This might be tricky to explain but I'll try anyway! I'm planning on converting my bungalow loft into habitable space and want to maximise the headroom without changing the roof pitch (45 degrees). The entire roof is being pulled off and a new cut roof built. The roof effectively comprises two pitched roofs that intersect at 90 degrees. I understand that the wall plate usually sits on top of the inner course of a cavity wall, with the rafter notched over it. Is it possible to use a wall plate that spans both courses of the cavity wall, tied to the inner course, but with the rafter notched over the outside course instead? My thinking is that this would move the rafter out by around 150mm, and if I maintain the 45 degree roof pitch, would give me around 150mm of additional headroom at any point under the sloping part of the roof. This would be a huge help in stair positioning, landing width, etc. Am I dreaming, or can it be done quite easily? cheers Derek
Derek, I'm sure I'll start something off here, but on your question as to whether you can span the inner and outer leaf, my answer would be "No", this is based on the fact that the cavity should remain unconnected to stop the transfer of water/ dampness. I know its going to be covered, but any ingress of water will leave you with big problems to cure in the future . As to your reasoning for doing this to increase headroom, why not just build the inner leaf higher and work around that? Good luck... Will
you can only count on the inside wall being able to take the weight of the roof. your idea of having the new rafters clearing the height of the inside wall at the position of the outside wall can be achieved by building up the by the required extra course of bricks. bridging the cavity at rafter level is no longer considered 'bad': in fact when I did my extension the building regs required it.
You often have to close the cavity these days which is slightly different to bridging it. You can't build up the inner skin to accept the roof at a higher level without working out how you will tie it together. The thrust of a tiled roof on an untied wall plate will make your roof spread an awful lot more than you would like.
Thanks for the replies, folks. Yoho, why is it that only the inner wall is strong enough to carry the roof weight? Surely if the outer course is built from the correct block (will be rendered finish), on the same foundation as the inner course, I can't see what the problem would be? The option to build up the inner course would presumably give me the problem of how to fix the floor joists, and tie them to the rafters. The joists couldn't sit on top of the raised wall, as then I've raised the floor as well as the roof height and gained nothing. Maybe I could build up the wall around each joist, resulting in the top of the wall looking like castle battlements, or use joist hangers. Even then, I don't see how the joists would be connected properly to the rafters. Regarding bridging the top of the cavity with a wall plate. I would have thought that damp bridging would only result when water had penetrated through the outer course, and found a way to bridge to the inner course when it was travelling down the wall cavity. Presumably, if there's water/damp at the top of the cavity, then I've got a leaking roof? Please excuse my ignorance, not my field I'm afraid! cheers Derek
Thanks for the replies, folks. Yoho, why is it that only the inner wall is strong enough to carry the roof weight? Surely if the outer course is built from the correct block (will be rendered finish), on the same foundation as the inner course, I can't see what the problem would be? AFAIK all roofs land on the inner wall. If the outer wall does not therefore take the roof weight, it may be of a less substantial contruction, or there may be different lintels used over openings, and I have seen this done. You didn't specify what the outer wall was, and if you don't know the details of the construction, or get it checked, you'd at least be safe using the inner wall. The option to build up the inner course would presumably give me the problem of how to fix the floor joists, and tie them to the rafters. The joists couldn't sit on top of the raised wall, as then I've raised the floor as well as the roof height and gained nothing. Maybe I could build up the wall around each joist, resulting in the top of the wall looking like castle battlements, or use joist hangers. Even then, I don't see how the joists would be connected properly to the rafters. (I'm assuming now that your existing ceiling joists can be used for the new floor) why do you need to tie the floor joist to the new rafters? You need to restrain the wall plate and tie the rafters to the wall plate...but if you didn't have either a ceiling or floor, what joists could you then tie the rafters to?! Take a look at the catnic website for the sort of products available for tieing all sorts of wood to wood or masonry. There are other brands too. The other alternative is to lower the ceiling but that might not be acceptable for the room below.
Thanks again Yoho. I didn't realise that rafters are not tied to floor/roof joists. I would have thought that a single wall thickness would not be strong enough on its own to resist the spreading of the rafters if they were only attached to the wall plate. I need to replace all the joists since at the moment they are only ceiling joists and not strong enough for floor joists. Some of the external walls are existing and some will be new. I can obviously influence the spec for the new outer walls from a roof loads point of view but some of the existing walls will remain and they are plain brick, inner and outer, early 1950's vintage. I don't know whether the outer course, and foundations, would be any less strong than the inner? cheers Derek
Where the pitched roof rafters land on the inner wall, it is - for perhaps 3 course of brick - a single wall... if it did/does make sense to raise your roof by building up the inner wall, you'd need to do the outer wall too to keep the same roof pitch...or have very deep fascia board! As long as the wall plate is tied well enough, and the rafters are tied to the wall plate (birds-mouth and nailed plus any other strapping necessary) they shouldn't spread.
How about closing off the cavity at plate with cavity close blocks or if not available regular clinker. set your wall plate as usual in 4x3 set the ceiling joists to wall plate over hanging the external wall by say 10", so creating a soffett then place a second wall plate running on top of the floor joists and secured with longer straps. rafters then birdsmouthed and placed tight up against joists and all neatly 4" nail secured to one another, externally remove excess timber from rafters and nib a horizontal cut on rafter levelled through to 2x2 secured to external brick/blockwork to form your soffet and joist ends cut true to take the facia board. If using 8x2 joists and 3" wall plate this gives you an extra 11" internally... I know it works as I have done it seen it done and had it specified on drawings so that rafters when forming a soffett clear the external cavity closing block on steeper pitched roofs. Another thought had you considered a gambrel shaped roof if cutting a new roof from scratch. Regards Robbo.
forgot to mention.... second wall plate also 4" nailed into the joists. You can set the joists at 400 centres and nog them out to stop any further movement after tosh nailing them into the first wall plate.
If you do it as robbo said ,you could put in longer 8x2s and longer rafters .The longer 8x2s would enable you to still be able to fix them to the rafters, The longer rafters will enable you to fix youre facsia and soffit in the orginal place,albeit with a wider soffit .
Robbo, it sounds like the ideal solution. I will need to talk to the planners and building control but if they go for it, I'm sure it will make a huge difference. It will also help restore the proportions of the building. The ground floor windows are right under the soffits at the moment, and with a 45 degree roof, it all looks top heavy. This will also give me enough extra ground floor wall height to put some decorative lintel features above the windows as well. Thanks very much! Derek
Have you considered a mansade roof ( think thats the name). This has 2 pitches, starting with steep angle, then shallower at top. Just a thought
Lovely Roof is a 'MANSARD'...a pitched roof which has a break in each slope,with a shallow top part and a steeper angle lower part,usually with dormers for the attic space. USA Gambrel Roof.
Yes, we did consider a Mansard roof but the planners didn't like it since it wasn't the original roof style. I've now had a chance to discuss Robbo's suggestion with my builder and he thinks it should be no problem so that's the way we'll go. Derek
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