5. Timber Floor
As discussed on the previous page, if you’ve gone for a concrete base then you’ve done your foundations and floor in one – congrats! You can move on to the walls!
However, if like me you have decided on a timber floor then you have an extra couple of steps to your build but on the upside it’s great practice for the walls. 😊
This is quite a long video as we look at screws vs nails, span tables, C16 vs C24 timber and more:
It’s worth taking your time with this stage as getting the floor as close to perfectly square as possible will set you up for the rest of your build. Having 90 degree corners will ensure your walls meet snuggly and even fitting your skirting boards will be a whole lot easier.
The price difference between C16 and C24 timber is negligible and some builder merchants only supply C24 so using it for the floor and roof can help move you down a timber width in the span tables and therefore keep more of the building’s height for you in terms of headroom.
I’ve created a more simplified version of the span tables than that shown in the video which you can download from the Resources page. This includes some deeper explanations and recommended timber widths. We’ll be coming back to these in the roof stage! 2023 update: there are now span tables freely available online!
Where to buy materials
For the general consumer in the UK, when you think of DIY you may think of B&Q or Wickes but for most building products they are bit more expensive (good for flooring, kitchens etc. though). 2023 update: Wickes has become more competitive the last few years and I now buy a lot of my materials from them, helped by their 10% trade discount which they aren’t too fussy about handing out 
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The level up from these are the national builder merchant: think Selco, Jewsons and Travis Perkins. Builder Depot is another but in the southeast of the UK only. Particularly for Jewsons and Travis Perkins you’re best off emailing them your materials list upfront and asking for a quote which will often be significantly lower than prices advertised on their websites. Even simply going in store to Builder Depot is cheaper than buying online.
After checking all those look around you for independent builder merchants as they are often the cheapest. Where I live now, near High Wycome, Country Supplies, a single branch supplier specialising in fencing offers the cheapest carcassing timber.
Things to bear in mind:
- Search on Google Maps for ‘timber/builder merchants near me’ to find the local ones.
- Suppliers name their carcassing timber differently, sometimes you’ll find it under ‘treated’ or sometimes under ‘studs and joists’ etc. The stuff we use for the outer shell of the building is not CLS timber, so don’t make that mistake.
- The cheapest place for one material, is likely not the cheapest for others so you can buy specific items from different place but the cost to you is more research, more time, more hassle and possibly extra delivery costs.
- Things are often cheaper in store. Builder Depot for example is cheaper when ordering over the desk.
- There are bulk buy discounts. Builder Depot goes down to £5.63 when ordering a pack of 121 timbers.
- Not all suppliers stock every item, length, thickness, timber strength etc.
- You may be able to hustle a trade card for discounts or join a club: landlords can join LNPG which gets you 10% off at Jewsons. Wickes’ discount I’ve mentioned above.
- Screwfix and Toolstation are best prices for screws and nails.
- I recommend this place for joist hangers, brackets etc.
As we go through the build, we’ll need to use more specialist companies for fascias, roofing, cladding etc.
*In the video I mention you need a trade card for Selco. This isn’t true anymore, anyone can go in or order online.
400mm vs 16" centres re-visited
If you’ve watched the series all the way through, you’ll know that I change my mind regarding joist and stud spacing in Part 16, due to plasterboard lengths not suiting 16″ spacing. It’s a little more of a difficult decision with the floor though as plywood is harder to find in a metric size. Both will work and trimming down some ply or plasterboard doesn’t take too long OR you can go with 16″ for the floor and switch to 400mm for the walls and roof 
. There are other spacings such as 450mm or 600mm which work too and can reduce the quantities of materials needed
Nails vs Screws re-visited
I explain the pros and cons in the vid but whichever you chose, you’ll end up with pretty much the same result – both do the job.
If your budget is limited to an impact driver then screws will certainly be the way to go over slaving away with a hammer. Your hands will thank me!
However if you can stretch your budget to a nail gun, the whole process will go a lot faster and as well as attaching timber together, it can be used for the OSB sheathing.
If you plan to finish your garden room with timber cladding, a first-fix nail gun can be used for ‘face fixing’ i.e. shooting a nail through the face of the cladding where the nails remain visible – larch is usually fixed this way for instance. However if you are planning on a ‘secret fix’ through the tongue of the cladding then a thinner, almost headless nail is required – a ‘brad’ or ‘pin’ nail which, yes, requires a second-fix nail gun which is how I attached my cedar cladding.
I now have both (DeWalt) but there are many accidents with nail guns so they aren’t for rookies. If you do get one or both, where eye protection and turn off the bump action, there’s just no need to go that fast as a DIYer. Overall my advice is to take it slower and go with an impact driver and screws.
By the end of this stage, you’ll have accomplished some of the hardest parts of the builds. From here on, things move forward more quickly and you’ll start to see your building take shape.
Task: Buy the timber, cut it, put it together!