Learn to Build – The First Floor
The First Floor
At this stage, take time to work out the level of the existing first floor of the housem in relation to the existing outer wall.
When you think that you have ascertained the correct level, break through the existing house wall and then you can confirm just how accurate your calculations were.
Use a long spirit level or a straight piece of wood with your spirit level resting on it. Now you have the exact level. Suspend the joists in strong metal hangers.
These metal hangers sit or should I say hang on the wall by a flat blade of metal.
Strapping and Restraint
All areas greater than 70 square metres must have lateral support in the form of internal partitions or cross walls.
All walls must be restrained at intermediate floor level, at wallplate level and down the rake of each gable by the provision of 1,000 x 30 x 5mm lateral restraint straps.
Crossing at least three joists or rafters with noggings below, 38mm wide by three quarters of the depth of the main timber.
These metal restraint straps can be notched into the joists where necessary to maintain a floor level.
Intermediate First Floors
The floor can be constructed using sawn lumber joists to the sizes as specified in the TRADA tables (Timber Research & Development Association) I should explain that for most homes that means that if 47 x 147mm joists are used the maximum spans are 3.06m at 400mm centres, 2.94m at 450mm centres and 2.61m at 600mm centres.
For 47 x 170mm joists these spans are 3.53m, 3.40m and 2.99m respectively and for 47 x 195mm, 4.04m, 3.89m and 3.39m.
Ideally, the joists, should be spaced at 400mm centres. Place the first joist about 25mm away from the outer wall of the extension, and thereafter at 400mm centres.
Providing the joists are thick enough and the flooring is strong enough, the joists may be spaced up to 600mm apart but, I don’t recommend it.
Wider spaced joists often leads to noisier, more springy floors.
Ensure that the joists are perfectly level in all directions. If there is any discrepancy in the heights of the joists, take the highest one as the correct one, and raise any lower ones to match.
A small piece of thin slate often does the trick.
As soon as possible nail noggins in between each joist to hold them steady and in place. The noggins can be cut from the same size wood as the joists but, or at least 75% of the height.
Make sure that you saw them perfectly square to ensure that the joists remain perfectly perpendicular.
Where joists lie beneath and run in the same direction as internal partition walling and beneath baths, they should be doubled up and bolted together.
The void between the joists should be insulated with a minimum thickness of 10kg/m3 of proprietary sound insulation quilt with 15mm plasterboard beneath and 20mm tongued-and-grooved softwood boarding or moisture-resistant chipboard decking to give at least 30 minutes’ fire resistance.
The joists should have solid blocking at each end between the hangers using timber at least 38mm wide by a minimum of three quarters of the joist depth.
Spans of over 2.5m up to 4.5m should have midspan strutting, either solid block noggins, timber herring bone or proprietary metal. Spans over 4.5 metres should have strutting at one third points.
Where joists are notched for pipework, the notches must be at the top of the joist, no deeper than 0.15 of the joist depth and positioned in a zone between 0.1 and 0.2 of the span.
Holes in the joist must be in the centre of the joist depth, must not be more than 0.25 of the depth and must be in a zone between 0.25 and 0.4 of the span.
There must be at least 100mm horizontally between any notches or holes.
Softwood or hardwood tongued-and-grooved flooring should be cramped up and either double or secret nailed to each joist. Nails should be two and a half times the thickness of the boarding and punched home.
Chipboard should have the tongued and grooved edges glued together and be laid with the long sides at right angles to the run of the joists.
They should also be glued to the joists. The boards should be nailed with ringshank nails two and a half times the thickness of the board or screwed at 200-300mm centres along the joists or beam-and-block floors.
Boards must run to a joist on all sides or be supported by a beam-and-block floor with an expansion gap of at least 10mm left where the boards adjoin or butt up to a rigid upstand or wall.
Manufactured and engineered timber T beams can be used in lieu of ordinary timber joists. They must be sized and installed in accordance with the manufacturer’s instructions. ‘I’ beams are never notched.
Instead, they are manufactured with punch holes, which can be taken out as appropriate.
First and intermediate floors can also be created using a concrete beam-and-block system, which is often strong enough to support blockwork upper part partitioning.
Once again the size, spacing and layout of the beams and blocks must be determined by the manufacturers.
Once the joists are in place, get the B.I. to do his inspection, and then you can continue the upward build. Don’t forget the wall ties!
I am sorry for having to blind you with science with all the above figures, but I am just trying to save you hours of research later.
Please do not let all these figures scare you off.
It is absolutely basic after reading it a couple of times, and you can always ask us or our fellow bloggers for clarification or help.