Size of timber

[andywatson]

Hi all, and thanks in advance for any help.

I am trying to work out the price of converting our garage, but I need to make sure I am using the right materials etc!

I have to raise the concrete floor by 155mm to match the rest of the house, and was planning on laying joists and then boarding over the top. How thick would the boards need to be, and how far apart?

As a side note since I will have about 150mm under the boards will I be able to use a cheaper insulation under the boards or should I buy more expensive stuff?

Cheers

Andy

 

[andywatson]

Spent a lot of time yesterday looking through previous posts, and it appears to be a bit more complicated than I initially thought.

My revised plan is to hang the joists, which should leave a couple of inches underneath that I can run a DPM (although there probably is one already under the concrete) and then all the electrics and plumbing.

From my reading 400mm centres are recommended, but I am a bit worried because the joists will be 4m long. Will 100mm*50mm joists be ok for this, or do they need to be thicker? As an alternative/additional support do I need to support the joists in the middle. I was watching a DIY program last night and they basically appeared to have nailed vertical "legs" onto the joists. Would that make any difference? I don't want my floor to be bouncy in the middle!

 

[Deluks]

You'll need 8x2 for that span, with intermediate supports you will get away with 4x2, use 3 paving slabs of 50mm per joist, with dpc between timber and slab. Better check with your bco first though. He'll probably suggest deeper joists/smaller support.

 

[andywatson]

Cheers for that.

8*2 are out of the question, since the concrete sub floor is only 6"(ish) below the level of the new floor!

I suppose I could then to back to my original idea of sitting the new joists directly on the concrete (which is both level & in good condition), albeit with a DPM between. I would imagine notching joists that are set on the floor would be no problem either.

A there any disadvantages of this? I am thinking the bigger boards would cost more, but this would probably be much faster and the price would be offset by not needing joist hangers. I can see that to get the floor exactly level with the rest of the house there might be issues with the thickness of the boards I put across the joists. But since I plan to ply/chipboard before then laying a solid wood floor (which is 22mm thick) then the actual thickness of the ply/chipboard will be almost irrelevant since I could lay the oak flooring directly onto the joists if I wanted.

Am I going along the right lines with my thinking here?

 

[Deluks]

Laying onto the floor will work, but will require more airbricks, as there will be no ventilation from joist to joist. What's the level of the ground outside, compared to the current garage floor level?

 

[andywatson]

The ground outside is exactly level to the existing garage floor.

The plan is to build a small wall across the garage door opening, then put in patio doors to give plenty of light into the new bedroom (a builder will be doing this). The small wall will bring the bottom of the patio doors up to the level of the new floor (so about 7" high).

From trying to read between the lines of your reply, it sounds like laying them onto the floor isn't what you would recommend, but I should go to your initial advice of supporting the joists midway as being a better solution?

Many thanks for your time.

Andy

 

[Deluks]

Timber joists on ground floor will need ventilation to prevent them from getting damp and rotten. So the exterior wall will need airbricks to let air in to circulate around the joists, so leaving space underneath will help this process along. As the area you have to work in is limited, you could just hardcore over the lot and just build a concrete floor, or block and beam. The best person to ask would be your builder as he can look it over and weigh up the pro's and cons.

Your first port of call should be to get a builder in to have a look at it. If you don't know of any, ask around friends and family etc for recommendations.