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Fitting flat roof joists

Can someone please do a drawing of how to fit/lay joists onto a flat roof on a utility room.
O/S dims are 8ft x 12ft. I know I have to attach a wall plate to the wall of the bungalow but am not sure how they are fitted to the other wall.

1 Answer from a MyBuilder Carpenter & Joiner

Best Answer

I'm not sure how to post a diagram, so I'll try and describe it.
As you say attach the wall plate on the O/S wall, normally by bedding it onto some mortar, make sure it is flat and level along its width and length. Make sure the wall plate is straight with a string line attached to each end of the outside edge of the wall plate (12feet?).
Make sure to pick the straightest piece of timber you can and you should be able to install it in 1 piece. If it has a bow cut it in the middle as it is very difficult to push/pull the bow out on wet mortar.
When the mortar has set attach the plate to the wall with restarint straps normally to three blocks below the plate, spaced 2m apart minimum.
The joists can join the main building in two ways.
1. If the wall plate of the flat roof is level with the wall plate of the existing house (can you make the two wall plates the same height?) then you will need to strip off some roof tiles and run the new joists through, butting and fixing them up to either the roof rafter or the ceiling joist, on top of the house wall plate.
You may find that attaching to the existing structure that the new joists don't always fall on a 400mm center for plasterboarding the ceiling. Its only a small ceiling so there won't be too much wastage, but if you have the ceiling height you could batten out the ceiling to fit with plasterboard dimensions.
2. Fix a ledger to the house wall. Do this with an expanding anchor bolt, or other appropriate masonry fixing.
The bottom of the ledger shold be level with the top of the wall plate. This makes it easy to level the joists. Make sure the ledger is not bowed along its length.
fix the new joists to the ledger with appropriate timber to timber joist hangers.
If your plate and ledger are straight and level, the you will have a nice flat ceiling!
Normally I would set the joists out at 400mm on center for the ceiling plasterboard, and set them in 50mm from the inside wall.
If you want/need a soffit don't forget to leave enough overhang on the ends of the joists. You will need to make a "ladder" on the two side walls which has the same amount of overhang as the front edge so that you get an even soffit all around the roof.
To do this cut some noggins from the same timber as your joists, all the same length.I normally space these at 600mm centers. Nail them onto the outside edge of each of the two end joists. You then need to nail another joist on the outside of the wall. This could be made up in one piece on the ground but it is more difficult to lift up. Then pack under the "rungs" of the ladder with wedges or mortar so the out side joist supporte and level/flat.
Don't forget to put some "furrings" on top of the joists so that the roof has a finished slope of 1:80. It could be more to make sure you get adequate water run off (1:40).


Answered 19th Mar 2019

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