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Specifying upper-storey floor structures

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How to construct upper floors and ceilings

With the scaffold at first floor level, the floor joists and any supporting beams can be built into the exterior walls. RSJs (reinforced steel joists) are often necessary to support an upper floor, especially if the ground floor is largely open plan with few interior supporting walls. Although it is possible to install RSJs and joists after the building is roofed, it is much easier to do so now when they can be lifted over the walls into position. There are three types of floor structure commonly used: the suspended timber floor, block and beam and the newer and lighter engineered timber I-joist floor.

Steel beams

When RSJs are being used to support floor structures, they should be coated off-site with a rust inhibitor (eg. red-oxide primer) and lifted into position before the floor joists are brought in. Both ends of the beam must rest on precast concrete lintels, or padstones, cut into the exterior wall, or on top, if it has only been built to the first storey so far. Make sure the padstones don’t create cold-bridging in the external walls – it may be necessary for a structural engineer to design thinner ‘spreader padstones’ to spread the weight.

The steel beam itself can be placed within the floor structure so that the joists can sit in the web of the beam. It should be a little less deep than the timber joists, so that they can be notched into the web, leaving a 10mm gap. This will allow shrinkage while preventing the floorboarding and metal from grinding together. However, with this method, solid bridging with noggings (horizontal lengths of timber) between the joists in the web will be needed to prevent movement.

Another way is to sit the RSJ directly beneath the floor and lay the joists across the top of it. It may be necessary to fix the joists on to timber plates bolted on to the beam. This method allows the joists to be installed in one length, but it does mean that the beam will have to be visibly boxed in with plasterboard in the ceiling below.

Lateral restraint straps

With all methods of floor structure, it’s important to support the external walls at each level, bracing them against the force of the wind and protecting them against any sideways movement, with lateral restraint straps once the joists have been fitted. The straps must be fixed across three joists, spaced at a maximum of 2m apart and notched into the top of the joists. Each one should then be screwed into the joists with solid noggings fixed between the joists underneath them. The hooked end sits over the external wall masonry, and becomes built in when the bricklaying is continued.

Suspended timber floor

When fixing timber floor joists, it’s important to make sure they are completely level and have adequate end bearings, if you are building them into the walls – although you can’t do this to the party wall of a neighbouring home. Alternatively, joists can be supported on hangars, but it is easy for one or two of them to be installed slightly out of line, creating a permanent dip in the floor. Laser levels are the best tool for ensuring this doesn’t happen. The table below shows the allowable spans for different sized timber joists.

Double joists

To support the upper room wall partitions, or to trim the stairwell, double joists should be nailed or bolted together. One of the problems with timber shrinkage later in homes is due to poor nailing, which can be avoided by using twisted nails, helical (self-cutting) wood screws and bolts at this stage, rather than simple galvanized round nails. If the positions of the room partitions don’t coincide with the joists, they can be supported on solid noggings (of the same depth) fixed between them. If the noggings are staggered alternately beneath the partition sole plate, you will be able to double-nail through the joist into the ends of each one.

Strutting

Timber joists need to be prevented from twisting and this is done with rows of noggings cross-braced between the joists. Using timber off-cuts as solid noggings or installing herring-bone strutting will both work. Strutting can be bought in steel  strap form or cut on site from 50 x 50mm softwood. The rows should divide floor spans between 2.5m and 4.5m through the middle, and greater spans over 4.5m into thirds by using two rows.

I-Joist floors

If you have joist spans of over 5m or are just keen to eliminate the problem of timber shrinkage and the cracking it causes to finishes, use timber engineered I-joists, eg. the Trus-Joint I-joist (TJI) or joists with metal webs. Manufactured off site like roof trusses as ‘I’ shaped beams, the thin part, or web, of the ‘I’ is made of oriented strand board (OSB) or steel diagonal bars, to make them much lighter than solid timber joists.

Solid joists creak when they shrink causing the floorboarding fixings to loosen. But with TJIs and metalwebbed joists, shrinkage doesn’t occur, so they are quieter to walk on.

As they are lightweight they are easier to install. The steel web type can have services running through them without any notching or drilling needed. Even 100mm toilet branch pipes can be run through them, an impossibility with any other floor system.

The only disadvantage for both types is that the floorboarding must be glued and screwed down for stability, which will make it difficult to take the floor up to get at wiring or leaking plumbing in the future. After the joists have been fitted, floor battens are laid across the top of them to keep them in place and upright. Bear in mind that, with this system, you can’t use the floor for storage or as a working platform until the permanent floorboarding is fixed down, as it won’t be stable.

Beam and Block

For the most robust floor structures, precast concrete ‘T’ beams have the advantage of being able to span greater distances than solid timber and don’t suffer from insect attack. The infill between beams can be done with medium-density (for sound insulation) wall blocks laid flat and grouted over with cement. Manufacturers will provide a layout plan showing how and where to position the beams. A block and tackle, teleporter or crane will be necessary for the installation.

First-fixing services

Before the ceiling below the floor structure is plasterboarded, the water pipes and electrical cables must be routed through. With conventional timber joists, there are limits to the size and positions of holes and notches to avoid weakening them. As central heating pipes are usually notched into the tops of joists, floorboards shouldn’t be fixed down yet, but laid loose until the pipes are installed, unless TJIs have been fitted. Make sure that hot water copper pipes are wrapped in fleece and don’t touch the joists or floorboards. If they do they will not only creak with expansion but char the wood over time.

Floorboarding

When using TJIs in a timber-frame construction, it’s best to fix the permanent floorboarding straight after they have been installed, to make the floor stable. All the first-fixing is done from underneath. Moisture-resistant particle board protected by a waterproof film should be used and the joints taped over. The film and tape are then peeled off on completion to reveal a clean and dry floor beneath. When fixing down the floorboards, glue the tongues of the joints and use helical type wood screws rather than nails. You will avoid a creaking floor later. The best quality screws are self-tapping and self counter-sinking.

Ceilings

As well as providing a decorative finish, ceilings also offer some fire resistance and sound  insulation to the storeys above. Standard plasterboard at 12.5mm with a plaster skim finish will be sufficient in a two-storey home, along with a high-density mineral fibre quilt between the joists for sound insulation. Otherwise, a thicker plasterboard or a sound-rated (colour coded blue) type of board should be fitted. Note these sheets are much heavier than the standard ones and always use plasterboard screw fixings that penetrate at least 50mm into the joists, rather than nails. For longer periods of fire resistance, use fire-rated plasterboard ( pink).

Ceilings will only provide you with good sound insulation and fire resistance if you don’t put in many recessed spotlights. Halogen spot-lamps produce a great deal of heat and if they are set into the ceilings, intumescent covers must be used as protective hoods over the back of each one, which is best done before the flooring is fixed in the room above. Alternatively, low energy tubular fluorescents and LED (light emitting diode) lamps can be fitted instead of halogens for a safer and more energy-efficient lighting solution.

2 comments

mjray
Posted on
28/08/16

I have a 30ish yr old Woolaway bungalow which I believe has a 'floating foundation'. Obviously a second storey couldn't be built onto it like that so over the years we have extended out and up. However, I now need a bathroom built into the attic of the original bungalow. So is it possible to just strengthen a small portion of the original foundations either side of the bungalow to achieve this? You have no idea how difficult it is to get an answer to this particular question and hope you may be able to help.
Yours hopefully
Marion Ray
email mjray@hotmail.co.uk

webmaster
Posted on
30/08/16

Hi mjray. I've posted your question in the Q&A section of the site as it's more likely to get a response there.

Andrew (Build It's digital assistant editor)

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