Levelling Floor Joists: How to Level an Old Floors

Summary: Levelling Floor Joists. This project is about levelling old timber floors, repairing and replacing timber joists. We show you how to lay floor boards over the leveled joists, how to cut the floor boards to the right shape and how to use a chalk line. Ultimately you will have a level floor with well fitted boards.

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Floors in older properties are very often out of level. Generally this is caused by a bowing of the floor joists which, very often, are not strong enough to fully support the weight placed on them. Over the years they bow and make life difficult when fitting new wardrobes or showers etc.

Rather than attempt to lift the floor by "packing out" between the boards and the joist ( the packing invariably works loose as the floor moves with use) it is easier, and more cost effective in the long run, to remove the floor boards and level the joists by fixing new, straight ones, to the existing. It is also possible to repair and replace sections of joists, in situ, without disturbing the entire floor.

Leveling a joist:

If, as shown above right, the brickwork itself is loose, damp and crumbling, then check the joist for wet and dry rot.

Bowed Floor Joist

Bowed Floor Joist

New, sawn timbers can be bought from your local builders or timber merchants. Each new joist should be checked by laying it on a level surface, on its narrow edge, before purchasing. There is little point replacing bowed timbers, with bowed timbers!

The existing bow may be the result of a weak timber, in which case simply fixing a new, straight one to the side will be effective. The new timber should be taken the full length of the old joist even if there is only a slight bow in the middle. Remember the old joist bowed because it is too weak, you must therefore give it as much strength as possible.

If the bow, or slope, is due to the base of a joist support (brickwork or timber) having crumbled away over time, then this fault must be addressed before a satisfactory repair can be effected. Old, crumbling bricks should be replaced, or jointed properly wherever possible. If it is the end of the timber that has crumbled, this will have happened for a reason. Look for signs of damp in the wall, and rot in the timbers. Get to the cause of these defects first and eradicate them.

If the existing joists offer little in the way of support, there is no point using them to support the new joists. You local builders merchants can supply "joist hangers", the correct size for your new joists. These are galvanised metal brackets, into which you slot a joist. They have a strong lip, which is then inserted into a brickwork joint, or fixed on a timber or steel support.

To level the entire floor, it is best to start with the two end joists. Then a string line can be fixed to the top of these two, new joists, giving you the level for the joists in between. The string line would be fixed in position x in the diagram below.

There is always the possibility, in older properties, that your floor joists have dropped at one end because the timber has become rotten and compressed. Thanks to some incredible, yet simple technology it is now possible to replace sections of joists and rafters, in situ. Take a look at the images below to see how this works.

Decayed timber joist and repaired timber joist

Decayed timber joist and repaired timber joist

Laying floor boards:

There are very few "square" rooms. The diagram below represents a typical room, within which the dotted lines represent a square section of the room. 1 & 2 are the walls of the room while A,B,C etc are the floorboards. D and G are the probable shape of cuts you will need to make.

Once your joists are in place and level as described above, floor boards can be fixed. It is important that the boards run at right angles to the joists regardless of the shape of the room and the first thing to do is set out the room so that you will get the first board down in the correct position. Using the 3,4,5 method of obtaining a right angle, which is fully explained in our Tips and Tricks pages, starting a project you will be able to mark a line across the new joists running between the points marked x on the diagram. We suggest a chalk line for this. See Using a chalk line:

It is also important that you work out the width of the room compared to the number of floorboards it will take to cover it. Measure carefully, or even use an offcut of board, marking the position of boards on a joist. This is to make sure you have a sensible cut at each end and not a full board at one end and a ½ inch cut at the other. Additional "trimming joists" may have to be inserted around fireplaces, chimneys etc to make sure the end of each board is well supported.

Lay your first board through the room, next to board A. Then, measuring carefully, mark and lay boards A, B, & C. Do not fix these last three boards. Laying the last boards, D and G against the walls 1 and 2, is a difficult job and can only be done properly if two or three boards, including the cuts, are pushed together and laid as one.

Work across the room fixing each board by your preferred method, until you get to board F. Then lay F, E and D, together with the cut G, in one go.

Floor Joist Plan

Floor Joist Plan

Cutting shapes:

Cuts D and G can be effected by, having laid A, B, C and D, E, F in position, unfixed, laying a full board against the walls 1 and 2, and marking the other side of the board onto boards C and D. This will give you the exact shape and size of the cut. Remove the marked boards, C and D and cut out the shape you have marked on them. Two new boards will be needed to replace them, while the cuts you have made are inserted at the same time. Remember to make allowance for the tongue on each board when you mark.

Cutting a Floor Board

Cutting a Floor Board

Using a chalk line:

The chalk line would be held on points x either by a bent nail at one end, with you at the other, or get the help of a friend. Pull the line tight, and then lift it and let it go. The "ping" of the chalk line will leave a mark across the top of the joists. Our demonstration photos below shows a standard chalk line, the chalk refill pot and how to "ping" a line. A very useful, inexpensive tool. If you are interested in purchasing one of these useful tools see our tools area below.

Chalk Line marking line on paper

Chalk Line marking line on paper

Chalk Line and chalk refil

Chalk Line and chalk refil

You might want to take a look at our video section on "using a spirit level" to watch a film on how to use this essential tool.

Don't fancy doing this project yourself? We work with Checkatrade to ensure that we recommend only reliable and trustworthy tradesmen.

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