AboutStuart Mawle Expertise Now been a carpenter & joiner for the past ten years in the UK. I can give avice / help on carpentry & joinery, health & safety, woodworking machinery, power tools and suppliers. I have served an indentured carpentry apprenticeship with a medium sized building contractor doing a wide range of domestic and commercial work. This has included office buildings, barn conversions, building extensions and renovations. During my career, I spent over three years as a wood trades technician at a college. My job involved joinery work, wood maching and helping CITB carpentry & joinery students` pratical projects. I am a member of Institute of Carpenters
Expert: Stuart Mawle Date: 5/8/2008 Subject: Plywood subfloor
Question I am redoing my kitchen and the problem I have is that half
of my kitchen is on a slab.Can I just glue 3/4" plywood over
the slab or do I need to drill and anchor it to the slab also.
Answer I can only base my answer on information provided, without personally seeing the job could lead me to a different answer. The main problem you got either way is you must have a barrier between the concrete and the underside of the plywood to stop the transfer of damp. If you don't then you could end up with bigger problems.
If it was me, I would use damp proof membrane as a barrier and float the plywood on top. The weight of the plywood with gravity will keep itself down. It would be ok to do this if you have to screw fixed the plywood to timber joists on the other sub floor section. You will need to check with current building regulation whether you are meeting the insulation requirement for floors. I am not up to date as I moved into joinery base work than site work.
A DPM is like a thick quality plastic sheet. With a floating floor you need to glue the plywood together (like chipboard flooring) obviously you need to create tongue and groove.
The easiest way to do it is to lay the plywood insitu, staggering the end joints to produce a strong job (if you don't you wont achieve anything). If you have to cut sheets down then do it with a circular saw making sure you produce accurate cuts/joints (within 1/16th of an inch +/-) and you need to have a 10mm gap for expansion all the way around between the plywood and the wall.
Make a note of where the pieces go then using a router with groover (cutter with a bearing is best) machine a groove all the way around in the plywood sheets to suit 1/4 inch ply. Cut strips of 1/4inch ply so that they are slight less than the overall groove in two sheets. I.E. if you got 10mm in each sheet then your strips need to be 18mm wide.
I would recommend sealing the underneath of the ply with something like Danish oil, primer or a timber preservative, allow to thoroughly dry and you should be using a WBP type ply for this work.
If the concrete has a rough surface you will need to lay a skim of soft sand to prevent the dpm from being puncture. Lay the plywood in, gluing the 1/4 inch ply into the grooves with waterproof pva. Make sure you only use 1, 1/4ply strip between the grooves on both sheets. I recommend Wudcare PVA 5min set glue. You may need to have to some kind of clamp ie flooring strap clamp to ensure you get plywood jointing up together properly.
For further information on clamps/glue visit www.axminster.co.uk. I hope this has help you. Stuart Mawle
