Laird Lugton Posted August 8, 2012 Report Share Posted August 8, 2012 (edited) We need some storage space and the loft has loads of space, we are only looking to put Christmas decorations etc up there, we have no water tank up there either. However, the house is a new build timber frame with the only solid walls being the exterior so the roof is supported by roof trusses. Having googled roof trusses I am aware that the "joist" I stand on is not load bearing and only there as a tensioner so I'm wondering if there are any structural engineers on here? I'm just looking for a quick answer at the moment as to whether it's feasible to put extra batons perpendicular to the "joist" (so that I retain the 270mm of insulation) and the chipboard flooring on top, would that be in the tolerances? What tolerances do they make on these trusses? If it's feasible I'll get a structural engineer in to do a proper calc for insurance purposes etc... My concerns are the weight of the flooring will weigh more than the items we want to put up there as the chipboard loft flooring is very heavy. The truss dimensions are span: 9500mm, trusses are spaced 600mm apart, pitch 37 degrees and the roof is slate which must weigh a ton! Cautious is my middle name Edited August 8, 2012 by Laird Lugton Quote Link to comment Share on other sites More sharing options...
bobt Posted August 8, 2012 Report Share Posted August 8, 2012 (edited) If you are "only" storing light stuff, xmas decorations and empty suitcases, it will be ok to board out an area, as to the insulation, why not replace the area with celotex or similar? 150mm gives the same as 300mm of rockwool. you can board the central area in the "A" part of the truss ok. edit for spellin Edited August 8, 2012 by bobt Quote Link to comment Share on other sites More sharing options...
Fisherman Mike Posted August 8, 2012 Report Share Posted August 8, 2012 (edited) 9500 o/a span is pretty long and right on the limit for your normal 35mm fink roof truss. Are the main members 47mm by any chance ? The bottom chord is generally to stop the main rafters spreading but also takes some of the load by transfer of the diagonal hangers. The only problem with loading the roof is potentially cracking if the ceiling is plastered or the nail heads popping if it is older Pre (97-98 ) and hasnt been fixed with drywall screws. I would be tempted to accurately sketch the roof truss and the roof layout, put the dimensions of the members on it, the spacing and the SOP and phone your local TRADA truss supplier ..Im sure for the price of a pint or two they would run it through the truss programmer and be able to tell you how and where you can load it. As bobt said above usually the central section either side of the hangers can be boarded. but its usual to run a couple of 75 x 50 longitudinal binders across the bottom chords at this position and fix the chip board to this rather than directly to the truss bottom Chord. This will save your insulation zone as well. Most trusses are designed to support a water tank over several trusses so you should be able to do something...but always check first. The weight of slates depend on their thickness, gauge and lap. Edited August 8, 2012 by Fisherman Mike Quote Link to comment Share on other sites More sharing options...
al4x Posted August 8, 2012 Report Share Posted August 8, 2012 slates are relatively light certainly when you compare them to tiles the difference can be massive, as Bob says celotex between the joists and then another layer running over the top and place the boards on that should be fine. You mention the weight of stuff being less than the boards so you will be absolutely fine, no need for calcs etc, your insurers would never ask for them. Quote Link to comment Share on other sites More sharing options...
Laird Lugton Posted August 8, 2012 Author Report Share Posted August 8, 2012 Thanks all for the informative replies. I've not heard of Celotex but will google it. If I can get rid of the Rockwool it would be a bonus as it's itchy stuff! I originally thought of putting extra batons perpendicular to the joist to build up the height to keep the 270mm of insulation. I've been up this evening to measure the trusses and they are 35mm. The depth of the bottom chord is approximately 120mm. On each truss at the Apex there is a sticker from the manufacturer with the job number and telephone number so I'll give them a call sometime this week to see if they can dig out the specs and tell me the load that maybe accommodated. Is there anything lighter than the chipboard for flooring. Is does seem very heavy! Quote Link to comment Share on other sites More sharing options...
ME Posted August 8, 2012 Report Share Posted August 8, 2012 (edited) Pah! Man up! Get up in the loft with the reciprocating saw, cut out the cross members you don't need and shove some asbestos sheeting up there to stick all your gear on. Rip out any unwanted rockwool insulation and make a tea cosy or a liner for your wax jacket. It can also double as a hearth rug and is totally fire proof. Asbestos can be got cheap, sometimes free from a skip. Also, chop up the unwanted truss and burn it in the fire - how's that for upcycling? Edited August 8, 2012 by ME Quote Link to comment Share on other sites More sharing options...
Laird Lugton Posted August 8, 2012 Author Report Share Posted August 8, 2012 ME, did you do my patio by any chance? Quote Link to comment Share on other sites More sharing options...
ME Posted August 8, 2012 Report Share Posted August 8, 2012 ME, did you do my patio by any chance? I do loft conversions, patios, legal advice, childrens parties and bar mitzahs Quote Link to comment Share on other sites More sharing options...
kent Posted August 8, 2012 Report Share Posted August 8, 2012 don't fret the weight of the boards is way offest by the extra stength they give. stuctural enginners go way over the top normal x3 the max load it could potentialy recieve. when they say non structural they just mean don't fully convert a loft leaving them in as is. if your ceiling boards start to crack or you get flexing you have some serious x-mas decs Quote Link to comment Share on other sites More sharing options...
ME Posted August 8, 2012 Report Share Posted August 8, 2012 Don't be tempted to store your jazmag collection up there. Structural engineers do go over the top but that amount of sticky paged magazines weigh a tonne. Quote Link to comment Share on other sites More sharing options...
Laird Lugton Posted August 8, 2012 Author Report Share Posted August 8, 2012 ME, we're not all like you. Embrace technology, everything can be stored digitally..... don't fret the weight of the boards is way offest by the extra stength they give. stuctural enginners go way over the top normal x3 the max load it could potentialy recieve. when they say non structural they just mean don't fully convert a loft leaving them in as is. if your ceiling boards start to crack or you get flexing you have some serious x-mas decs I do wonder if a structural engineer will say no to cover himself.... Quote Link to comment Share on other sites More sharing options...
Fisherman Mike Posted August 8, 2012 Report Share Posted August 8, 2012 Celotex or Kingspan are brilliant but not really suited to insulation in the horizontal plane of the roof so should not be substituted for a Mineral wool. Both products are impervious so could possibly give rise to interstitial condensation and mould growth on first floor ceilings If you are going to remove the horizontal insulation and replace it with celotex it should only be done in the rafter plane and even then it brings with it other problems as you are converting the roof space from a cold roof to a warm roof. You will need to maintain a minimum 50mm air gap from the underside of the sarking felt to the insulation and if you have a depth of rafter of 120 then 150mm insulation (which is what you will need to reach the elemental u value of 0.20 obviously wont go) unless you introduce two layers 1 between and one over the rafters. The depth of Insulation you have now will provide a U value of 0.16 at 270mm, 270mm is the required thickness to meet Part L. ( better than the 150 celotex) Dont forget that if your house is new it will have been designed to comply with current Part L of the building regulations or what ever the equivilent is in Scotland. The designers would have designed it to comply with statutory EPC criteria and perhaps even Code 3 sustainable homes. You may also compromise any NHBC or similar New Home warranty if you change the insulation system without getting Building regulation approval. Its a bum I know but thats the way it is. Best stick with the devil you know... Are you sure your current insulation is not Non Itch varient Most Home builders are now using it in their sustainable homes and have been for some time Quote Link to comment Share on other sites More sharing options...
Fisherman Mike Posted August 8, 2012 Report Share Posted August 8, 2012 ME, we're not all like you. Embrace technology, everything can be stored digitally..... I do wonder if a structural engineer will say no to cover himself.... Structural Engineers dont design roof trusses though its done using Trada certified design software..even then its generally only a component design which they will warrant only if the reccommended bracing they provide is carried out. Roof trusses are not designed for floor loading generally unless they are attic trusses. You will be safe with normal storage loads but dont over do it. Quote Link to comment Share on other sites More sharing options...
Gerry31 Posted August 8, 2012 Report Share Posted August 8, 2012 Nice reply very accurate and concise. Its ever so easy to get carried away with DIY without considering the possible outcome. And its always us builders who get called cowboys Quote Link to comment Share on other sites More sharing options...
Laird Lugton Posted August 9, 2012 Author Report Share Posted August 9, 2012 (edited) Celotex or Kingspan are brilliant but not really suited to insulation in the horizontal plane of the roof so should not be substituted for a Mineral wool. Both products are impervious so could possibly give rise to interstitial condensation and mould growth on first floor ceilings If you are going to remove the horizontal insulation and replace it with celotex it should only be done in the rafter plane and even then it brings with it other problems as you are converting the roof space from a cold roof to a warm roof. You will need to maintain a minimum 50mm air gap from the underside of the sarking felt to the insulation and if you have a depth of rafter of 120 then 150mm insulation (which is what you will need to reach the elemental u value of 0.20 obviously wont go) unless you introduce two layers 1 between and one over the rafters. The depth of Insulation you have now will provide a U value of 0.16 at 270mm, 270mm is the required thickness to meet Part L. ( better than the 150 celotex) Dont forget that if your house is new it will have been designed to comply with current Part L of the building regulations or what ever the equivilent is in Scotland. The designers would have designed it to comply with statutory EPC criteria and perhaps even Code 3 sustainable homes. You may also compromise any NHBC or similar New Home warranty if you change the insulation system without getting Building regulation approval. Its a bum I know but thats the way it is. Best stick with the devil you know... Are you sure your current insulation is not Non Itch varient Most Home builders are now using it in their sustainable homes and have been for some time I think the easiest thing to do will be to leave the insulation as is as I don't want to have damp problems or the cost of buying extra insulation. It's a huge roof, the footprint is 9.5m by about 18m and I only want to floor a portion so will stick to the original plan of taking a layer of insulation off and leaving the bottom layer of insulation between the joists. Then I'll fix batons perpendicular to the joist so the floor is raised and I can get the remaining insulation down between the raised batons without crushing it. The insulation maybe non-itch, but my hands after I've touched it feel as if they have small splinters in them, next time I'll wear gloves!! Will phone the truss manufacturer this morning to see if they can help with loading... Nice reply very accurate and concise. Its ever so easy to get carried away with DIY without considering the possible outcome. And its always us builders who get called cowboys Hence the reason I want to check what load it can take! Edited August 9, 2012 by Laird Lugton Quote Link to comment Share on other sites More sharing options...
Cosd Posted August 9, 2012 Report Share Posted August 9, 2012 The tongue and groove chipboard floor boards you buy for loft floors will distribute the weight over the trusses. This would be sufficient to hold any weight which you can carry up the loft ladder to put in the loft. Go for it..... Cos Quote Link to comment Share on other sites More sharing options...
Gerry31 Posted August 9, 2012 Report Share Posted August 9, 2012 Hi it was by no means ment as a critisim you have done the right thing but you would be surprised how many dont. I was once called to a house where the owner was complaining he couldn't open the bedroom doors upon inspection I found he had removed a support wall in the loft in order to build a train set up there Quote Link to comment Share on other sites More sharing options...
Fisherman Mike Posted August 9, 2012 Report Share Posted August 9, 2012 (edited) Hi it was by no means ment as a critisim you have done the right thing but you would be surprised how many dont. I was once called to a house where the owner was complaining he couldn't open the bedroom doors upon inspection I found he had removed a support wall in the loft in order to build a train set up there Im sure it wasnt seen as a criticism....Ive been called back to many houses we have built over the years to patch ceilings were the plaster has cracked because the loft storage area has been overloaded with years of accumulated junk. One chap who had a flood in his last house even tried to store 600 sand bags in his loft. ............he could quite easily carry them up the ladder one at a time... He thought that because he had an access hatch it was meant for storage..I tookan age to explain to him that it was for access to the water storage tank. He thought that it was for future use as an observatory and he could just cut all the bits hanging down to get his telescope in.! even seen cowboy Builders cut away the hangers of trusses to give more storage space and some even hammer the chipboard flooring through the bottom chord and the ceiling below with 6ins nails... Like I said modern trusses are designed for ceiling loadings not floor loadings but if you are sensible and follow the Truss manufacturers reccommendations you will be fine if you have the original job number and supplier its likely they will be able to print out a 3D layout of the size and position of supplimentary bearers and boarding. Modern fink layout trusses are engineered so that each component is dependant on each other and the loadings are synchronised through the jointing plates. They are engineered to fine tolerances so that the timber sections can be kept as small as possible and make them cheap to produce, dont be tempted to remove any of the components therefore. One thing Ihave learned in 36 years in construction is " if you are going to do a job do it properly the first time" Edited August 9, 2012 by Fisherman Mike Quote Link to comment Share on other sites More sharing options...
Gerry31 Posted August 9, 2012 Report Share Posted August 9, 2012 Well said I totally agree Quote Link to comment Share on other sites More sharing options...
Laird Lugton Posted August 9, 2012 Author Report Share Posted August 9, 2012 Gerry, I didn't take it as criticism! I agree though that do a job properly the first time, it has taken me a while to find that out Quote Link to comment Share on other sites More sharing options...
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