complicating factors that I'm hoping the hive mind can help out with.

Here's an overall view, looking from our loft to the neighbouring house. The brick pillars are two chimney stacks running up from the lower floors through to the roof. We need to build in three parts: between the pillars and to either side.

Here's a closer view of the centre, and looking up and down. At its highest point this area is 8' tall (250cm), and the area between the pillars is 4' wide (120cm). Looking left and right of the pillars.

The really awkward bits are the roof trusses (Pic1 | Pic2) and especially these deep pits next to the pillars (Pic1 | Pic2). Do these pits serve any purpose, or are they just like that to square off the chimney breasts in the rooms below? They measure 2' on each side, and are about 4' deep.

Our first idea was to put up a stud wall and clad it on both sides with fireproof board, but now we're leaning more towards using these lightweight Thermalite concrete blocks (looks like it would be easier, cheaper and stronger). Assuming that we use the Thermalite blocks, the main questions that we've got are:

1) Should the wall be one or two blocks wide? How do we ensure that the wall is stable, especially in the central area? We would build on top of the existing party wall, which looks like it is two standard house-bricks wide.

2) What do we do about the pits by the chimney stacks? The wall would have to span these pits, although it wouldn't be particularly tall or heavy at that point. How about covering them over with tightly fitting fireproof board? Would it be safe to build on top of that?

3) How do we deal with joining the block-work to the wooden rafters? Should some kind of membrane or compound go between the block and the adjoining rafter? Should they actually be attached in any way?

Any pointers, ideas or online resources very gratefully received.

posted by boosh to home & garden (7 comments total)

To simplify things I think you need to build on one side of the chimney or the other. A simple stud wall, floor to roof with 5/8 drywall on BOTH sides will provide the fire rating. That way you can ignore the trusses and the pits. The pits look like they are corbelled, probably because they relate to fireplaces or stoves on lower levels. I would not use the Thermalite. Each block weighs 6.4Kg!!!!!! You would be adding several hundred kilos, so unless you are planning to buttress the ceiling below it will sag and crack. When that happens the mortar joints in the new wall will crack.
posted by Gungho at 7:09 AM on January 1

I agree with Gungho (IANAContractor, but I've restored several old/victorian homes).

Just build on your side of the chimneys. I think building on the other side would be impossible, unless you have access through the neighbor's house. The trusses on the near side of the chimney seem to be in pretty good shape, (in the "up" photo) and, other than being peaked, it looks like it would be pretty straightforward to frame out that area with studs and add insulation. Cladding the "other" side will be a bit tricky, but it doesn't have to be finished in the same manner as your side. If the neighbors want to finish their side later that would be up to them.

Good luck. I live in an apartment now, and this questions makes me miss working on my houses. Oh, no, wait, it doesn't.
posted by Fuzzy Skinner at 7:45 AM on January 1

I'm not sure where the OP is, but in the UK, a thermalite block (also known as an aerated concrete block) weighs a lot less than a kilogram. Medium density blocks weigh considerably more than thermalite blocks.

1] I would have thought that one brick would be sufficient. If you're careful you could add some wall ties to the existing brickwork, and then build them into your new wall. You can get some that screw into an existing wall, and then have a V shaped protrusion to concrete over.

2] I have no idea. Unless maybe you built a "lid" for the hole out of bricks, somehow.

3] Joining block and rather is going to be difficult. I would seal the join with fireproof sealant (it puffs up when exposed to high heat) and then use a tie to screw into the block and the rafter. One thing to keep in mind, though, is that your rafters should not be supporting your wall. Your wall should be able to stand up under it's own weight. It shouldn't need fixing to something else.

Check out your local Wickes store, and see if they do a Good Idea leaflet. They seem to cover an awful lot of eventualities. Also, try getting a builder in to see the job, and ask them how they'd approach it.

I am not a builder, nor am I experienced in any way with this sort of work. I just used to work in a DIY shop. Good luck.
posted by Solomon at 7:55 AM on January 1

The 6.4kg weight comes directly off the link provided. Perhaps you are thinking of smaller-sized blocks?

Personally, I'd frame it with 2x4's (or even, if the firecode would allow, 2x3's to save a little money and weight), with drywall on each side. Insulated if noise or temperature were concerns. It's just a partition wall (not holding up any weight), so it doesn't need to be as overbuilt as most walls, just whatever the minimum is that will get you the fire resistance you need.

But I'm not a contractor, and I live on another continent where building practices and regulations are quite different, so adjust accordingly. Have you considered hiring a contractor or a building inspector for an hour to come, look at it, and talk you through how the work should be done? Just offer to pay in cash what they would charge by the hour, and it shouldn't be hard to find someone.
posted by Forktine at 8:15 AM on January 1

just whatever the minimum is that will get you the fire resistance you need.

If you're going to be blocking off a conduit between two residences, I think you'll probably want to keep the thermal & sound insulation figures in your solution. Do it right the first time.

I agree with Forktine--frame around the chimneys. No way your going to be able to keep masonry aligned with a chimney, particularly as old as those look. Over time the house will settle & shift, the chimney will break away from the walls and require constant attention to keep it sealed. If you frame it, you can give yourself a comfortable amount of wiggle room.
posted by Civil_Disobedient at 10:12 AM on January 1 [1 favorite has favorites]

I agree with the above people that the blocks are too heavy for a attic and would be a pain just to get them up there.
As for codes and fireproofing I can only say what I would do in California under the IBC building code.
In old houses we see a lot of wood mounted on chimneys and fireplaces and even if it was already there we are not permitted to add more directly to the masonry (1"-2" inch clearance to combustibles). Note in the US (and the IBC) sheet rock is considered combustible (it's covered with paper).

I would do steel studs with cement board (on the chimney) and sheet rock/cement board covering the rest. Mostly fireproof, no rot, and easy. Just run the track up, down and over on a vertical plane then fill in with metal studs as needed. You can use the really light studs as there is no load bearing issues to deal with. Use fire caulk for the tiny gaps as suggested.
Get some SHARP snips and a screw gun/drill, good gloves and glasses then go for it.

You would have fire resistant wall, light weight, no rot and it could be opened in the future (just unscrew) if needed by you or the neighbors.

I keep saying rot as I can see water damage in the pictures on the rafters and stains on the bricks.
As for the pits I would do a cement board and plywood cover for fireproofing and strength (step on safe).

I am a contractor, but I live on another continent where we measure in inches but drink in pints.
posted by blink_left at 12:58 PM on January 1

Thanks everyone.

The steel studs/board/fire caulk option looks ideal - thanks a million for the idea and the link, blink_left. Now lets see if I can get this thing constructed without putting my foot through the ceiling.
posted by boosh at 3:41 AM on January 2