Building Extension with Pre-Fab Wall Frames

I am adding a fairly simple rectangular extension on the back of my 1960s triple-fronted cream brick veneer single story house. I am going to continue with footings rather than a slab, in keeping with the elevated floor level of the existing house. As time is a significant factor for me, I am seriously considering getting my wall frames pre-fabricated. However, I have been warned by one builder mate that sometimes it's hard to get the new to join up with the old precisely due to various minor inconsistencies between the plans and reality. He thinks that with an extension it's wiser for me to build the frames on site so I can adapt to slight variations easier.

I don't mind building them myself, however there is limited space to work in, and also because I won't be on a slab the only real option I have is to have the frames lying across the joists as I construct them. I've done this before, and found it an irritating way to work.

I am using trusses for the roof, therefore it might make sense since the trusses will be pre-fab to get the same company to build my walls as well.

What are your personal experiences? Any recommendations from experience? Tips?

Thanks in advance,

Zac

Even if you do a whole house with pre fab frames, you end up having to butcher the odd one or two. I think it's an easy quick way of getting them done and will probably cost you less if you factor your time in. Then just be prepared to make some alterations at the interface if necessary. As long as you tell them the exact stud height, you should be OK.

I had no experience with framing whatsoever, prior to our reno and I found it very straightforward to build the frames on the floor joists. I laid down a some oregon planks I had to make it a bit simpler to hold the top and bottom plates up, and everything else just slotted straight in.

Also it gave me the capacity to make adjustments as I went along, particularly with windows and extra openings. Getting pre-fab frames would have been a frig, as we had some height issues and didn't have much space outside of the reno anyway.

I did have some photos of how we set it up in my reno thread, but sadly they have gone the way of the dodo.

Yes, I was following your progress Namtrak, I enjoyed all the regular photos and updates before they were swallowed in a black hole.... you must be just about ready to start your next house am I right?!

Hi Zac

I used pre-fab frames and it is a very easy way to do it. The ones I got, came with the bracing etc. and were even notched ... you probably wouldn't do that youself!!

They come in sections, so if you are really concerned about the join to the existing house ... then just leave a 1m gap at the house end of each wall and build you own custom frame to fill it.

But really, my house is 100 years old and I managed with a few shimmes etc.

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Also it gave me the capacity to make adjustments as I went along, particularly with windows and extra openings. Getting pre-fab frames would have been a frig, as we had some height issues and didn't have much space outside of the reno anyway.

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Yes it's a good point. I compared the price of pre-fab and found I couldn't buy the timber for the price they were giving me the frames for. Then I had to pay someone to make them. But if you're doing it yourself, it probably doesn't matter if it takes you a bit longer.

I'm just saying that you don't have to feel as if the frames come and that's it - you can always knock them apart and modify as and when required, which we had to do a fair bit here. Probably 80% went straight in and the rest required some sort of mod or complete rebuild.

So I guess it was worthwhile for us because we got the timber much cheaper than I could have, 90% of the labour was included in the cost and for the ones that needed to be modified, all that was needed was the occasional new nogging or an extra stud here and there.

I found that because the pre-fab guys get the materials so cheap it worked out similar for me to do it my self or have them made. So if a significant number will go straight in it's worth it.

For a simple rectangle I would thing it's the go.

What you're saying there SilentC is very much along the lines of what I was thinking. And a good point about the wood being so much cheaper than what you could get it for. I'm also short on space to even store the wood necessary to build the frames!

One point of concern for me is that the old part of the house has a pitched roof, with normal ceiling joists at 450 centres. The roof on the extension as stated previously is going to be built from trusses. Firstly, will the trusses most likely be at 450 centres? 10mm plasterboard on the ceiling needs to be hung at 450 centres max. Also, I know some people attach battens to the truss bottom chords (say 30mm or 50mm? I'm not certain) and attach the plasterboard to the battens. This is the kind of thing I need to firmly decide well before ordering the wall frames, as it will mean an important wall height variation. What is your recommendation regarding the ceiling?

Also, how long does it take for an order of walls and trusses to be completed? A few weeks or so?

Yes I am! SWMBO and 3 girls are NOT!

And yes, the prefab stuff was always cheaper. No matter how much I tried to knock people down on price - counting nails, bracing etc

Prefab is the way to go. Just make sure you are accurate and there will be little issue.

I got quotes for my reno and the cheapest carpenter was $3600, whereas the frames delivered was $1100.

if you have limited space onsite i would definatly go prefab... get the manufacturer to hold the frames and trusses till you have bearers, joists flooring down.. then get them delivered and up. ur trusses will be at 600 centers, common ceiling scenario is run furring channel fixed perpindicular to truss bottom chord at 450 centers

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Originally Posted by zacnelson

One point of concern for me is that the old part of the house has a pitched roof, with normal ceiling joists at 450 centres. The roof on the extension as stated previously is going to be built from trusses.

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You might want to check to outside roof line carefully if mixing "sticked" roof to truss roof. The older style roofing sits a little lower as the rafters are bird's mouthed over the top plate whereas trusses sit on the top plate. Not an insurmountable problem, but one to be aware of if you want the external roof lines, and the internal ceiling heights, to match between the old and new..

The issue with old houses is you get stump sag.The house ends up out of level and twisted big time.
Believe it or not some of the work on old houses is appalling in todays standards.
You may find the plate line is out of parallel or wildly out of square even between different walls.
You would really need to do a full as constructed (ASCON) set of plans including levels to know what the hell is going on.

Custom frames allow you to adjust to the anomalies in the old house that maybe 3 inches out of level, the roof sagged, walls out of plumb, out of parallel, bowed in any of those orders.

The best way to match old to new is have new returns perpendicular to the old at the join so your not lining up old out of plumb, twisted etc etc.

A truss roof is hard to adjust to a 3inch difference in one end to the other.

Another is Scotching in the new trusses with the old roof that maybe sagged.

Of course a professional will be able to "humour" the building but it all takes a lot of thinking and care.

The chippy that did my extension 10 years ago got prefab roof trusses and built the walls himself.
my house is b/v and 30 years old
The out come was this.
He measured the wall height at only one end of the existing house.
And so the day I came home from work to see all my wall frames up
they were sitting 30 mm low to the top plate on the south side of my house
He said he would look at it when the roof trusses were put on.
The final outcome was my tiled roof has big woofs in it on both sides as the extension followed the existing hips. Even after I sent him back up there for another day to try and fix!
If you want to do it right make your frames yourself to factor in all the errors that have accumulated over the last however many years.
Then with your roof get a real chippy to build you one to match if your following existing hips...
The problem with older roof is they sag and although it may look fine there could be a good sag in it that the truss manufacturer cant measure or allow for.

If your creating new hips and don't have to match ceiling heights from the old to the new exactly then prefab walls (with the 1 or 2 metre add in on site)and roof trusses will probably work fine.
cheers RileyP
A sparky ...wondering why I paid a nailgun cowboy to rush things through without taking the love and care I would of done for free.
SHMBO didnt have the faith in my carpentry prowess I suppose....
Time have changed!:) and the holiday house will be built by me!

The thing I learnt about extensions are these:
Do not attempt to match an new roof with old eg follow existing hips.
Do not attempt to extent a ceiling into the new area ( I didn't try this but dread the results I would have got...)
If your roof is tile and you need more tiles they will never be the same colour so pending the size of your extension to house ratio
Toss your whole tile roof and replace with metal as the cost will be about the same as paying for the tiles, the extra trusses needed for tile and respraying your whole roof.
I hope this helps
cheers Rileyp

Thanks guys, you've all been so helpful. As it happens RileyP, I do in fact have a tiled roof in the existing house, and I intend to have the extension in colourbond and to replace the entire old house roof with colourbond at the same time. I have a number of reasons for this;
1) I LOVE the look of colourbond
2) 45 year old house, roof needs re-pointing, gutters falling apart, and fascia boards all rotten so large-scale changes required regardless
3) It would be impossible to match the old tiles anyway

I also think that with changing roof cladding, it should make it easier to account for some slight variations that will no doubt appear on one side of the house, where I am following the exisiting roof line. The process of re-battening and so on should allow for some fine adjustments to compensate.

I shall be very meticulous in measuring things for the truss manufacturer, in particular the profile of the rafters to ensure the roof line doesn't vary.

How long does it typically take for the walls/trusses to be built?

After you get your quotes, whichever truss manufacturer you decide upon should come out and measure up onsite. I asked each of the places where I got quotes if they would come out and measure up, and one that said they dont do site visits didnt get the business. The one I ended up running with came out twice to measure up.

My only advice is that they take multiple measurements to cross check squareness etc. The pitch length on our house was different and wasnt taken into account, and the existing wall height was also different in a couple of places - needed some adjustments when the trusses went on.

The trusses had a 3 week lead in, so I ordered them 4 weeks before I needed them. Time will depend on how much work the trusser has, for every quote I got I asked how much lead time there was for a job.

Cheers

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If you want to do it right make your frames yourself to factor in all the errors that have accumulated over the last however many years.

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OK, let's look at what part a wall frame plays in the building. Apart from the obvious, it controls the ceiling height/height of the trusses and it controls the footprint of the building.

What I'm struggling to come to terms with is why people think that every part of the wall frame needs to be custom made to account for errors in the existing building? Presumably, the average extension being somewhat rectangular and under a single roof line, your new wall frames will interface with the old building in two places that count: the two external walls where the trusses will sit.

Now assuming you measure the wall heights correctly at these two points, what is the problem with making all the wall frames up to the same stud height? Would you really have different stud heights to compensate for a sag in the old roof? What is that going to do to the ceiling, window heights etc? Would you really put trusses out of line to match an existing roof line?

There are only two other changes that you might make to a wall frame to fit with an existing building: square and plumb (assuming that you don't need to change wall length). Why would you need to custom-build frames to deal with that?

The only time I can think that you would need to tailor stud lengths individually would be to compensate for an en-even floor - but you're not going to have that problem because you will make sure your floor timbers are in order, right?

Added to that the fact that you'll be getting the frames cheaper than you could make them, even if you have to spend 4 weekends mucking about at the interface, I really can't see the logic of this advice.

If you're really concerned about it, you could get all the walls in the new part made up but leave a section out on either side and build these on site to suit.

I agree with you completely SilentC.

Also Namtrak, it's good to know they are willing to come and cross-check measurements, that's re-assuring.

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There are only two other changes that you might make to a wall frame to fit with an existing building: square and plumb (assuming that you don't need to change wall length). Why would you need to custom-build frames to deal with that?

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Yeah thats right. It is those points. The is other things like window placement if frames have to shortened or lengthened relative to walls off shooting of other walls which may have particular length to accommodate a benchtops, cabinets, etc

If the back wall is out of plumb then there will be gaps top to bottom but you can fix that though it is a bit of messing around.

If one side the house is out of parallel with the other side then when you try to line up the existing you will have a kink if the side walls of the new are to be lined up with old side walls. When you build it you will see it even though the back wall measured it may not be the same for the new back wall if the side walls are out.

If the house is square, level and plumb you should be fine.

My point is most chippies would want to build the frames to suit the "old" house on site

About your love of colourbond.

The thing I dont like about it for the roof is how ordinary the white bird poo looks under the antenna
I think colourbond is excellent for everything other than the roof which should in Australia be zinc..
Its also the most reflective which makes it the coolest in summer and you cant really see the bird ???? either oh and its cheaper!

The things that sets one metal roof apart from another is the use of a scallop cutter on the ridge and hip capping's.

I personally don't like it but roofs that have this done look special and it takes a while to pick it but when you you do you go ohhh!
I think most hire places have them available.
cheers Rileyp
http://www.steelselect.com/image_man...7ff7d8fde3.jpg

Yes, it's interesting in books I read about roofing they always refer to cutting the ridge and hip capping like that, and yet I NEVER seem to observe it when I drive around admiring other peoples' roofs! (Like the guy in the colorbond ad)